Back Focus Troubles #Absolute_Encoders


M Hambrick
 

Hi David

In my sketch, item C is a custom adapter that I purchased from Precise Parts. On the Precise Parts web site I specified the A-P 92 Stowaway flattener (92FF) on the scope side, and on the camera side I specified the SBIG FW8S-STXL Filter Wheel with Dovetail noseplate. The dimension "C" on the sketch is the effective length of the adapter. The determination of this dimension is discussed below. 

If you look on the sketch you will see dimension "X". This distance (64.1 mm or 2.254") is the specified backfocus distance from the rear face of the 92FF to the CCD sensor surface. This dimension comes from the specifications for the 92FF from Astro-Physics.

This nominal backfocus distance is the sum of dimensions A + B + C + D where:
  • A = The OPTICAL backfocus distance from the face of the camera/filter wheel to the CCD surface. This dimension is fixed and comes from the specifications for the camera and filter wheel. Make sure to use the optical backfocus distance and not the mechanical backfocus. I made that mistake once and had to get the adapter modified.
  • B = The thickness of the dovetail nose piece on the filter wheel. This dimension also comes from the specifications for the camera.
  • D = The thickness of a 1 mm spacer ring (0.040"). It is recommended that the custom adapters should be designed so as to allow the actual backfocus distance to be tweaked to optimize the flatness of the images in the corners. The adapter length is therefore specified assuming that there will be one spacer ring to get the nominal backfocus. You can remove the spacer ring to get the nominal backfocus - 1 mm, and you can add an additional spacer ring to get the nominal backfocus + 1 mm.
  • C = This is the effective length of your adapter and is equal to X - (A + B + D)

I ordered the spacer rings from Precise Parts because Astro-Physics does not offer them - Yet. I believe I saw a post where they plan to do so soon. These spacer rings have to fit inside the custom adapter and rest against the shoulder. They were not cheap, and I would suggest asking Astro-Physics when they will be available before you go to Precise Parts. If you go the Precise Parts route, you will specify a washer/shim on the scope side, and on the camera side you must specify the A-P 92FF/92TCC Canon EF camera adapter. The effective length is 1.0 mm.

I hope this helps

Mike  


M Hambrick
 

Hi David

I am out of town until tomorrow evening. I will send some information when I get back home.

Mike


On Jun 11, 2021, at 8:02 PM, David Diaz <night.skywatcher@...> wrote:

Mike

Awesome chart; thanks.

Can you please tell me how to communicate to PreciseParts items ‘C’ and ‘D?’

Thanks!

David D


David Diaz
 

Mike

Awesome chart; thanks.

Can you please tell me how to communicate to PreciseParts items ‘C’ and ‘D?’

Thanks!

David D


Andrew J
 

Hi Linwood.

I was also concerned about the 2.75" vs. 2.67" back focus and which one I should use. I called Tele Vue and talked with David and asked him which number I should use for a KAF-16200 sensor with 34.6mm diagonal, and he told me to go with the 2.75" value. I hope this is correct, as I already placed my order with Precise Parts for the 2.4" to ATIK EFW3 M54 adapter. If he gave me the wrong number, then I will be about 1mm to long even if leave out the 1mm shim. I will not be a happy camper if they gave me the wrong number.

Good info about guiding from the rings. If I have to go the OAG route then I will have to start all over again with the spacers and likely have to go back to PP again for another adapter. I just receive a new 1600 mount with encoders. I haven't set it up yet as I am still waiting on a few parts. I have been following the unguided discussions on the board with interest. Maybe I will get lucky and be able to image without guiding. 

Thanks for the heads up about the spacing above the saddle. The good news is I think I have a solution already for this issue. I had the same problem with a Lunt 80mm solar scope. If I mounted the scope directly to the saddle the dew shield would hit the saddle and it would interfered with the etalons. To solve this problem I got a Male to Male D Series Adapter (DMM12) and two D Series Female to V Series Female adapters (DPA-VPA-FF) from ADM. This got the scope up off the saddle by about 1.5 inches. It also gave more flexibility when it came to balancing the scope as the Lunt is very back end heavy. Assuming I can get a Vixen style dovetail to fit the Tele Vue Rings, I should be OK. ADM has a VDUP7-V Series dovetail that should work, but I am waiting to get the rings so I can check the measurements with ADM. 

Thanks for all the helpful info.  Glad to hear you are happy with the scope.

Andrew 


Don Anderson
 

No problem PM me anytime. 
Do you need my email address?

Don Anderson


On Thursday, June 3, 2021, 09:24:28 a.m. MDT, Andrew Jones <andjones132@...> wrote:


Hi Don.

I read your post with interest. What set me down the path to finally get a definitive answer to how to calculate the Back Focus with Filters is that I just acquired an TV NP101is OTA, the little brother of the NP127is. Your setup is similar to mine. I have an ATIK 16200 with an EFW3 FW. I ordered the TV NP101is Field Flattener (LCL-1069), which per TV has a recommended Back Focus of 2.75" (69.85mm) for the ATIK 16200 34.6mm diagonal sensor. I also ordered TV's Set of 6 Accessory Tubes for 2.4" (TLS-2245). I plan use two of the spacers in the set to for the recommend 1.375" (34.9mm) spacing between the focuser and the FF. The remaining two spacers (0.500" and 0.250") will be placed after the FF. The remaining spacing will have to filled with a Precise Parts Adapter as TV does not make an adapter to go from their 2.4" accessories to the M54 thread I need to connect to the ATIK EFW3. I ordered the the Precise Parts adapter 1mm short and will use the shims included in the 6 piece spacer set to the required Back Focus. I could have just ordered a longer Precise Parts Adapter and skipped the two spacers after FF, but I wanted to keep at least two places where I could add shims as you can only use one shim for each threaded connection. Plus the shorter PP Adapter was cheaper... 

I have some questions about attaching this scope to the 16" DOVELM162 saddle that I have on my AP 1600 mount. I also need to figure out a guide scope solution. I have the 10 x 60 Vario Finder with Quick Release Guider Bracket Kit. (1060VGKIT) that I use with my TEC140, but the only way to use this scope with the NP101is is to commit the cardinal sin of mounting the guide scope to the rings. Although, this may be a less of a sin for a wide field scope. I may have to go OAG route, but I would like to see if I can make what I have work first. Do you mind if I PM you with a few questions about your setup?

Andrew

 


ap@CaptivePhotons.com
 

> have an ATIK 16200 with an EFW3 FW. I ordered the TV NP101is Field Flattener (LCL-1069), which per TV has a recommended Back Focus of 2.75" (69.85mm) for the ATIK 16200 34.6mm diagonal sensor.

 

>Although, this may be a less of a sin for a wide field scope. I may have to go OAG route, but I would like to see if I can make what I have work first. 

 

Andrew, one last chime in and I’ll leave you to it…

 

I have that scope, and I think you will love it.  Their backfocus distance recommendation is a bit unusual, as they later say for “larger [than 35mm] formats, use 2.67in instead of 2.75in”.  I talked to TV and when describing my setup with a ASI6200 (which is 35mm format) he kept saying 68mm (2.67” = 67.8mm).  I made the mistake of ordering from precise parts without ability to adjust, but minimum length (with OAG below) and am at 68.7mm, and I think ended up slightly too long based on corner star shape.  So I think you are wise to preserve some additional adjustment room.  In my discussions with them (David Nagler if I recall, not sure) they emphasized a certain amount of variability unit to unit.

 

I added a Celestron OAG – which is definitely NOT made for that OTA’s backfocus as it is really thick at 29mm – as I use it with a C11 as well.  I added it after using a 60mm guide scope on top of the rings.  On the TV double ring set the guide scope was pretty secure there, but I thought I would get better guiding both from a flex standpoint and magnification with an OAG, and I did, improving by maybe 20-30% of RMS (with the usual caveat that there’s so much randomness night to night it is hard to trust such measurements).  Plus I can disconnect from the TV101is and move the entire setup to the C11 without losing focus.

 

My real reason for writing though is the mounting of all this.  The TV Ringset is very short, and does not give any room above the dovetail, indeed negative room to the focus knobs.  And especially with a filter wheel and OAG this is all very back heavy, and I find the dovetail forward enough that the focuser would hit the saddle.  I ended up putting in a pedestal to raise the rings about a half inch or so to allow the focus knobs room if I slid the dovetail forward in the saddle.

 

For perspective here it is mounted on my MyT (still waiting for the AP1100 but the issue will be similar as they don’t even extend above the dovetail much less saddle without some kind of pedestal).

 

Linwood

 


Andrew J
 

Hi Don.

I read your post with interest. What set me down the path to finally get a definitive answer to how to calculate the Back Focus with Filters is that I just acquired an TV NP101is OTA, the little brother of the NP127is. Your setup is similar to mine. I have an ATIK 16200 with an EFW3 FW. I ordered the TV NP101is Field Flattener (LCL-1069), which per TV has a recommended Back Focus of 2.75" (69.85mm) for the ATIK 16200 34.6mm diagonal sensor. I also ordered TV's Set of 6 Accessory Tubes for 2.4" (TLS-2245). I plan use two of the spacers in the set to for the recommend 1.375" (34.9mm) spacing between the focuser and the FF. The remaining two spacers (0.500" and 0.250") will be placed after the FF. The remaining spacing will have to filled with a Precise Parts Adapter as TV does not make an adapter to go from their 2.4" accessories to the M54 thread I need to connect to the ATIK EFW3. I ordered the the Precise Parts adapter 1mm short and will use the shims included in the 6 piece spacer set to the required Back Focus. I could have just ordered a longer Precise Parts Adapter and skipped the two spacers after FF, but I wanted to keep at least two places where I could add shims as you can only use one shim for each threaded connection. Plus the shorter PP Adapter was cheaper... 

I have some questions about attaching this scope to the 16" DOVELM162 saddle that I have on my AP 1600 mount. I also need to figure out a guide scope solution. I have the 10 x 60 Vario Finder with Quick Release Guider Bracket Kit. (1060VGKIT) that I use with my TEC140, but the only way to use this scope with the NP101is is to commit the cardinal sin of mounting the guide scope to the rings. Although, this may be a less of a sin for a wide field scope. I may have to go OAG route, but I would like to see if I can make what I have work first. Do you mind if I PM you with a few questions about your setup?

Andrew

 


Don Anderson
 

Hello Andrew
I have been following your thread on this with interest since I went through this exercise some years ago when setting up my imaging train on my Tele Vue NP127is refractor. The concepts can be difficult to get and keep straight in ones head and it is very easy to get confused especially when adding correcting optics and allowing for filter refraction. Mike gave a very good description of how to handle correcting optics such as reducers and field flatteners that are introduced downstream of the focuser.
As you and Mike discussed, spacer tube selection is not necessarily critical when setting up for native F/L imaging or for imaging with correctors in the train as long as one has lots of back focus to work with. However, most scopes have limited back focus capacity and now a days astro-imagers are trying to cram more gear into their imaging trains. Further, scopes other than premium ones have focusers that are not as robust as one would like and can not handle even moderately heavy imaging trains without significant draw tube sag if the draw tube is racked out a significant distance. For these and even for really good focusers, it is best to limit the drawtube extension when at focus to as little as practical to minimize sag.
I have a Tele Vue NP127is refractor with the Tele Vue stock 2.4" focuser. My imaging train consists of a Starlight Xpress SXVR H-694 camera attached to an SX filter wheel. I can image at native (660mm f/l) using spacer tubes or I can image at 580mm f/l using the TV NPR1073 .8X FR using appropriate spacer tubes to meet the 55mm reducer BF spec as well as the necessary spacer tubes to bring the sensor into focus. Tele Vue recommends setting up the imaging train so that the draw tube extends out no more than 25mm at focus. I started by detersmining the scope back focus from the back of the rear lens cell (The NP127is is a Nagler-Petzval design) which for the NP127is is 278mm (10.96"). Using the rear of the lens cell as a datum, I calculated all spacer dimensions from that datum point. I made sure for all setups that I selected the spacers that would position the draw tube 12-19mm (1/2"-3/4") out from a fully racked in position when the sensor was at focus. I have attached a drawing to illustrate what I am talking about. Another setup which I have yet to try, is imaging with my 2X Powermate (1320mm F/L). This should be easy since the back focus distance behind the Powermate is pretty flexible. I just need to fit within the 10.96" overall scope back focus.
I hope this is of interest.
Regards


Don Anderson


On Wednesday, June 2, 2021, 11:26:47 a.m. MDT, Andrew Jones <andjones132@...> wrote:


Update: I mentioned in my original post that I hand contacted a Telescope and Eyepiece manufacture and got two different answers to the following scenario.

 

Assume there was correcting element that moves with the focuser that requires 50mm of fixed back focus and the imaging chain had a camera and filter wheel (with no filters) with a physical length of 25mm. In this configuration a 25mm spacer would be required to achieve back focus. If a 3m thick filter that adds 1mm of back focus is installed in the filter wheel, what would be the length of the spacer needed to achieve the correct back focus?

 

One person told me 24mm, another said 26mm.

 

This discrepancy is what led me to post the question here. I am happy to report that the person who told me 24mm tracked me down today and changed their answer to 26mm. They explained that adding the filter to the optical path increased the total back focus of the system. Introducing the filter increases the total back focus of the correcting element from 50mm to 51mm. Therefore the length of the spacer required to achieve the correct back focus for this scenario would be 26mm (50mm native BF + 1mm BF from filter – 25mm physical length of other components = 26mm spacer). I have to say, I was really impressed that the company would take the effort to contact me to correct their mistake. This is a rare thing.

 

This correction helped validate my initial thinking that adding a filter that adds 1mm of back focus changes the total back focus of the system as the starting point for the spacer calculation, before subtracting the physical distance of other components in the imaging chain. It is easy to get this backwards and add the 1mm to the components that are subtracted from the native back focus of the correcting element. This is the mistake the person made who said I would need a 24mm spacer.

 

I found this explanation helpful so thought I would pass it along. It also demonstrates how easy it is to make a mistake that would lead to a 2mm error in the calculation.

 

If adapters are ordered 1 – 2mm shorter than required and shims are used to achieve the correct back focus as described in Dale’s post then this mistake is easy to correct. If adaptors are ordered to the exact calculated length as I use to do, this can be a cost mistake to correct. After talking to the person yesterday who told me 24mm, I thought I was going to have to order a bunch of new adapters because I had ordered them to exact length and if what he told me was correct, it meant that all my adapters were 2mm to long. Thankfully, it appears I might be OK if I ignore the tolerances. Ordering them shorter than needed is definitely the way I plan to go in the future.

 

I hope this was useful.

 

Andrew


Sébastien Doré
 

I always wondered what the impact would be on these systems when a Crayford focuser is used and the primary mirror is locked at a specific focus position, effectively swapping the variable and fixed portions of the optical chain. 

I too had that question on my mind for quite some time for the EdgeHDs (I own the C8). 

From my readings, my understanding at this point is that with a crayford-type focuser in the optical chain, the backfocus spacing from the last optical element still needs to be pretty accurate (to < 1 mm) to optimize the field flatness over the entire sensor area / illumination circle. 

Hence, with a crayford on these scopes, the focus point needs to be pre-set with the primary mirror focuser prior to locking the clutches and should then only be fine-tuned by the crayford (auto)focuser to microns precision. The backfocus spacing tolerance for field flatness being an order of magnitude or so greater than that of focus point, field flatness should not, in theory, be impacted (much) by the change in backfocus spacing from the crayford reaching the focus point.

But as I said, this is my understanding only. If someone has a better explanation, I'm very interested in hearing it too.

Sébastien


Andrew J
 

On Wed, Jun 2, 2021 at 08:11 AM, ap@... wrote:
the majority of what the external focuser on the SCT is doing is restoring the same focus length (and so same back focus length), not so much changing it.
ap,

Thank you for that explanation. Using a focuser to correct or compensate for these changes makes sense in a standard SCT without any correcting elements in the light path. Moving the Primary Mirror or moving camera sensor should have the same net result.

I think this becomes more complicated when using an Edge HD system due to the glass correcting element in the baffle or adding the .07x focal reducer. 146.05mm is a very specific number. I always wondered what the impact would be on these systems when a Crayford focuser is used and the primary mirror is locked at a specific focus position, effectively swapping the variable and fixed portions of the optical chain. It is not something i necessarily need an answer to as I no longer own and Edge HD, but just curious. 

Andrew


Andrew J
 

Update: I mentioned in my original post that I hand contacted a Telescope and Eyepiece manufacture and got two different answers to the following scenario.

 

Assume there was correcting element that moves with the focuser that requires 50mm of fixed back focus and the imaging chain had a camera and filter wheel (with no filters) with a physical length of 25mm. In this configuration a 25mm spacer would be required to achieve back focus. If a 3m thick filter that adds 1mm of back focus is installed in the filter wheel, what would be the length of the spacer needed to achieve the correct back focus?

 

One person told me 24mm, another said 26mm.

 

This discrepancy is what led me to post the question here. I am happy to report that the person who told me 24mm tracked me down today and changed their answer to 26mm. They explained that adding the filter to the optical path increased the total back focus of the system. Introducing the filter increases the total back focus of the correcting element from 50mm to 51mm. Therefore the length of the spacer required to achieve the correct back focus for this scenario would be 26mm (50mm native BF + 1mm BF from filter – 25mm physical length of other components = 26mm spacer). I have to say, I was really impressed that the company would take the effort to contact me to correct their mistake. This is a rare thing.

 

This correction helped validate my initial thinking that adding a filter that adds 1mm of back focus changes the total back focus of the system as the starting point for the spacer calculation, before subtracting the physical distance of other components in the imaging chain. It is easy to get this backwards and add the 1mm to the components that are subtracted from the native back focus of the correcting element. This is the mistake the person made who said I would need a 24mm spacer.

 

I found this explanation helpful so thought I would pass it along. It also demonstrates how easy it is to make a mistake that would lead to a 2mm error in the calculation.

 

If adapters are ordered 1 – 2mm shorter than required and shims are used to achieve the correct back focus as described in Dale’s post then this mistake is easy to correct. If adaptors are ordered to the exact calculated length as I use to do, this can be a cost mistake to correct. After talking to the person yesterday who told me 24mm, I thought I was going to have to order a bunch of new adapters because I had ordered them to exact length and if what he told me was correct, it meant that all my adapters were 2mm to long. Thankfully, it appears I might be OK if I ignore the tolerances. Ordering them shorter than needed is definitely the way I plan to go in the future.

 

I hope this was useful.

 

Andrew


ap@CaptivePhotons.com
 

Andrew:


> In this case the last correcting element does not move with the focuser and distance between the last correcting element and the sensor will change as the focuser is repositioned. In examples where the last correcting element does not move, why is dialing in a specific back focus distance using spacers so critical if the distance is going to change depending on the position of the focuser?

In a terrestrial lens focus distance (indeed even focal length) changes with distance to the subject, due to angles of the corresponding light rays changing with distance.  In Astro work, everything is effectively at the same distance from us (parallel rays, effectively infinity) and the primary reason we actually need a focuser (beyond initial setup) is to correct for temperature change, which is primarily a dimensional changes in the focal path.  While more complex optics make it more complicated (and in theory temperature could affect the glass not just dimensions of the OTA and train), the majority of what the external focuser on the SCT is doing is restoring the same focus length (and so same back focus length), not so much changing it.


Andrew J
 

Hi Mike.

This is really good example of the two "types" of back focus, fixed vs. variable. The X back focus distance is fixed will not change when the focuser is repositioned. In this case I could understand the argument for getting the back focus distance as close as possible per the specs of the Field Flattener. However, the Y back focus is variable and will change when the focuser is repositioned. When the back focus is variable, I believe the documented back focus is a "recommended" value to help ensure focus can be achieved within the travel limits of the focuser. In the case where the back focus distance is variable it probably is not worth stressing over every mm of back focus. As long as it is close and still within the travel distance of the focuser then that should be good enough. 

I think you nailed the explanation of the two different types. This has helped me get my head around back focus and understanding the difference between fixed vs. variable back focus. Thank you for your detailed post. 

Andrew


Andrew J
 

On Tue, Jun 1, 2021 at 02:01 PM, Dale Ghent wrote:
at it's a good idea to not fixate on a single exact number when it comes to back focus dista
Hi Dale.

I think this is really good advice. In the past I always ordered my Precise Parts adaptors to be exactly the calculated length based on the scope/correcting element's published specs minus the other elements in the optical path. Probably not a good practice as even PP states their parts have +/- 1mm tolerance. Combined with the +/- tolerances of the other components in the optical path you mentioned and it is probably never going to be the calculated distance. I actually sent a note to Precise Parts last night on how to create shims using their configurator for non-standard thread sizes. Agena Astro Products sells Fine Adjustment Spacer Rings for standard thread sizes. Going forward, I will follow your advice and always order my Precise Parts 1mm short and use shims to get to the correct length. 

Andrew


Andrew J
 

Hi Jeff.

You are right, I picked a bad example. I remembered after my initial post that the FF on the TEC140ED moves with the focuser and because it is the last correcting element before the sensor means that the distance between this element and the sensor will not change when the focuser is repositioned. So, again bad example.  My understanding is that the back focus is determined by the LAST correcting element in the light path before the camera sensor. Therefore, I should have used the example of the TEC140ED without the FF which I think has a recommended BF of 170mm.

Another example I am familiar with is the Celestron EDGE HD scopes that have a correcting element in the baffle tube of the scope. Celestron states in their Edge HD White Paper that back focus for the EDGE HD 925, 1100, 1400 is 146.05mm. However, it is fairly common for people to add Crayford focusers to the back of the EDGE HD scopes and lock the primary mirrors. In this case the last correcting element does not move with the focuser and distance between the last correcting element and the sensor will change as the focuser is repositioned. In examples where the last correcting element does not move, why is dialing in a specific back focus distance using spacers so critical if the distance is going to change depending on the position of the focuser?

Andrew


Andrew J
 

Hi Linwood.

Thanks for the OPT link. I hadn't seen that before and is quite helpful. I have book marked it for future reference. 

Andrew


M Hambrick
 

Hi Andrew

I will try to throw my two cents worth into this discussion, and hopefully will not confuse you even more.

First, If the filters are said to add 1mm of backfocus, I would interpret this to mean that they will add to the total backfocus length so that instead of 85 mm your backfocus will be 86 mm.

Second: To your question about why the backfocus specification matters I would say that unless you are using a telecompressor or field flattener in your imaging train then the backfocus probably doesn't matter (as much) because as you stated, you will adjust the focuser until the image is in focus, and that is as good as you will get.

Take a look at the drawing below. You will see that there are actually two backfocus dimensions; dimensions X and Y. If you are using a telecompressor or field flattener, dimension X from the back of the compressor or flattener is critical and will determine whether you get pinpoint stars across the whole image, or if they are points in the middle and elongated at the corners. Just about every field corrector or compressor on the market will have a specification for the total backfocus, and if you have filters you will have to account for them in determining what the actual backfocus is. As Dale has said, it is better to be a little bit undersize and to make up any difference using shims. Once you have assembled your imaging train, the distance from the rear of the corrector to the sensor is fixed, but it will still be necessary to adjust the focuser to reach focus.

If you are not using a field flattener, then dimension Y is really what you are interested in because it will give you an idea of how far out the focuser has to be racked to achieve focus. You may decide that if you have to rack your focuser a long way that it is better to add an extension to your imaging train. In the sketch below you can see that I have to rack out the focuser on my Stowaway about 2-1/2". I am considering adding 2" thread-on extension in front of the flattener so that I do not have to rack the focuser out so far.

The worst case scenario is if you can not rack your focuser in far enough to reach focus.

I hope this helps.

Mike


Dale Ghent
 

What Linwood said. I'll add that it's a good idea to not fixate on a single exact number when it comes to back focus distance. There are manufacturing variations in the objective or mirror of the scope as well as the corrector that will combine to slightly alter what the back focus distance turns out to be. In other words, the distance on paper will land you pretty close and on slow scopes it might be just fine, but faster focal ratios will require some more tuning. In it end, it might be that the BFD wasn't 80.8mm, it might be 80.2 or 81.1 or something close, but not exactly what the documentation says.

With that in mind, I always slightly undersize my ensemble of adapters and extensions by 1 or 2mm. I then use shims, available in an assortment of diameters and sub-1mm thicknesses, to bump the sensor out some more until it its inside the corrected field. When fine-tuning this distance, the on-paper value isn't what you follow, it's what you see in the corners of your frames.

On Jun 1, 2021, at 16:00, ap@captivephotons.com <ap@CaptivePhotons.com> wrote:

The best example I have seen is the spacing guide in this web site about half-way down. It shows what happens to an in-focus image when spacing is too short or too long.



https://optcorp.com/blogs/astronomy/how-to-set-the-correct-back-focus



The center will always look good, since you can reach focus in the center. But without proper backfocus the corners are then distorted (well, they are always somewhat distorted but the right backfocus gives them the minimum distortion, which is sort of its definition).



My understanding of a filter is it adds, meaning if the backfocus from OTA to camera was specified as 146.5mm (e.g. my C11) then when I add a 3mm Chroma filter, I add 1/3rd that thickness, and now use 147.5mm in total.



I find that the most confusing things about backfocus are all the (legacy I think) descriptions of 55mm and DSLR spacing, where the ASSUMPTION is you are starting with a specific set of camera and T-Adapter lengths, and so they only talk about the rest of the equation.


The second most confusing is the point of measurement. My Refractor is measured from the shoulder of the male threads of the last component, i.e. the point closer to the OTA at the base of the threads. My SCT is measured from exactly the opposite, on its male threads you measure from the end of the threads, the further point from the OTA. This is a detail often hard to find, but can vary by 10mm or more. Indeed in that opt article it shows the backfocus measurement point matching my refractor, but WRONG for my SCT.



And yes, backfocus helps keeps Precise Parts gainfully employed. 😊



Linwood




Jeffc
 




On Jun 1, 2021, at 12:35 PM, Andrew Jones <andjones132@...> wrote:




My second question is, why does Back Focus spacing matter? When using a refractor with a Crayford style focuser, it is not clear why getting precise Back Focus spacing is so important when the camera sensor will be moving in or out to achieve focus anyway.


My understanding is the flattner goes on the focuser drawtube and the camera attaches to the flattner.  At least that is the how the AP and just about every other flattner I’ve seen works… it threads on to the draw tube.  

Ie.  path of the photons is:
Front lens -> OTA -> focuser body -> focuser draw tube -> flattner -> filter wheel -> camera -> sensor. 

The focuser focuses incoming light on the flattner , similar to an eyepiece.  The space between the flattner and the sensor is critical and should be fixed .. and likely/hopefully  not affected by temperature changes. 

The FSQ 106n (which I’ve used) has elements fixed inside the OTA which did not move with the focuser but this was a different optical design than the typical doublet or triplet refractor. 

Flattners and reducers afaik will not deliver consistent FOV over focus changes which would be required as temperature changes. 

-jeff 


ap@CaptivePhotons.com
 

The best example I have seen is the spacing guide in this web site about half-way down.  It shows what happens to an in-focus image when spacing is too short or too long.

 

https://optcorp.com/blogs/astronomy/how-to-set-the-correct-back-focus

 

The center will always look good, since you can reach focus in the center.  But without proper backfocus the corners are then distorted (well, they are always somewhat distorted but the right backfocus gives them the minimum distortion, which is sort of its definition).

 

My understanding of a filter is it adds, meaning if the backfocus from OTA to camera was specified as 146.5mm (e.g. my C11) then when I add a 3mm Chroma filter, I add 1/3rd that thickness, and now use 147.5mm in total.

 

I find that the most confusing things about backfocus are all the (legacy I think) descriptions of 55mm and DSLR spacing, where the ASSUMPTION is you are starting with a specific set of camera and T-Adapter lengths, and so they only talk about the rest of the equation.


The second most confusing is the point of measurement.  My Refractor is measured from the shoulder of the male threads of the last component, i.e. the point closer to the OTA at the base of the threads.  My SCT is measured from exactly the opposite, on its male threads you measure from the end of the threads, the further point from the OTA.  This is a detail often hard to find, but can vary by 10mm or more.  Indeed in that opt article it shows the backfocus measurement point matching my refractor, but WRONG for my SCT.

 

And yes, backfocus helps keeps Precise Parts gainfully employed.  😊

 

Linwood