Re: Back Focus Troubles #Absolute_Encoders

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.

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.



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