Flexure Measurement - How Much is Too Much ?


Mike Dodd
 

On 6/23/2021 10:03 AM, Worsel via groups.io wrote:
Mike

Do you recall when you last created your PEM curve?
Less than a year ago.

Ray answered my question on the PEMPro support forum about reading the curve from the mount, so I know what that looks like (5.45 arcsec peak-to-peak). Tonight I'll run another curve with PEM enabled to see what the corrected tracking error is.

--- Mike


Worsel
 

Mike

Do you recall when you last created your PEM curve?  The curve should be redone every so often (I do my 1100 annually).

The PEM curves are in Documents/CCDware/PEMProVx, where x is the version of PEMPro that you have, 2 or 3 most likely.

Bryan


Roland Christen
 

APCC itself does not have any impact on tracking.
APCC Pro has modeling which can be used to supply a custom tracking rate to the mount. That is another kettle of fish which itself will not change the characteristics of the mount's tracking behavior. It simply adjusts the tracking rate to compensate for various things such as atmospheric refraction, polar misalignment, instrument flex, etc.

Rolando


-----Original Message-----
From: Mike Dodd <mike@...>
To: main@ap-gto.groups.io
Sent: Tue, Jun 22, 2021 1:31 pm
Subject: Re: [ap-gto] Flexure Measurement - How Much is Too Much ?

On 6/22/2021 12:20 PM, Roland Christen via groups.io wrote:
> The 1200 mount does not have encoder capability. It does have PEM which
> can drop the periodic error down to the 1 arc sec level.

I have PemPRO, and used it to create a PEM curve, which is enabled all
the time. At the moment, I can't find the curve, but I did create and
apply a curve.

Assuming PEM is enabled, would APCC improve tracking on my 1200 beyond
what I have now?

Thanks, Roland.

--- Mike








--
Roland Christen
Astro-Physics


Mike Dodd
 

On 6/22/2021 12:20 PM, Roland Christen via groups.io wrote:
The 1200 mount does not have encoder capability. It does have PEM which
can drop the periodic error down to the 1 arc sec level.
I have PemPRO, and used it to create a PEM curve, which is enabled all the time. At the moment, I can't find the curve, but I did create and apply a curve.

Assuming PEM is enabled, would APCC improve tracking on my 1200 beyond what I have now?

Thanks, Roland.

--- Mike


Roland Christen
 

The 1200 mount does not have encoder capability. It does have PEM which can drop the periodic error down to the 1 arc sec level.

Rolando



-----Original Message-----
From: M Hambrick <mhambrick563@...>
To: main@ap-gto.groups.io
Sent: Tue, Jun 22, 2021 10:31 am
Subject: Re: [ap-gto] Flexure Measurement - How Much is Too Much ?

Hi Mike

Does your 1200 mount have encoders ? I understand that these will take you to the next level and beyond as far as tracking goes, especially for permanent setups as you have. I can't remember exactly what combination is needed (Encoders, CP4, Keypad), but there was a discussion thread a while back where Roland talked about building a pointing model to allow unguided imaging. Maybe Roland can comment on this.

I already have a CP4 with my non A-E 1100 mount , and I plan to purchase a set of encoders as soon as my name is called on the wait list. I am also planning to upgrade my keypad as soon as they offer upgrades to existing keypad users.

Mike 

--
Roland Christen
Astro-Physics


Roland Christen
 

First you would need to characterize your mount's tracking. You can do this by measuring the PE using PEMPro. The resultant curves will give you a good idea what your moment to moment and long term tracking errors are. It also allows you to create a compensation curve which is then downloaded into the CP3 controller. When you turn on the PEM function it should result in more accurate and smooth tracking. The only other thing that affects your tracking would be your local seeing. You can characterize that by using the Guiding Assistant in PHD guide software. Then it's just a matter of setting the guiding software properly according to your seeing.

Rolando



-----Original Message-----
From: Mike Dodd <mike@...>
To: main@ap-gto.groups.io
Sent: Tue, Jun 22, 2021 10:09 am
Subject: Re: [ap-gto] Flexure Measurement - How Much is Too Much ?

On 6/21/2021 4:50 PM, Roland Christen via groups.io wrote:
> To get really good images takes a lot of effort, very good tracking of
> the main optics, and a steady and hyper-accurate mount.

Hello, Roland. I am very interested in this, and wonder if you would
advise me if there's anything I can do to my 1200 mount, S/N 1200589, to
improve tracking. I have the CP3 box, S/N 0374, and I installed the
V-chip last year.

My first thought is APCC. Will APCC improve tracking on _any_ A-P mount,
or just newer models? I've read your posts about unguided imaging using
APCC, and this is very appealing to me, but I don't know if my mount can
achieve that.

Is there anything in the CP4 box that will improve tracking? If not, I'd
rather not spend the money on CP4.








--
Roland Christen
Astro-Physics


Mike Dodd
 

On 6/22/2021 11:31 AM, M Hambrick wrote:
Hi Mike

Does your 1200 mount have encoders?
No, and it is my understanding that I cannot add encoders to it. I would love to be proven wrong.

--- Mike


M Hambrick
 

Hi Mike

Does your 1200 mount have encoders ? I understand that these will take you to the next level and beyond as far as tracking goes, especially for permanent setups as you have. I can't remember exactly what combination is needed (Encoders, CP4, Keypad), but there was a discussion thread a while back where Roland talked about building a pointing model to allow unguided imaging. Maybe Roland can comment on this.

I already have a CP4 with my non A-E 1100 mount , and I plan to purchase a set of encoders as soon as my name is called on the wait list. I am also planning to upgrade my keypad as soon as they offer upgrades to existing keypad users.

Mike 


Mike Dodd
 

On 6/21/2021 4:50 PM, Roland Christen via groups.io wrote:
To get really good images takes a lot of effort, very good tracking of
the main optics, and a steady and hyper-accurate mount.
Hello, Roland. I am very interested in this, and wonder if you would advise me if there's anything I can do to my 1200 mount, S/N 1200589, to improve tracking. I have the CP3 box, S/N 0374, and I installed the V-chip last year.

My first thought is APCC. Will APCC improve tracking on _any_ A-P mount, or just newer models? I've read your posts about unguided imaging using APCC, and this is very appealing to me, but I don't know if my mount can achieve that.

Is there anything in the CP4 box that will improve tracking? If not, I'd rather not spend the money on CP4.


Jeffc
 

Fwiw.. Some time back I moved towards using OAGs. (e.g. ST-8300m + STi on SBIG OAG)  
At most focal lengths (less than 1100mm) I don't have a problem finding stars in the OAG with the STi camera.   
I've also used the Lumicon GEG OAG with a DSI-Pro guide camera.. worked pretty well; rarely had issues finding stars.

With the Mach2, I'm not even bothering with the OAG.   (I hope I no longer need to use guidescopes either, but if i did, i would probably look for an OAG compatible with the imaging camera.)

-jeff

On Mon, Jun 21, 2021 at 9:48 PM M Hambrick <mhambrick563@...> wrote:
Thanks for the link to your web site Mike. You have some really good information in there. 

I am going to try to get some more flexure measurements over a longer period of time next time we get a clear night here. I noticed in your example of differential flexure that you were getting shifts of multiple pixels. Although I only had one 10 minute subframe, the shift that I measured was much less than one pixel, and the stars in that image looked good.

I am trying to look at everything I can to try to improve my images. Until now I have concentrated all of my efforts on are focusing and guiding. I have not seen anything in my images so far to suggest that I have a problem with flexure, but I want to get an idea of how much there is.

Mike


M Hambrick
 

Thanks for the link to your web site Mike. You have some really good information in there. 

I am going to try to get some more flexure measurements over a longer period of time next time we get a clear night here. I noticed in your example of differential flexure that you were getting shifts of multiple pixels. Although I only had one 10 minute subframe, the shift that I measured was much less than one pixel, and the stars in that image looked good.

I am trying to look at everything I can to try to improve my images. Until now I have concentrated all of my efforts on are focusing and guiding. I have not seen anything in my images so far to suggest that I have a problem with flexure, but I want to get an idea of how much there is.

Mike


Roland Christen
 

Looking good is one measurement. All the stars are round is comforting, and that's all that some want in their images. I have seen round stars in posted images on CN where people have imaged with mounts that have huge periodic errors, imaged unguided with those mounts, imaged with small short guide scopes etc., and had roundish stars. Broke all the rules and still had round stars. But if you look closely at full size you can see they lack sharpness and resolution, and most of them are out of focus because they also did not refocus critically each frame as the temp dropped. I compared one of my images of M57 with one I saw posted on CN. The stars were round on the CN image but they were 3 times as big and the resolution in the nebula was also really poor. Almost the same focal length, image scale and aperture!

To get really good images takes a lot of effort, very good tracking of the main optics, and a steady and hyper-accurate mount.

Rolando



-----Original Message-----
From: Mike Dodd <mike@...>
To: main@ap-gto.groups.io
Sent: Mon, Jun 21, 2021 3:15 pm
Subject: Re: [ap-gto] Flexure Measurement - How Much is Too Much ?

On 6/21/2021 3:29 PM, M Hambrick wrote:
> Roland has stated on multiple occasions that using a separate guide
> scope is not the ideal way to do guided imaging because of flexure. How
> much flexure is considered excessive ?

ANY flexure is too much! :-) You can read about my experience here:
<http://astronomy.mdodd.com/flexure.html>

> ...after a single 600 second exposure I measured the
> shift in the position of the guide star centroid to be 0.053 pixels in
> the X-direction, and 0.304 pixels in the Y-direction. This doesn't seem
> to be too bad in the X-direction, but I am not so sure about the
> Y-direction. Does anyone have any thoughts on this ?

The proof of the pudding is in the eating. If your images look good,
then your flexure is so small you shouldn't have to worry about it.

--- Mike








--
Roland Christen
Astro-Physics


Mike Dodd
 

On 6/21/2021 3:29 PM, M Hambrick wrote:
Roland has stated on multiple occasions that using a separate guide
scope is not the ideal way to do guided imaging because of flexure. How
much flexure is considered excessive ?
ANY flexure is too much! :-) You can read about my experience here: <http://astronomy.mdodd.com/flexure.html>

...after a single 600 second exposure I measured the
shift in the position of the guide star centroid to be 0.053 pixels in
the X-direction, and 0.304 pixels in the Y-direction. This doesn't seem
to be too bad in the X-direction, but I am not so sure about the
Y-direction. Does anyone have any thoughts on this ?
The proof of the pudding is in the eating. If your images look good, then your flexure is so small you shouldn't have to worry about it.

--- Mike


M Hambrick
 

Roland has stated on multiple occasions that using a separate guide scope is not the ideal way to do guided imaging because of flexure. How much flexure is considered excessive ? I believe that it was also Roland who suggested a way to detect flexure by using the information box in MaxIm DL to measure the position of the guide star in the main imaging camera at the start and at the end of the guiding session. The assumption is that the guide star position in the guiding camera will not change, but if there is flexure, it can be seen as a shift in the position of the star in the main imaging camera.

I tried this the other night. It was a very short session because the seeing was so bad, but after a single 600 second exposure I measured the shift in the position of the guide star centroid to be 0.053 pixels in the X-direction, and 0.304 pixels in the Y-direction. This doesn't seem to be too bad in the X-direction, but I am not so sure about the Y-direction. Does anyone have any thoughts on this ?

Mike