11 replies to this topic

[Tbear1]

I’m using Starsense on my CGEM mount and 9.25 SCT with the field reducer at the moment. 

 

After a Polemaster polar alignment and checked with PHD2 drift align my polar error is about 2 arc minutes. 
 

After calibrating my Starsense camera and doing an auto four alignment, the Starsense tells me the Polar alignment is way off, sometimes as much as a degree and suggests doing an ASPA which in my opinion would actually make the polar alignment worse.  When I do a GoTo for example to Vega, it isn’t in my field of view. It’s close, but just outside my field of view. Adding reference points still doesn’t bring it into my field of view. I have a DSLR and am using CPWI. 
 

I feel that my poor GoTo performance is related to Starsense thinking my polar alignment is so far off, although I know it tries to compensate for it when it models the sky. 
 

Can anyone shed any light on this? I know I can plate solve to find/center targets however sometimes without a field reducer when imaging a deep space object there simply aren’t enough stars in the field of view to plate solve. Therefore I need a pretty good go to accuracy so I can then manually center the target. 
 

 By the  way, my guiding is pretty darn good which supports having a decent polar alignment. 
 

This one has me baffled - your comments would be greatly appreciated.  

[mlord]

My understanding is that, while guiding and go-tos will work with a non-perfect polar alignment, there will be more "stair-casing" while tracking, than with a better polar alignment.

[deansjc]

Assuming you're following all of the steps documented under "best practice" (reference many documents on NexStar alignment here on CN) I would try skipping the PA using the Polemaster and following even an auto 2 star as with the HC and in place of the DSLR for now, your a mid-range FL eyepiece.  Using that view in combination with your finder scope/red dot finder, see if you can improve your precision by having a GoTo fall within view.  Using an 8", I've never been uable to get the target in view.  

 

Ensure first that your finder is aligned to the tube and secure, etc.  You could skip for now any PA routine .  

 

If this works, I would suspect that the Polemaster requires calibration to the mount.  (I have no experience with that.)  Perhaps the HC routine for PA will deliver a decent PA.  My experience suggest it will.  A long exposure at low ISO will confirm achievement of a good PA.  It would leave you then needing to decide what's up with the Polemaster.

 

The most encouraging item in your post, to me, is that you are close.  So, it's a matter of precision in your routine and possibly calibration of the polemaster.  (Even though one would think that the Polemaster should deliver better results.)

 

John

[Tbear1]

My understanding is that, while guiding and go-tos will work with a non-perfect polar alignment, there will be more "stair-casing" while tracking, than with a better polar alignment.

Thanks for your reply. Fortunately my polar alignment according to the ‘ gold standard ‘ which is PHD2 drift alignment is decent and my guiding is fine. My issue is the accuracy of my goto after using Starsense. 

[Tbear1]

Using an 8", I've never been uable to get the target in view.

John

Thanks for your reply. I haven’t tried just using the hand controller with the Celestron alignment and polar alignment routines. ( Although I hate that method, it would be worth a try - that’s why I bought the StarSense ). 
I have used Polemaster for years and it’s great. I double checked it using PHD2 drift alignment which is very accurate in measuring drift in both axis. Also, as I mentioned, my guiding is decent. 
 

I'm interested in what yin said ‘ using an 8 inch you have never been able to get the target in view ‘ - is that by using a StarSense camera or by using the Celestron two star and ASPA method?

Edited by Tbear1, 05 December 2024 - 06:54 PM.

[deansjc]

Jeez, did I say I couldn't?  That was the opposite of what I intended - sorry.  A typo muddled that.  I meant to write that I've never been unable.  Translate: it always worked well enough or me to just do a minor nudge.  My GoTo was consistently visible even after auto two star.

 

I'm a bit rusty, but while you are troubleshooting this, I'm very sure that you can build the model by syncing your targets once centered.  It might be that if you align and then add a few additional points to your model, especially in the part of the sky you are observing, your model will improve.

 

ASPA is a bit of a clutzy method, I agree.  But I have used it to achieve very good results.

 

I am a bit stumped as to what is impeding the quality of the model, but you do require a good PA for best results.

[Tbear1]

Jeez, did I say I couldn't?  That was the opposite of what I intended - sorry.  A typo muddled that.  I meant to write that I've never been unable.  Translate: it always worked well enough or me to just do a minor nudge.  My GoTo was consistently visible even after auto two star.

 

I'm a bit rusty, but while you are troubleshooting this, I'm very sure that you can build the model by syncing your targets once centered.  It might be that if you align and then add a few additional points to your model, especially in the part of the sky you are observing, your model will improve.

 

ASPA is a bit of a clutzy method, I agree.  But I have used it to achieve very good results.

 

I am a bit stumped as to what is impeding the quality of the model, but you do require a good PA for best results.

Thanks for the clarification. I’m wondering if I used some backlash compensation if it would help the StarSense model. ( Then turn it off when guiding ). I have tried added reference points but it didn’t improve much. I do understand the concept of syncing on a target but there again you have to slew it to the center while viewing through a red dot finder which in my view defeats the purpose. Also, I do deep space photography and you can’t see deep space objects with a red dot finder! 
 

I can plate solve but sometimes especially without a field reducer there are not enough stars in the field of view to do that and I must manually center my target  through a series of test images. Obviously I need to at least have part of the target in the field of view to do this. 

[deansjc]

I'm a novice re much of this, transitioning from an 8SE to a 72mm reflector on an HEQ5Pro, so much to learn. I doubt that backlash comp is a factor.  You have two alignment methods available: red dot (hopefully calibrated well, but it's not a precision instrument and prone to misaligning; the other is live view.  I find the latter is better to use after the red dot gets you close but it does require a nearby bright reference.  

 

What you are describing are common challenges.  If you resolve your initial challenge to get your target in your frame, you'll be way ahead.

 

John

[Tbear1]

Well I believe I ( unknowingly) solved my goto issue although I still have a hard time believing my own solution. 
 

Tonight I calibrated the StarSense Camera twice and the offset values were very close to each other. However when I did the four point auto align the goto was still way off. I purposely didn’t add more reference points because quite frankly I’ve been using my setup for 2 years and never had the need. 
 

After using Polemaster and then checking polar alignment using the PHD2 drift alignment routine, it was again around 2 arc minutes. Decent but not perfect. Always been good enough for what I do. 
 

The temperature was dropping and in between my attempts to align again I instinctively tightened down the counterweight knobs ( which were quite tight already ) and the dovetail  knobs holding the ( 20 lb ) tube to the mount. Although snug, I was able to tighten them more than I expected; especially one of them. Understand that they were not loose, but they were not socked down as hard as possible. 
 

The very next four point auto align produced a goto to Vega on the edge of my camera field of view ( which was 58 arc minutes by 39 arc minutes ). I added a reference point on that very spot and did another goto and Vega was dead center. I did a goto to Saturn which was in a different part of the sky and then Jupiter which was in another part.of the sky. Both were dead nuts center, and I mean dead nuts!
 

I shut everything off and repeated this 2 more times ( excluding calibrating the camera ) with identical results. I simply can’t believe tightening those knobs cured the problem. However, that was the only change I made.

 

In hindsight, when I returned home from the summer, while trying to perfectly balance the scope I had loosened them to move the tube slightly and of course I re-tightened them but perhaps not enough? This was the first time since last spring I used the system and it was fine back then. Perhaps the dovetail wasn’t  perfectly seated on the mount and the extra torque on the knobs made the difference. All I can say is that it works fine now. A simple solution to a very elusive problem. 
 

The one thing that doesn’t make sense is the poor Polar error that the four point auto align always says I have. Quite frankly it has always said that and I’ve always just ignored it because my goto’s have always been good as well as my guiding. 
 

Sorry  for the long post but I spent many hours trying to resolve this and can finally sleep again! Thanks to those that offered suggestions. They were certainly on my list to try. 

[deansjc]

Nice!  Yep, hard to believe, but if you were moving from your final alignment point to another part of the sky, there would be a good chance that a shift of the rig would occur and it takes so little movement to "nudge" the GoTo target out of view.

 

I'd still want to figure out the PA variance between drift technique, PoleMaster and ASPA.  Good news is that close is good enough, as stated by PHD2 folks.  

 

We doth obsess too much :-)

[Tbear1]

So to close out this saga, the main cause of my problem  was one or both of the knobs on the saddle not being tight enough. Socking them down  significantly solved the totally off the wall goto I was getting. ( it’s embarrassing to admit that ). 
 

The discrepancy the StarSense camera was indicating relative to polar error after its automatic four point alignment was significantly reduced after I recalibrated the camera using a star in the south and assuring the approach to centering the star finished in the correct direction  to limit the effect of backlash.  ( This was advice from Celestron’s tech support with emphasis on removing the backlash ). I also added one reference point after the auto align. 
 

My goto are now finding its targets and the StarSense Camera polar error typically reads between 2 and 4 arc minutes ( the actual error per PHD2 is around 2 ).

 

My logic for baring my soul here is to give back to the community for others that might have similar issues. I hope some day my experiences will make someone else’s life a bit easier. 

[deansjc]

Your pennance is to take many hours of worthy images or conduct many hours of pleasurable visual work.