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Fixing Guiding on CGEM DX

astrophotography equipment mount
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#1 Yassinem

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Posted 17 September 2019 - 05:15 AM

I have been dealing with guiding issues for the past few months.  I can't seem to get my total RMS error to 1" or less.  At first I was thinking it was the weight, I have a CGEM DX Mount with 1100 Edge HD, and had a ST80 on it along with all my camera equipment I was pushing 44 lbs, the mount can only do 50 so I was getting really close to the max payload limit.  I use this was a hyperstar setup so for most of my imaging I need a guide scope. And I'm using either the Orion Starshoot Autoguider Camera or the asi120mc-s with the same results.

 

Given the hyperstar reduces my focal length to 560mm I can get away with a smaller guide scope. So I removed the ST80 and my dovetail bar, replaced with a 50mm guidescope which reduces the weight by a good 7 lbs.  But I'm still getting the same guiding problems, it helped a little I would say but nothing significant.  I've looked to see if the motors needed tightening and made some adjusments but again no big improvements.  Also ran the guide helper in PHD2 and applied the suggestions but again no major improvements.

 

As far as polar alignment I think I'm pretty good there, I've got a polemaster and I spend quite a bit of time trying to get polar alignment as perfect as possible.  My RMS Error is somewhere betwewn 1.5' and 2" but I can't really take exposures longer than 2 minutes without seeing the stars elongated.  I would like to be able to take 4 minute exposures (without the hyperstar) but to do that I really need to get the RMS error below 1".   So I'm at my whits end, attaching my guide logs from the last session.  Any help would be greatly appreciated. 

 

Guide Log From 9/14: https://drive.google...WCcWVH-WqvgysJB



#2 dcollier

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Posted 17 September 2019 - 08:51 AM

Are You using Periodic Error corrections, that might help?  Also Look at your set up.  Use this as a starting point and let the brain tweak it from there. http://uncle-rods.bl...g-your-phd.html

 

I hope this helps? 

 

                -Dave 



#3 dcollier

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Posted 17 September 2019 - 09:01 AM

Another problem might be  Pixel scale = 4.77 arc-sec/px,  It likely is not be possible to 1"  RMS with this set up.  What is the Pixel scale with the ST 80?  are you using a focal reducer on your guide setup?  I use an exposure  to 1-2 seconds and get acceptable results. 

 

        -Dave 



#4 Yassinem

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Posted 18 September 2019 - 08:56 PM

I have the PEC application but haven't tried it yet, will probably try it this weekend. 

 

As far as the pixel scales here is what I'm working with (what I've tried):

 

ASI120MC-S (3.5 microns):

 - Orion ST80 (400mm): 1.8 arc-sec/px

 - Orion 50mm (162mm): 4.46 arc-sec/px 

 

Orion Starshoot Autoguider (5.2 microns):

 - Orion ST80 (400mm): 2.68 arc-sec/px

 - Orion 50mm (162mm): 6.62 arc-sec/px

 

No focal reducer on the ST80, using the basic setup.  

 

I also just got the ZWO off axis guider as well I was hoping to try.  At that point my focal length would essentially be 2000 mm (.7x Focal reducer @ 2800 mm) So my pixel scale would be 0.36 arc-sec/px.

 

I guess I'm not familiar with how this affects PHD2, is there any documentation out there that can help guide me?



#5 dcollier

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Posted 19 September 2019 - 07:18 AM

S,o If you are working with an SCT  at f/10 or f/7 then I would use the ST 80 and the ASI-120.  Start with the settings in the Uncle Rod Link and then calibrate on that and use the brain Icon to tweak them. Exposure is also key 1-2 seconds is about right.  Any faster than that and you will just be chasing the seeing and will see lots oscillations.  I usually start with 2 seconds.  I was able to get in the 1" range without using PEC by just following Rods guidance and also this link.   https://openphdguidi...stPractices.pdf .  Make sure you are reasonably polar aligned.  No need to be obsessive about it  1'-2' will work fine.   The Orion finder  guide scope will work with focal lengths of the main scope under 1500 mm.  Smaller pixels will be better in most cases.  Just those two links worked for me.   A documentation link is also provided.  Good luck

 

          -Dave 

 

https://openphdguidi.../documentation/



#6 KLWalsh

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Posted 20 September 2019 - 10:18 PM

Have you hypertuned the mount?
I noticed a LOT of improvement from hypertuning my CGEM when balancing the scope, in the smoothness of movement of the mount, and in improved tracking. I polished the ring gears, replaced the bearings, and replaced the lube.

Using PHD2 with ASCOM also help a lot with guiding.

#7 Yassinem

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Posted 21 September 2019 - 12:11 AM

No haven't hypertuned this mount yet I'm a little nervous about doing it myself and I don't know if I want to spend the money on an older mount like this one. But I do have some good news, following the advice here I have been able to get guiding around 1.15" consistently which is the best I've been able to do in a long time.  I feel like I'm getting close to that 1" mark!



#8 Yassinem

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Posted 21 September 2019 - 05:19 AM

I was finally able to get below 1" tonight after messing with the settings, guiding looked really good.  But I've got a new problem on my hands :( Crown shaped stars.  Was very strange looking and I verified it wasn't my guiding they were showing up even at 5 second exposures. Back to the drawing board...



#9 KLWalsh

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Posted 21 September 2019 - 08:39 AM

No haven't hypertuned this mount yet I'm a little nervous about doing it myself and I don't know if I want to spend the money on an older mount like this one. But I do have some good news, following the advice here I have been able to get guiding around 1.15" consistently which is the best I've been able to do in a long time. I feel like I'm getting close to that 1" mark!


I’ve had my original version CGEM since 2012. It was ‘ok’, but I hypertuned it myself last year. SO glad I did. Works WAY better now; it’s like a brand-new mount.
I admit there were some tricky steps to the hypertuning, but Ed Thomas at Deep Space Products was always quick with his replies to my emailed help queries. If you’re not at all handy with tools, send it to Ed and pay the $s to have him do it.
But if you know which end of a wrench is which, you can just follow along with the dvd Ed includes with the kit and diy.

Edited by KLWalsh, 21 September 2019 - 08:40 AM.



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