Troubleshoot a mount for star trailing

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Raed
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Troubleshoot a mount for star trailing

#1

Post by Raed »


Hi everyone,

So I solved my first and second problems (hue on the images and walking noise thanks to responses i got here) and now just started to take 3 minute exposures. I've used the mount a few times so far, and did the mistakes everyone warned me about, and heard the unpleasant sound of slight grinding when RA axis balancing with the release not fully applied (background info).

I am using an IOptron Cem25p, and in the last two times, i heard a soft ticking sound when tracking. Out of 60 x 3 minute shots, 10 or so images had some star trailing consistently across the images.
Details:
  • Cem25P polar aligned (no star alignment since I use EKOS and guide scope to correct alignment using its module)
  • Tracking: Auto-alignment disabled. Guide scope only used when aligning initially
  • Image exposure: 180 seconds
  • Issue: Random images show slight star trails. Not in order, and not in steps. Just random set show this. Trails always in the same direction.
  • Performed PEC check on the mount, gave it is ok.
I am wondering if the mount is damaged, or 3 minutes requires autoguiding, and how to troubleshoot to determine what could be the problem for "random trails".

Thanks
Raed
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JayTee United States of America
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Re: Troubleshoot a mount for star trailing

#2

Post by JayTee »


Your mount is most certainly not damaged. The fact that you have an 83% keeper rate on a 3 minute unguided exposure (on an entry-level mount) means you are doing many things "very" right. At this point guiding will help you regain most likely 8 of those 10 lost exposures. There are always a couple throwaways on every batch. Plus, it will allow you to take even longer exposures. Even all the way up to the limit of your local light pollution.

Let us know if you need help with your auto guiding setup.

Cheers,
JT
∞ Primary Scopes: #1: Celestron CPC1100 #2: 8" f/7.5 Dob #3: CR150HD f/8 6" frac
∞ AP Scopes: #1: TPO 6" f/9 RC #2: ES 102 f/7 APO #3: ES 80mm f/6 APO
∞ G&G Scopes: #1: Meade 102mm f/7.8 #2: Bresser 102mm f/4.5
∞ Guide Scopes: 70 & 80mm fracs -- The El Cheapo Bros.
∞ Mounts: iOptron CEM70AG, SW EQ6, Celestron AVX, SLT & GT (Alt-Az), Meade DS2000
∞ Cameras: #1: ZWO ASI294MC Pro #2: 662MC #3: 120MC, Canon T3i, Orion SSAG, WYZE Cam3
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∞ EPs: ES 2": 21mm 100° & 30mm 82° Pentax XW: 7, 10, 14, & 20mm 70°

Searching the skies since 1966. "I never met a scope I didn't want to keep."

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Raed
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Re: Troubleshoot a mount for star trailing

#3

Post by Raed »


Thanks JayTee, I will try to get auto guiding working (2 USB ports when I need 3, and hub did not work). I will see how it goes and let you know.
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KathyNS Canada
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Re: Troubleshoot a mount for star trailing

#4

Post by KathyNS »


Raed wrote: Mon Oct 19, 2020 8:59 pmTrails always in the same direction.
That is an important clue. Unfortunately, it is missing some data: what direction?

The direction of star trails tells you where to start looking. Trails in declination indicate an imprecise polar alignment. Trails in RA indicate problems tracking, such as periodic error. Trails that appear to rotate around the centre of the image or some other point indicate field rotation. The corrective action is different in each case.

Three minutes is a bit long to go unguided. Most people give up before they get to one or two minutes.
Image
DSO AP: Orion 200mm f/4 Newtonian Astrograph; ATIK 383L+; EFW2 filter wheel; Astrodon Ha,Oiii,LRGB filters; KWIQ/QHY5 guide scope; Planetary AP: Celestron C-11; ZWO ASI120MC; Portable: Celestron C-8 on HEQ5 pro; C-90 on wedge; 20x80 binos; Etc: Canon 350D; Various EPs, etc. Obs: 8' Exploradome; iOptron CEM60 (pier); Helena Observatory (H2O) Astrobin
Raed
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Re: Troubleshoot a mount for star trailing

#5

Post by Raed »


Thanks Kathy,

Eyes can be deceiving :) Having the images processed, and based on FWHM, 22 of the 60 images have low quality scores with shape < 0.5 (using APP). Much more than i flagged with just visual inspection.

If I remember correctly, its probably tracking related as they stretch in the direction of the RA axis, but will check to make sure.

JayTee,

One thing worth mentioning is I'm using the Redcat for the scope, which should be very forgiving in alignment :).

Thanks dears...
Raed
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Re: Troubleshoot a mount for star trailing

#6

Post by Raed »


Hi Dears,

So i finished the processing of the final capture. I think even without the 22 of 60 images, its still pretty ok. I love this...starting to hate the moon.. :)
Attachments
Veil-RGB-session_1-lpc-cbg-mod-St_fin_RS.jpg
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