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INDI Library v1.9.8 Released (29 Sep 2022)

Bi-monthly INDI Library released with new drivers and bug fixes.

Differences in main scope / guide scope polar alignment

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Hi,

I've been using kstars and ekos during lasts months, and I'm in love with it. I'm always do the polar alignment routine with the main optical tube (c8 with 6.3 focal reducer), but last night I tried with the guidescope , and there's significative differences... With the main scope ekos gives me a polar error of 2' 13"



and with the guide scope an error of 9' 11"...



OTA and guide scope are properly aligned...

why this big difference?

thanks

jordi
3 years 9 months ago #34848
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What are the pixel sizes and sensor dimensions of both cameras? What are the focal lengths of the two telescopes? My guess is that the focal length of the guide scope is much smaller and the pixels larger yielding a larger field of view and bigger pixels in terms of arc seconds. This decreases the accuracy of the algorithm detecting the star centers leading to an increase in the polar alignment offset.


Clear skies, Wouter
Wouter van Reeven

ASI6200MM and 7 slot 2" filter wheel with a SkyWatcher Esprit 80 ED on a SkyWatcher HEQ5-Pro
ASI1600MM-Pro Cooled and 5 slot 1.25" filter wheel with an 8" TS Ritchey-Chr├ętien on a SkyWatcher EQ6-R
3 years 9 months ago #34850

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Main scope:

focal length: 1280 mm
camera: canon EOS 4000D
sensor size: 22.3 x 14.9 mm
sensor resolution: 5184 x 3456
pizel size: 4.29

guide scope:
focal length: 230 mm
camera: QHY 5L-II Mono
sensor size: 1/3 inch
sensor resolution: 1280 x 960
pizel size: 3.75

one doubt: on the main scope I'm not taking pictures at full resolution, I'm taking pictures at 2592x1728. These are the values that I've write in the indi control panel, both for "Image settings" and "image info":





If I put 5184 x 3456 on the "Image Info -> CCD Information" the images are cropped (I have selected 2592x1728 on the camera), is this ok?

thanks for your response
3 years 9 months ago #34857
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I have similar guiding setup as you, and routinely get under 2' PA error. I have a ZWO 60mm f/4.6 guidescope (276mm focal length) and a QHY 5L-IIm guide camera (same as you), which I use for PA.
3 years 9 months ago #34878

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The first thing that comes to mind is flexure in the guide scope causing it to follow a non-circular path as you rotate.
-- Ken
RPi3 Ubuntu Server 20.04, Windows 10 AMD64, AAEON UP Core Ubuntu Desktop 20.04
Avalon M-Uno, EQ6 Pro, Atik420, ASI1600MM-C, ASI120MM-S, DBK21AU04, ZWO EFW, Optec TCFSi
Vixen R150S, GSO RC8, ST80
3 years 9 months ago #34897

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The camera is well attached to the guide scope, and the guide scope is well attached to the main scope, there's no movement, it's like a single block. Anyway, repeating from one day to another the same PA routine gives me exactly the same values for main and guide scope (+-2' / +-9'), if it was flexure the value for guide scope the PA error would never be the same?
3 years 9 months ago #34899

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