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Sharp star struggles with my new OSC camera


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Hi all!

Been trying for months to get sharp stars through my ASI533MC-Pro on an 8"RC scope and still not getting it right! I managed to get lovely sharp stars on my Canon 60D before this new purchase, so i'm struggling to see what may be wrong. Few things i've done listed below:

- Checked collimation every session both indoors with a cheshire and then on a star near zenith.

- Use Bahtinovgrabber and a mask to make sure focus is as close as possible to perfect.

- Tried both with and without focal reducer.

- Checked guiding, which is always under 1".

- Checked polar alignment with sharpcap and levelled mount.

Issues are annoying showing up on subs of pretty much any length, 60-600s. I've attached a couple to show.

Any help or tips are hugely appreciated, this is my biggest learning block so far!

 

2021-02-28_00-05-23_M81 Bode's_-10.00_150.00s_0002.jpg

2021-02-28_02-06-57_Hercules Globular Cluster_-10.00_60.00s_0014.jpg

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I have a similar issue with my new RC8, although it is not as bad as this. I believe it is due to tilt on the focuser - this effect is exaggerated by using a reducer inserted in the focuser tube. Although the collimation is pretty good, the focuser alignment is definitely wrong on mine. (You can see the offset reflection on a very bright star shown by the mirror reflection being out of line).

I put a simple laser collimator in the focuser and this shows the reflective plane of the secondary does not line up with the focuser. Rotating the collimator shows the offset is consistent. I am hoping to get an M90 tilt adjuster to compensate. Unfortunately this is down to focuser being attached to the primary mirror - but very slightly out of alignment.

I would wait for some 'expert' opinion on this though. I am new to the RC8 so I might be wrong!

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M13 is definitely out of focus. The stars all look like they have some degree of coma but all pointing the same way. For a start I'd just try the mask and visually get it right. Don't rely on Bahtinov grabber. I've never had much luck with software trying to tell me Bahtinov spikes were good or not.

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On 28/02/2021 at 16:02, Weaver209 said:

guiding, which is always under 1"

Hi

I agree with @david_taurus83 that focus looks off, but remember you'll need really good seeing and clear skies to be able to guide the rc; I think you're aiming at below 0.5" pixel. Maybe you could bin the pixels and/or use an OAG?

The tilt; you could probably just put a small piece of paper either left or right before tightening the camera to the focuser. Elegant? No, but may help diagnose. However, if you can get steady enough seeing and get the necessary guiding accuracy, my guess is that you'd hardly notice. Any which remains visible should be easily corrected in processing.

Cheers and HTH.

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Edited by alacant
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