collimation woes, or is it something else?

[bomberbaz]

Hello all, I have been having a few issues with my collimation. The scope is a RVO Horizen ED 60 which according to the RVO website goes through rigorous testing prior to leaving the shop. 

I am wondering if part of the issue is down to my method of attaching it to the camera, see directly below. However I have now removed the std 1.25" eyepiece type connector and attached the camera via a 58/42 screw type 20mm spacer.

The problem is the star are comet shaped (see other picture below) and was hoping someone has had this issue before and can tell me outright what the issue was for them, saves me speculating and hopefully allows me to come to a quick conclusion of my own problem.

Cheers

all

steve

[knobby]

Hard to tell on my phone but the streak appears to be in the same direction across the frame ?

What exposure length was this ? Could it be tracking ?

[pharscape]

I've had images like that when the guiding SW tries to correct a shift due to something loose or dragging cables. Are all your images like that regardless of exposure time or where you point the scope? If it only seems to happen with long exposures, I'd suspect a flexure/tracking/guiding issue.

If you are concerned it's a collimation issue then try rotating the main scope and camera in the rings. If the aberration rotates as well the problem is likely a collimation issue somewhere between the primary lens and the camera sensor (something not square on to the optical path.).

[bosun21]

Definitely looking like a tracking issue as opposed to a collimation one. The star shape seems to permeate through the entire image and are all unidirectional. As I have no experience with this mount I will leave it to the SGL members who do. Good luck.

[barbulo]

+1 for the guiding issue. You could plate solve the image and see in which axis the elongation is. RA axis = wrong sideral rate or backlash. DEC axis = bad PA.

[bomberbaz]

2 hours ago, knobby said:

Hard to tell on my phone but the streak appears to be in the same direction across the frame ?

What exposure length was this ? Could it be tracking ?

It is in the same direction is the cometing.

Exposure length 3 minutes, my tracking has been pretty decent. In dec it has been very tight, it's the ra thats been naff.

2 hours ago, pharscape said:

I've had images like that when the guiding SW tries to correct a shift due to something loose or dragging cables. Are all your images like that regardless of exposure time or where you point the scope? If it only seems to happen with long exposures, I'd suspect a flexure/tracking/guiding issue.

If you are concerned it's a collimation issue then try rotating the main scope and camera in the rings. If the aberration rotates as well the problem is likely a collimation issue somewhere between the primary lens and the camera sensor (something not square on to the optical path.).

Hmm, just checked the Alt clamp, it was quite loose and there was more flex movement than normal. (There is always a small amount but definitely more)

There is a built in rotator into the focusing unit, I shall take a couple of subs with it one way then rotate 90 degrees to see if the comet (assuming it is still there after tightening the clamp) rotates too.

Anyway, thanks for the pointers gents, think I might be getting somewhere here. 

Edited June 3, 2022 by bomberbaz

[ONIKKINEN]

The elongation appears to be in DEC as plate solved by astrometry.net here: https://nova.astrometry.net/user_images/5963191#grid

Do you have a guide log or just some guide statistics from the night of capture? Looks like your DEC axis is struggling, but dont know how to advice on this particular mount on how to fix. If backlash in DEC is particularly bad you may need to guide in one direction or not guide in DEC at all but would be easier to advice with some guide stats.

[bomberbaz]

Ok so had a little play tonight and here are my findings.

The comet rotates with the camera, one way or the other it follows the rotation.

As regards exposures, 180 seconds or 10 seconds, it is there.

Not sure if I am right here but this appears to point towards a image train/collimation issue rather than tracking!

Oh and sorry should have pointed this out earlier, my previous OTA (which should be tried in place of the horizen) didn't suffer from any such issues.

[ONIKKINEN]

8 hours ago, bomberbaz said:

Ok so had a little play tonight and here are my findings.

The comet rotates with the camera, one way or the other it follows the rotation.

As regards exposures, 180 seconds or 10 seconds, it is there.

Not sure if I am right here but this appears to point towards a image train/collimation issue rather than tracking!

Sure sounds like a mechanical issue with tilt or an optical one with collimation then, yes. Just by coincidence the effect happened to be along the dec axis.

[bomberbaz]

More testing done at request of vendor. (Not that it was needed as under warranty)

With and without the FF, the comet star shapes are there.

Rotation of the focuser makes the comet stars follow the rotation.

V/tired now but will put up pictures tomorrow of the focal train so it is more apparent.