TheSkySearchers

The attached shows the top right corner of a recent image I captured. The stars are lopsided all facing a way from the centre of the image. I'm confident the collimation is good and the focus is good too. Can anyone tell me what the cause is please?



It's M108, almost!

Thanks

Regards

Graeme

That looks like coma. Is this a new development on your SCT? Could also be coma + field curvature.

If this is new, did anything change like camera spacing?

I agree with Ruud. Coma.
Quite normal for a normal standard SCT if it occurs in all corners. Deflection by the weight of the camera could be a cause.

Yep looks like tilt to me.

Might look into a tilt ring.

https://optcorp.com/products/t2-tilt-ad ... gKhRPD_BwE

They are a bear to set up though, fair warning.

Ruud wrote: ↑Sun May 10, 2020 10:55 am That looks like coma. Is this a new development on your SCT? Could also be coma + field curvature.

If this is new, did anything change like camera spacing?

Thanks Rudd.

Yes, it is a new development. As it goes, there's been quite a few changes recently! I added a guide scope mounted on a second dovetail bar on top of the OTA. Also, the black coat paint started to flake and I removed the corrector plate to clean it out. Also, the OTA has been posted to Yorkshire and back in a futile attempt to ascertain the reason for the faulty black paint coating. And finally, the camera fell out onto the brick patio a few weeks ago!

John Baars wrote: ↑Sun May 10, 2020 1:53 pm I agree with Ruud. Coma.
Quite normal for a normal standard SCT if it occurs in all corners. Deflection by the weight of the camera could be a cause.

Thanks John

I've always used the same Canon 600D and I have only just noticed this effect in my images. Other than ensure all the fittings are tight is there any other solution if the weight of the camera is the cause?

UlteriorModem wrote: ↑Sun May 10, 2020 3:37 pm Yep looks like tilt to me.

Might look into a tilt ring.

https://optcorp.com/products/t2-tilt-ad ... gKhRPD_BwE

They are a bear to set up though, fair warning.

Thanks Tom

That looks like a prescision adjustment tool. Now I know the cause, I'll try setting up more tightly and carefully. If that doesn't work then I'll look into trying a tilt ring.

Cheers chaps.

Regards

Graeme

looks typical from a non edge sct and a full frame, is this in all corners or just one ?, how was your star test inside and out ? a reducer corrector should clean that up .

yobbo89 wrote: ↑Sun May 10, 2020 4:37 pm looks typical from a non edge sct and a full frame, is this in all corners or just one ?, how was your star test inside and out ? a reducer corrector should clean that up .

Oh wait, that was not the full frame?

Can he post a full frame image? I may have been off base.

yeah op, mentioned he attached an image of the top right section .

yobbo89 wrote: ↑Sun May 10, 2020 4:37 pm looks typical from a non edge sct and a full frame, is this in all corners or just one ?, how was your star test inside and out ? a reducer corrector should clean that up .

Thanks Yobbo.

I don't recall seeing this at all before the session of 07/05/20. I will have to go back and study a few images from the past.

What's a star test?

yobbo89 wrote: ↑Sun May 10, 2020 5:55 pm yeah op, mentioned he attached an image of the top right section .

Yea my bad for skimming the thread. Thanks for the correction.

Graeme1858 wrote: ↑Sun May 10, 2020 6:17 pm

yobbo89 wrote: ↑Sun May 10, 2020 4:37 pm looks typical from a non edge sct and a full frame, is this in all corners or just one ?, how was your star test inside and out ? a reducer corrector should clean that up.

Thanks Yobbo.

I don't recall seeing this at all before the session of 07/05/20. I will have to go back and study a few images from the past.

What's a star test?

So, I searched for star test inside and out! Didn't know there was a significant difference between which side of focus a doughnut could be! So I only did the one collimation adjustment and I don't know if it was inside or outside. I made a pin hole in tin foil, put it over a torch and put the torch at the end of the garden 40m away.

Here is the post adjustment image:



Two new concepts learnt in one thread!

Thanks

Regards

Graeme

A better artificial star is to use a stainless steel ball bearing stuck in some modelers clay and the clay stuck on a fence post 50 to 100 meters away. Obviously, the sun needs to be out for this to work.

JT

My new hand set has arrived! Hooray! So I set up last night to sort out my collimation.

After aligning I chose Dubhe as my test star as it was close to the Zenith.

Here's moving in uncollimated.
Here's moving out uncollimated.
Here's moving in collimated.
Here's moving out collimated.
I'm happy with that!

Regards

Graeme

By jove, I think you've got it.

JayTee wrote: ↑Tue May 12, 2020 10:53 pm By jove, I think you've got it.

Yeah, it looks ok. So the camera must have not been in properly last time or something.

Also, I think I need to collimate more frequently.

Regards

Graeme

Hello Graeme,

Please remember that after you collimate your telescope with the out of focus image at this level, you should move it closer to focus, such that the Poisson spot is visible so that you can judge its centering. Eventually, moving to in focus at high magnification to be able to judge the even illumination of the airy disk.

And, it is critical to always judge the adjustments after recentering the star in the field of view. Do always finish the last focuser move of an SCT anti-clockwise to seat the primary properly on the baffle.

Jim