irregular star shape-newtonian

alacant · June 18, 2018

Hi everyone. I'm trying to diagnose this; stars not too bad centre frame, worsening toward the top and right. The collimation looks fine and the star test is dead centre. It's not coma I don't think as I'm using a sw 4 element cc on a sw 250p, f4.7. Canon 700d. Attached jpg is a copy of the original .CR2.

Tilt?

Cheers and TIA for any pointers...

Peter Drew · June 18, 2018

Why all this current obsession with edge of frame star images? The image on this post is of the Ring Nebula which itself looks fine to me.

david_taurus83 · June 18, 2018

I too have this. Top left and bottom stars appear ok where my right hand side stars are elongated. I tried adding a spacer to my MPCC but I think it made it worse so I suspect focuser tilt. We just ain't got the long clear nights now to tinker with allan keys!

Edit: Nice Ring Nebula btw

spillage · June 18, 2018

If you can rotate the camera 180 and check to see if the issue move or stays put in the same position.

Mark.

alacant · June 18, 2018

7 minutes ago, spillage said:

rotate the camera 180

Hi. Yep, the frame looks the same; worst top right.

35 minutes ago, david_taurus83 said:

adding a spacer

Tried that too but of course the tilt -if that's what it is- will still be present.

In my case, it seems I need to move the bottom left of the sensor (top right of the snap) away from the focuser base.

spillage · June 18, 2018

Have you been able to work out which corner of the focuser relates to the image. You may be able to undo the focuser and put a small spacer in that corner and see if that helps. That is if you focuser does not have the adjustment screws.

Astrobug · June 18, 2018

Is your camera bayonet tight? In the past mine was tilting the whole camera.

Try to make a picture on the other side of the meridian and see how the star shape change.

With a loose bayonet gravity is a real nightmare.

Tim · June 18, 2018

There is more to collimating a Newt than just aligning the primary and secondary. You also need to collimate taking allowance for the focuser, and any fittings you have between that and your camera.

It is possible to get that effect if your secondary mirror is slightly rotated with respect to the image plane, can be hard to pick up as well, and easy to miss, or even make worse, if you use a laser alone to collimate with.

Do you know how to square your focuser to the image plane?

alacant · June 18, 2018

12 minutes ago, Tim said:

square your focuser to the image plane

Hi. I think so. I have measured the distance from the top of the tube on the opposite side of the tube using a laser in the drawtube. It's the same distance as the mid point of the focusers barrel.. Evidently, it's not important but I've done it anyway. I think!

Tim · June 18, 2018

It's hard to explain isn't it

This sentence in that link is the key one

Quote

The important thing is that the focuser and the secondary are lined up, as seen from the focus.

spillage · June 18, 2018

There is also this video which may help with focus alignment.

emyliano2000 · June 18, 2018

I had the same problem after I replaced the stock focuser on my 200p. Ended up stripping the OTA and step by step checking absolutely everything I could think of and in the end my problem was solved. I took a few photos of my workflow.

1. First I put the OTA in a horizontal position

2. Second I rotated the ota until the focuser plate was level on one side

3. Third I checked if the focuser plate was level on the other side. It wasn't, so I had to insert some shims between the plate and the ota to make it level.

4. I checked again the level on the other side like i did at no. 2

5. I squared the focuser plate to the OTA as shown in the video posted by @spillage

6. I fixed the focuser on to the focuser plate. The focuser drawtube wasn't aligned with the OTA so I had to tweak it until I got it right. (Each focuser might be different)

20180515_211213.mp4

7. I centered the secondary mirror holder to the ota.

8. I checked if the center point on the primary is centered.

9. I took the primary mirror out of the cell and I stuck some cut offs of double sided adhesive pads inside the cell to hold the mirror in place (I was always loosing my collimation after a meridian flip). The adhesive pad that I used is a bit thicker than the cork pads that are in the cell and this way I didn't have to remove the cork pads.