Poor Collimation or something else?

[matt_baker]

Okay, so here's some pre-context.

I went out last night to try APT for the first time, along with plate solving to make my life easy and so I could stay in the warm while it does its thing.

This took me a while to figure out but once the ball got rolling, it was running perfectly.

I start running the sequence on M13 but noticed something off about the stars.

They all had a distinct shape to them. Would this be down to poor collimation, poor guiding, differential flexure?

I have a feeling it may possibly be down to my laser collimator, since the laser probably isn't collimated itself.

Could anyone help diagnose the issue?

[alacant]

11 minutes ago, matt_baker said:

They all had a distinct shape

Hi. Really? Are you sure it wasn't more accentuated toward one edge(s) or corner(s)? From which part of the frame is this taken?

Is this a 130pds with B-MPCC and a Nikon? The Baader is susceptible to astigmatism with even the slightest of tilt or incorrect spacing.

I'm sure we'll soon have this fixed.

Cheers.

[matt_baker]

Actually, I think toward the center of the image it's okay but it starts to go funny just out from the center.

And yes this is the MPCC mk.3 with the 130PDS and a Nikon D5300

I've attached the raw file if you'd like to take a look for yourself

Matt

L_0057_ISO800_120s__NA.NEF

Edited May 5, 2019 by matt_baker

[alacant]

Hi. Yeah, it's worse along the bottom edge and the astigmatism changes depending where else you are in the frame, you are slightly out of focus, the camera is tilted, the coma corrector (cc) is not at the correct distance from the censor or you are out of collimation. Or some combination of the above.

First, make sure the collimation is perfect. There are two methods of connecting the cc one via m42 and one via m48. Please see the attached. Make sure that the focuser thumb screws are not bearing against the undercut on the cc causing the sensor to be non-orthogonal. Of course, be sure to read the common collimation myths.

HTH

Edited May 6, 2019 by alacant

[matt_baker]

7 hours ago, alacant said:

Hi. Yeah, it's worse along the bottom edge and the astigmatism changes depending where else you are in the frame, you are slightly out of focus, the camera is tilted, the coma corrector (cc) is not at the correct distance from the censor or you are out of collimation. Or some combination of the above.

First, make sure the collimation is perfect. There are two methods of connecting the cc one via m42 and one via m48. Please see the attached. Make sure that the focuser thumb screws are not bearing against the undercut on the cc causing the sensor to be non-orthogonal. Of course, be sure to read the common collimation myths.

HTH

I've measured the t2 adapter depth and it's 8.5mm, which goes in line with the 46.5mm flange distance to equal 55mm backfocus.

What I think I may do is drill another hole on the focuser for an extra thumbscrew to make sure it's in there tight.

Although I use a laser collimator, I've recently read that they need collimating themselves, so I'm going to make a jig and collimate the laser

I'll update you on the  situation once I can image again

Matt

[alacant]

9 minutes ago, matt_baker said:

I use a laser collimator

Hi

You may want to consider a Cheshire sight tube. This one also has cross hairs to nail both primary and secondary mirrors.

Cheers

 

[matt_baker]

19 minutes ago, alacant said:

Hi

You may want to consider a Cheshire sight tube. This one also has cross hairs to nail both primary and secondary mirrors.

Cheers

 

Are cheshires better than laser?

[Guest Tuomo]

1 hour ago, matt_baker said:

Are cheshires better than laser?

They are if you lazor is cheap and/or not calibrated. I learned this hard way by buying cheap ebay lazor. When you rotate your collimator in focuser, does the red dot stay in one place? If not, there is problem with laser and/or focuser.

[alacant]

2 hours ago, matt_baker said:

Are cheshires better than laser?

Hi. More accurate and easier, so for me at least, better. A barlowed laser comes close, but you're then adding even more variation to something which really doesn't need it. Just my €0.02.

HTH.

[pietervdv]

Triangular shaped stars are often caused by pinched optics. Is your primary mirror still properly supported? 

Pieter 

[matt_baker]

3 hours ago, pietervdv said:

Triangular shaped stars are often caused by pinched optics. Is your primary mirror still properly supported? 

Pieter 

I'll be sure to check this out and loosen the clips to a business card

[cletrac1922]

I use laser for my 10" Dob, and very rarely need to re-collimate, even though out 2 or 3 times per month with my club, doing presentation primary schools, scout/guide groups

What I have found with laser collimator, is does not fit snugly like 1.25" eyepiece does in focuser

Had a new member a while ago, showed up one club night with a second hand 8" solid tube dob

Collimation was way out, and was not able to align

When looked down the tube, noticed that the secondary mirror was to one side

Using bit of paper, measured the veins, and found secondary off centre

After adjusting veins, able to do proper collimation

Appears previous owner had been fiddling, and why got rid of it

John

[glowingturnip]

just to mention, the tolerances on those MPCC diagrams are there for a purpose - through experiment I found I needed at least 57.5+1 on my M48.  Could probably even go 0.25 to 0.5mm longer for better results, but I'm running out of thread.

I wrote a piece on collimation for another chap a while back - basically you'll need a cheshire *and* a well collimated laser if you're going to get it nailed - hope this helps -