out of focus stars Out of focus stars in the corners

[peeb]

Hi All,

Just wondering if anyone could shine a light on this issue I have.

I am getting an uneven field of view as you can see from the attached images, I am getting out of focus stars. If I rotate the camera physically the out of focus stars move to the next corners? My focus in the center of the image is spot on as reported by the Bahtinov Grabber. The camera is attached by the way of a screwed in to the camera extension tube which hopefully runs snugly into the focuser.

My scope is a Skywatcher 190mm MakNewt and I am using a QHY8L ccd camera. My scope is polar aligned (Polemaster) and balanced, all housed in a permanent setup.

Many thanks in advance.

Paul

 

Edited August 18, 2019 by peeb
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[Nigella Bryant]

I'm no expert but it looks like coma. 

[michael8554]

Not clear from your post, is the distortion in all 4 corners? 

Then yes it's Coma. 

Either crop your results or fit a Coma Corrector. 

Michael 

[vlaiv]

This scope is Maksutov Newtonian - so it should not have coma - no coma corrector needed.

There are couple of things that can be wrong - first is collimation. I have no idea how to collimate MakNewt, but since it has secondary as well as spherical primary. Primary can be tilted - that will "move" it off optical axis with respect to corrector plate. I don't think corrector plate can or needs to be collimated - but I might be wrong there.

Secondary can probably also be collimated.

There should be resources online to both check collimation and how to perform it. Here is an example of a thread that deals with MN collimation - might be of interest:

https://www.cloudynights.com/topic/590912-collimation-on-a-maksutov-newtonian/

Other than that - it can be sensor tilt. Although there is a "snug fit" for camera adapter inside focuser - either focuser can be at an angle, or maybe there is enough play to cause tilt.

First thing to do would be to examine each corner and also check collimation. Tilt is probably last thing to consider if it is indeed snug fit.

[JamesF]

I'm sure there are some threads on fettling the MN190 here, too.  One or two people have referred to them in the past.  Definitely worth a search.

James

[michael8554]

6 hours ago, vlaiv said:

scope is Maksutov Newtonian - so it should not have coma - no coma corrector needed.

Interesting - I don't have one so searched for this, the articles I saw said they suffered from Coma!

Looks like Coma, quacks like Coma......... 🙂

Michael 

[Merlin66]

Compared with field curvature????

 

[vlaiv]

2 hours ago, michael8554 said:

Interesting - I don't have one so searched for this, the articles I saw said they suffered from Coma!

Looks like Coma, quacks like Coma......... 🙂

Michael 

You are right, according to

https://www.telescope-optics.net/Mak-Newton.htm

There is some coma in MN design, but it should be less than standard newtonian. Although it has spherical primary it looks like meniscus corrector in front is introducing some coma in the system. I've never heard that people use coma corrector with this scope, even for imaging.

Quote

Similarly to the Schmidt-Newtonian, alignment of the Maksutov-Newtonian is more complex than that of all-reflecting Newtonian, due to the presence of lens corrector. Ray tracing indicates that sensitivity to decenter of the Maksutov corrector is similar to that of the Schmidt; however, its sensitivity to tilt is several times higher.

Above quote from linked page (at the bottom) does suggest that tilt could be culprit of funny stars in the corner as these scopes tend to be sensitive to it.

Images that I see taken with this scope, usually don't have coma in the corners, but they might be cropped. However, people did have corner star issues with collimation of this scope, like in this thread:

 

[alan potts]

On 19/08/2019 at 13:05, vlaiv said:

You are right, according to

https://www.telescope-optics.net/Mak-Newton.htm

There is some coma in MN design, but it should be less than standard newtonian. Although it has spherical primary it looks like meniscus corrector in front is introducing some coma in the system. I've never heard that people use coma corrector with this scope, even for imaging.

Above quote from linked page (at the bottom) does suggest that tilt could be culprit of funny stars in the corner as these scopes tend to be sensitive to it.

Images that I see taken with this scope, usually don't have coma in the corners, but they might be cropped. However, people did have corner star issues with collimation of this scope, like in this thread:

 

Strange Vlaiv, I have seen many an advert claiming Coma free, dam good scopes, I love mine for visual and should get it on the mount and give it a go with a camera, fast too.

Alan

[michael8554]

1 hour ago, alan potts said:

Strange Vlaiv, I have seen many an advert claiming Coma free

Agreed Alan, which is why I was surprised that research suggested otherwise.

But peeb hasn't said yet whether it's all corners or just one.

Still looks like coma to me though...

Michael