Jump to content

Stargazers Lounge Uses Cookies

Like most websites, SGL uses cookies in order to deliver a secure, personalised service, to provide social media functions and to analyse our traffic. Continued use of SGL indicates your acceptance of our cookie policy.

sgl_imaging_challenge_banner_30_second_exp_2_winners.thumb.jpg.b5430b40547c40d344fd4493776ab99f.jpg

Recommended Posts

Hey guys, I have a 140 mm APM APO refractor with a TS field flattner. I also use a ONAG for on axis guiding. With the ONAG i'm over the recommended distance for optimum use of the field flattner. Because of this (I think) I get stars with coma at the edge of the field. Is this the problem or should I look for something else too? with the refractor I was hopping for pinpoint star near the edge of the field of view.  I have attached a image from last night  (raw).

m332.jpg

Share this post


Link to post
Share on other sites
1 hour ago, dan_adi said:

So I am too close?

I'd say yes, but I've no idea what a ONAG even looks like, you say you're over the recommend distance..

What's the rest of the setup, camera, filters etc

Share this post


Link to post
Share on other sites

The setup is like this: field flattener + 2 mm connection + ONAG + 2 mm connection + filterwheel + g3 16200 camera

required distance for field flattener about 87 mm

ONAG needs about 66 mm

g3 16200 + filterwheel needs 33 mm

 

Share this post


Link to post
Share on other sites

I will test tonight if it s clear. If I’m too close that means good news, just add some spacers :)

Share this post


Link to post
Share on other sites

I would say your too far away and need to reduce spacing. I've never had any luck with that diagram above and have seen a couple of references that it is the opposite way round. Best way to work it out is to just put the camera on with spacers to the recommended back spacing and experiment from there. Once you find the optimum spacing measure it with a caliper and then factor back into it your ONAG.

Share this post


Link to post
Share on other sites
8 minutes ago, david_taurus83 said:

I would say your too far away and need to reduce spacing. I've never had any luck with that diagram above and have seen a couple of references that it is the opposite way round. Best way to work it out is to just put the camera on with spacers to the recommended back spacing and experiment from there. Once you find the optimum spacing measure it with a caliper and then factor back into it your ONAG.

That's interesting...I've never had bad stars to say if it's right or wrong..always used specs and measured with vernier calipers..

Is be interested if it's the other way around thou,  don't like promoting duff info

Share this post


Link to post
Share on other sites

Distance is the issue for sure.

Actual prescribed distance for flattener only works if flattener is matched for that particular scope - otherwise it is general guideline and one should try different spacing to get good results.

Above diagram should be right in principle, but again depends on optical configuration of field flattener. I don't have enough knowledge on different flattener designs (if there are indeed different configurations) to be able to provide more detailed insight into all of that but I suspect that both things: spacing and above diagram should be taken as guideline more than fact unless specified exactly for flattener / scope combination by manufacturer.

In any case, answer is trial and error. There might even be a case where you can't find "perfect" distance - there is always some level of aberration in the corners. This can happen if flattener does not correct large enough field to cover whole sensor. 16200 is full frame sensor with 34.6mm diagonal - certain flatteners might have difficulty to correct for stars that far off axis (not saying that TS one is such flattener - but it can happen).

Share this post


Link to post
Share on other sites
1 hour ago, newbie alert said:

That's interesting...I've never had bad stars to say if it's right or wrong..always used specs and measured with vernier calipers..

Is be interested if it's the other way around thou,  don't like promoting duff info

Yes, I agree. The best results I have had have been by adhering to the recommended distance. I have always thought I could get better results if i experimented but it usually ends in folly! As vlaiv points out, the diagram may be correct for some flatteners but opposite for others. May depend on if the lens elements configurations being concave or convex at the last element.

Share this post


Link to post
Share on other sites
On 06/11/2019 at 08:34, dan_adi said:

I get stars with coma at the edge of the field.

Technically, that's not coma.  It's just defocused stars.  Verify this for yourself by defocusing a star on axis.  It's nice and circular.  Now move that defocused star to the edge, it becomes oblate/elongated.  That's why you should always star test exactly on axis.

Share this post


Link to post
Share on other sites
15 minutes ago, Louis D said:

Technically, that's not coma.  It's just defocused stars.  Verify this for yourself by defocusing a star on axis.  It's nice and circular.  Now move that defocused star to the edge, it becomes oblate/elongated.  That's why you should always star test exactly on axis.

Technically it's astigmatism :D

Angle is too small to make elliptical cross section of converting light cone (there is small contribution of this effect, but here primary effect is astigmatism).

There is in fact a bit of coma from what I can tell - pure astigmatism is symmetric aberration, so star shape should be elliptical, but here there is a bit of coma in the far corners.

image.png.e660227c71bea8a7cf32a61804b7d7d6.png

In any case, this mix of aberrations is due to wrong distance of flattener to sensor.

Share this post


Link to post
Share on other sites

Thank you guys for the quick replies. I will get back with an answer as soon as the clouds go away. 

Share this post


Link to post
Share on other sites

In case I can not reach optimal distance from flattener to camera do you guys think if I replace my stock APM focuser with my optec Leo low profile focuser I might have succes? 

With the low profile focuser the imaging train will be shorter and I will have more backfocus available. Does this make sense? Or no matter what my backfocus is , I must respect the flattener optimal distance to camera sensor.

Share this post


Link to post
Share on other sites

How much back focus you have is only important if you can't reach focus for needed sensor - flattener distance. I think you should have no problems with amount of back focus needed as flatteners often move focal point inward (particularly those that act as reducers as well).

Share this post


Link to post
Share on other sites
5 hours ago, dan_adi said:

I must respect the flattener optimal distance to camera sensor.

That is all that matters.  Any extra in-focus added by a low profile focuser isn't going to change that.  As @vlaiv says above, if you can reach focus with the current focuser and flattener, then there's nothing to be gained in swapping focusers.

Share this post


Link to post
Share on other sites

I have made some progress, I have removed a small spacer (between camera and the on axis guider) and now I'm on the other side of incorrect placement as in the second picture posted by newbie alert. The spacer I think is about 4 mm. So I guess I should order a new spacer. At least now I know I can reach the optimal distance from flattener to camera and still be able to use my on axis guider.

Thank you for the support, the troubleshooting was faster with your help. Below is a new image with incorrect distance(m33, 900 sec exposure, raw image, full moon :) )

m33corected.jpg

Share this post


Link to post
Share on other sites

Sure would be nice if field flatteners came with a helical focuser to fine tune the separation.  Something like the old Tamron 2x Macro teleconverter would be nice:

spacer.png

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.


  • Recently Browsing   0 members

    No registered users viewing this page.

×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.