Streaky stars continued

[Carl]

I have just managed to upload a larger image showing full frame.
Please remember this is not a focus test just a test to work out what is happening to the corners.
Richardo re your earlier comments
1. Collimation was correct last month, will check again Thursday night
2. Tightness of fittings. Will check again Thursday night
3. Distance from chip to reducer is aprox 50mm using radial guider, aprox 100mm using modified extension tube. I get wider field of view with extension tube but same problem with stars.
4. Stars have exact same shape and length of ellongation regardless of exposure time.
5. Focuser and mirror alignment could be an issue. After all it is a Celestron C8, I may be asking too much of it.
6. Last polar alignment 2 months ago star remained centre of illuminated cross hair for over 10min. My mount is on a permanent pier, rediculously rigid. Will check again Thursday night
7. I have an 80ed with a Hotech field flatener, I will do exactly the same tests on it.

Any assistance greatfully accepted

[Hagar (Doug)]

Hi Carl, I kept out of the last thread because I couldn't see enough of the image to hazard an educated guess. Now that I can see the whole image I would think you have two problems here to cope with. nI suspect the elongation is a field curvature problem and you might find a field flattener of some description might help. The second I suspect is an aligment issue placing the right side of the image out of focus graduating to the outer edge. This may be a mirror flop issue which is fairly normal for small SCT's without mirror locks. I don't know if there is any easy fix for this problem.

Good luck with it.

[joecool (Mark)]

My thoughts... Only because I have the same problems.

Collimation is fairly close or you would see different sized pacmen in each corner. (Though it is slightly less in the top left??? Mirror flop? May need a mirror lock but this does not always prevent the tilt; I have similar problems but fortunately to a lesser extent).
Same with tightness of fittings, or again, it would be differential. I think that tightness comment from the previous corner shot you uploaded was because the commenter did not realize it was a corner. That first shot definitely looked out of focus in one corner if that was the case.
I've been using the spacings as per the instructions provided with my Meade 6.3 reducer and get the same results on my 10" f6.3. I suspect it is over-correcting the coma induced arrow heads I have all around the edges at prime focus. What I really need is a curved film plane, just like in the old days at the big observatories! Or a new Meade ACF or RCX!
Focus is OK too or the globules near the keyhole would have noticeably softer edges. Due to the coma be sure to focus on a star near the center of the image, um like that star near the keyhole (um, like eta carinae).
No field rotation to speak of. Stars half way out from the center are still round.

I have seen images from one bloke with the same scope as me using an MPCC coma corrector to good effect. So I got one. Unfortunately it only goes into a 2" barrel, so I have also got a JMI EV1cM to take it. Checked it all comes to focus fine, but now the clouds have rolled in over Adelaide. New equipment curse at work there! Sorry fellow Adelaidians!

Mark.

[joecool (Mark)]

Oops. I just thought about it. Collimation would not cause different sized pacmen. I think it only changes how sharp the image is??? Dang. I don't know enough either...

Please help!

Mark.

[renormalised (Carl)]

I concur with what Doug has said, now that I can see the whole piccie. You will also have a little bit of coma there as well, but the flattener should take care of that. I would also check how well your image train is aligned as well as checking for any mirror flop. Things can work loose or twist awry during the night and sometimes they may not be on dead straight in the first place.

[richardo (Rich)]

Ok Carl, I still would like to know what camera....
But I took a guess and went for the canon 5D @ 1.34 arcsec/pix... not that it should make too much difference when just viewing how your collimation is through CCD inspector...
Here's a screen shot to give you an idea of perhaps what's going on.
collimation looks to be an issue.
And also by just eye balling your image, I'd be experimenting with your reducers spacings... most of your issue will be the spacing... add a little bit on, take a little bit off... etc
I've seen similar when i was squaring my SX CCD cameras face plate to my optics in conjunction with my coma corrector.

But I don't know enough about the C8 and its mirror flop issues or quality of its focuser/ hardware to comment much further.

Hope you get it sorted soon.

Rich

[Paul Haese]

Certainly corrector is not at the right distance. The stars in the corners are out of focus while the stars in the center of the field are focused. So that needs sorting out with some correct spacing.

Also there is apparent field rotation in the corners. If you look at each corner the stars are rotating in the same direction. That means your PA needs looking at just a little more or you have some slop or movement with the camera. Also just check the way you align your subs too. This could be contributing to make the effect of PA more evident.

Get these sorted and you are well on your way.

[gregbradley]

If you are using a Canon 5D on an SCT you are probably not going to be able to get round stars at the corners with any reducer.

You need a flattener, not a reducer. And a 5D is a 35mm sized sensor so that requires a corrected circle of about 44mm or so.

That is quite large and generally that would mean you need a 3.5 inch or larger focuser.

You will not be able to achieve with anything less.

What camera is this?

If its an APS sized sensor then I think you should be able to get all stars in focus. I am pretty sure there are plenty of images around where this is the case.

All reducers/flatteners have a specified metal back distance. The definition of this term is the distance from the last metal flange to the chip.

For example 86.2mm usually with a stated tolerance like +/-6mm.

You are moving it 50mm to check. That is a huge variance. They are usually way more sensitive to the spacing than that - more like +/- 6mm.

Also I am not sure what effect the radial guider has. Is that rotating as the scope is tracking? That may explain your rotating stars whilst your PA is accurate (if it proves to be - it would be the first thing to check).

Most likely you are expecting round stars to the corners that is not possible with that setup if you are using a 5D.

Greg.

[Hagar (Doug)]

Hi Carl, Here are the curvature and collimation maps for this image.
It does indicate problems with both collimation and field curvature which relates back to spacing problems. I have quite an array of adapters which I can bring out and see if we can get it a bit better.
Let me know.

[Carl]

Greg Thanks for your input.
I'm using a canon 500d. with a celestron 6.3 focal reducer and flattener plus a radial guider bnetween. maybe i need to ditch the radial guider and have spacers .
I've tried imaging also with a 2 inch adapter between the radial guider and camera. From memory my distance from reducer to chip is around 100mm.

Cheers carl

Originally Posted by gregbradley

If you are using a Canon 5D on an SCT you are probably not going to be able to get round stars at the corners with any reducer.

You need a flattener, not a reducer. And a 5D is a 35mm sized sensor so that requires a corrected circle of about 44mm or so.

That is quite large and generally that would mean you need a 3.5 inch or larger focuser.

You will not be able to achieve with anything less.

What camera is this?

If its an APS sized sensor then I think you should be able to get all stars in focus. I am pretty sure there are plenty of images around where this is the case.

All reducers/flatteners have a specified metal back distance. The definition of this term is the distance from the last metal flange to the chip.

For example 86.2mm usually with a stated tolerance like +/-6mm.

You are moving it 50mm to check. That is a huge variance. They are usually way more sensitive to the spacing than that - more like +/- 6mm.

Also I am not sure what effect the radial guider has. Is that rotating as the scope is tracking? That may explain your rotating stars whilst your PA is accurate (if it proves to be - it would be the first thing to check).

Most likely you are expecting round stars to the corners that is not possible with that setup if you are using a 5D.

Greg.

[Carl]

Originally Posted by Hagar

Hi Carl, Here are the curvature and collimation maps for this image.
It does indicate problems with both collimation and field curvature which relates back to spacing problems. I have quite an array of adapters which I can bring out and see if we can get it a bit better.
Let me know.

Thanks Doug. I have sent you my contact details via members private email

[gregbradley]

Originally Posted by Carl

Greg Thanks for your input.
I'm using a canon 500d. with a celestron 6.3 focal reducer and flattener plus a radial guider bnetween. maybe i need to ditch the radial guider and have spacers .
I've tried imaging also with a 2 inch adapter between the radial guider and camera. From memory my distance from reducer to chip is around 100mm.

Cheers carl

Hi Carl,

You are using a reducer plus a flattener or did you mean reducer/flattener? They are 2 separate items and not the same thing.

Reducers reduce the F ratio for example in your case from F10 to F6.3.

Flatteners correct for the curvature of the lens/corrector/mirror and give pinpoint stars corner to corner. They usually don't affect F ratio or only slightly. Their job is to make sure stars in the corner are in focus at the same time as stars in the centre of the image. I was not aware of a flattener for an SCT.

Flatteners and reducers don't usually work together. So if you are using 2 separate items that may be your trouble.

Greg.