Whats going on here? (desperate for help!)

[buikimaging]

Hello all,

I posted a thread in the imaging help section but without any succes.

I got hold of a new Explore scientific 80mm apo triplet with a field flattener for my imaging needs.

After a couple of tries I couldn't take away the bad star shape in the corners.

I got rid of any movement in the focusser and extender, using locktite to secure the screws in the focusser itself.

When i run my images through CCDinspector, I almost cringe.

curvature of 107.4%???

tilt of 68%??

what is going on? does anyone have any idea? 

The image I ran through CCDinspector is made with a full spectrum Sony A230 DSLR.

I'm running on my last fumes, the scope is almost 1 month old but it never took a shot without curvature or tilt.

Thanks for any input!

[wuthton]

When you rotate the camera does the tilt-

A- Stay in the same orientation, so it's either focuser slop or an optical defect?

or

B- Move with the camera which will be a chip orthogonality problem?

[buikimaging]

When you rotate the camera does the tilt-

A- Stay in the same orientation, so it's either focuser slop or an optical defect?

or

B- Move with the camera which will be a chip orthogonality problem?

However i rotate the camera around the optical axis, the star deformation is always in the top right and top left of the frame. Whatever I do.

[jonathan]

When you have eliminated the obvious then maybe look at possible imperfections - you say it doesn't change when you rotate the camera so probably not that, possibly something in the tube or lens elements?  Could it be an alignment problem of the camera mount?

[Zakalwe]

Are you sure that the camera sensor is at the correct distance from the focal reducer? If not, then you will get mis-shappen stars.

Is the focal reducer the correct one for the scope?

Does the focal reducer have any field flattening effects? A triplet will have a curved focal plane, especially is has been reduced to a fast focal speed.

[buikimaging]

Are you sure that the camera sensor is at the correct distance from the focal reducer? If not, then you will get mis-shappen stars.

Is the focal reducer the correct one for the scope?

Does the focal reducer have any field flattening effects? A triplet will have a curved focal plane, especially is has been reduced to a fast focal speed.

It is not a focal reducer, only a flattener with about 3mm incorrect spacing, which would be to the intrafocal. So I would have 3mm extra extension in the form of Delrin rings. The faint stars in the frame are also a shape of what I can only say as: Doughnuts.

Every single star is a doughnut shape in the frame, from bottom of the shot to the top, left to right

[Steve Ward]

If stars are doughnuts then there's an obvious focus issue ... some images would be helpful too ... 

[buikimaging]

If stars are doughnuts then there's an obvious focus issue ... some images would be helpful too ...

I focussed using a bahtinov mask, perfect focus is acquired.

Image that was used: http://stargazerslounge.com/uploads/monthly_08_2015/post-44447-0-15419500-1440418456.jpg

The star deformation in the corners is clearly visible

[Zakalwe]

It is not a focal reducer, only a flattener

Ah, thanks for the correction.

with about 3mm incorrect spacing, which would be to the intrafocal. So I would have 3mm extra extension in the form of Delrin rings. The faint stars in the frame are also a shape of what I can only say as: Doughnuts.

Every single star is a doughnut shape in the frame, from bottom of the shot to the top, left to right

3mm is a lot.

Get the spacing correct first. Sometimes it pays to play around with the spacing to get the best star shapes.

[Tim]

However i rotate the camera around the optical axis, the star deformation is always in the top right and top left of the frame. Whatever I do.

So the scope is probably ok, just need to look at how the camera is mounted.

Can you show us some pics? Give a break down of each part in the train.  When you rotate the camera, does the focuser itself rotate as with some triplets, or do you just rotate the camera? Does the flattener rotate too?

[buikimaging]

So the scope is probably ok, just need to look at how the camera is mounted.

Can you show us some pics? Give a break down of each part in the train. When you rotate the camera, does the focuser itself rotate as with some triplets, or do you just rotate the camera? Does the flattener rotate too?

I can not provide pictures at this moment. I was just at the telescope dealer explaining everything and tried to rule some things out. Focusser slop, flattener, telescope. We are going to rule out the flattener first, as it is a budget model, its the first that comes in mind, bad glass, bad alignment of the glasses inside the flattener etc. It is and will be a budget model.

As for the imaging train:

It's the Triplet > 5cm extender (ruled out already)> astronomik CLS-CCD > flattener > T-ring > DSLR.

The telescope dealer (Robtics) provided me with a TeleVue TRF2008 reducer/flattener to test if it indeed the flattener causing the issue. Giving a TeleVue TRF2008 for testing purposes... Great service!

Do first things first, check the flattener.

[Tim]

You know you are going to want the TeleVue after trying it

[buikimaging]

You know you are going to want the TeleVue after trying it

Unfortunately, yes will require some more saving, but I will need it, its not like I have FSQ money next couple of years (student). I will keep everyone updated as soon as I have some images with old and TeleVue flattener.

[wuthton]

If the flatener doesn't rotate with the camera then it is unlikely to be the source of your problems.

Can you try a different camera?

[buikimaging]

If the flatener doesn't rotate with the camera then it is unlikely to be the source of your problems.

Can you try a different camera?

It does rotate with the camera: flattener is attached to camera with T-ring. The camera is always in the same position relative to the flattener, so rotating the camera means also rotating the flattener. It's a solid connection, no rotation is possible between camera and Flattener.

[libraryman]

First thing is to take 10 x 10 sec images unguided in a rich star field free of glob clusters and nebula, measure them all in CCD inspector......then look at the curvature.

Whilst it may be still bad you will at least have a good idea of what it is really like...guiding and other things will skew any result in CCD inspector, so you don't really know what's going on..

Ray

[buikimaging]

First thing is to take 10 x 10 sec images unguided in a rich star field free of glob clusters and nebula, measure them all in CCD inspector......then look at the curvature.

Whilst it may be still bad you will at least have a good idea of what it is really like...guiding and other things will skew any result in CCD inspector, so you don't really know what's going on..

Ray

Thanks for the tips,

I thought you should eliminate any other errors, guiding and polar alignment. Otherwise you are just measuring guiding errors, bad polar alignment and such. The flattener should be perfect so we will know for sure very soon if the problem is with the flattener or anything else

[buikimaging]

I've got some great news!

As we tried to work out some spacing with Delrin rings we managed to get the FWHM down to 2.80" and 4.2" on the edges so field curvature went down to 40% or so. still not ideal but we need more spacing, FWHM is still decreasing in a 20sec frame with spacing ranging from 2 till 3.5mm.

I have to find something to get 4-5mm spacing but that's for another thread, did not try the Televue yet don't want to get spoiled and will save me money for now. Took a shot at IC5070 last night, did not do a star alignment but its a bit off my frame, I'll post it in the deep sky imaging thread.

Thanks for all your inputs!

[johngwheeler]

It sounds like you know what you are doing with your optical tests, but could the problem simply be a poor example of the telescope itself? I bought an ES ED80 CF Triplet a couple of weeks ago (which I think is what you have, although without the field flattener), and wasn't very impressed visually. It seemed to have an unacceptable amount of field curvature and poor edge contrast.  The mechanical precision of the focuser rotation was also very poor, allowing easy miscollimation of the focuser to the optical axis.

I returned it to the dealer.

I know many people say good things about Explore Scientific, but my experience left me unimpressed with both optical and mechanical quality. Maybe I was just unlucky!

I hope you have better fortune in fixing your issue.

[wuthton]

It sounds like you know what you are doing with your optical tests, but could the problem simply be a poor example of the telescope itself? I bought an ES ED80 CF Triplet a couple of weeks ago (which I think is what you have, although without the field flattener), and wasn't very impressed visually. It seemed to have an unacceptable amount of field curvature and poor edge contrast. The mechanical precision of the focuser rotation was also very poor, allowing easy miscollimation of the focuser to the optical axis.

I returned it to the dealer.

I know many people say good things about Explore Scientific, but my experience left me unimpressed with both optical and mechanical quality. Maybe I was just unlucky!

I hope you have better fortune in fixing your issue.

The tilt rotates with the camera which pretty much excludes the telescope from the guilty parties.

[Zakalwe]

I have to find something to get 4-5mm spacing

Something like this will do the job:

http://www.ebay.co.uk/itm/Blue-Fireball-T-Thread-Spacer-Ring-4-5mm-Extension-S-T4-5-/121235523448?hash=item1c3a334778

Teleskop Express also have them, I think.

[Tinker1947]

If you want spacing which allows for 100th of a mm then there are 2 adjusters on the market depending on your space to start of with, i used one with with my CCD and Coma Corrector......

http://www.firstlightoptics.com/adapters/baader-varilock-29-lockable-t-2-extension-tube.html

http://www.firstlightoptics.com/adapters/baader-varilock-46-lockable-t-2-extension-tube.html

[buikimaging]

Thanks for all the input: I just posted a picture on the imaging section: http://stargazerslounge.com/topic/251743-best-shot-so-far-ngc7000/

For the ES triplet having bad field curvature, would make sense because... It's still a triplet, not a flat field Quadruplet with a perfect flat field to begin with. About the optic quality of the scope, look at both my images, no CA what so ever on none of the stars, only field curvature due to bad flattener spacing. Maybe the scope you had was made on a Monday morning?

The focusser mechanical slop I can't agree with, I locked my focusser from the beginning, undid the screws on the inside of the focusser tube and locked them up with some Locktite and no more focusser slack, I percieve this telescope as a quality piece of kit for the price, my guiding wasn't too good because I had to touch the camera itself to open the shutter, but as can be clearly seen, the difference the spacing makes is humongous!

I can live with this amount of field curvature, I will find some 4-5mm spacers from TS optics or ASToptics to try get even better spacing! The Vari-Locks won't work because 23mm is already WAY off for my DSLR which already has about 51mm backfocus