C11 Flattener

[Rick]

Ugh. Did they just send the same one back?

[mickw]

It looks like it's a slight improvement, but still not enough.

Are you rotating just the flattener or flattener and extension tube/spacer ?  Could be the spacer.

If they actually did test this one before sending it out, could the fault be with your C11, focuser etc. ?

I think you need to try it in another C11 before shoving it up TS's bottom

[MarkS]

Ugh. Did they just send the same one back?

I'm wondering the same thing myself. I should have added my own unique identifying feature on it before returning it.

Are you rotating just the flattener or flattener and extension tube/spacer ?  Could be the spacer.
If they actually did test this one before sending it out, could the fault be with your C11, focuser etc. ?

The optical train is:
C11 - SCT adapter - flattener - extension tubes - Canon T ring - Canon
The flattener has a male M48 thread on one end and a female M48 thread on the other, so it can be removed leaving everything else:
C11 - SCT adapter - extension tubes - Canon T ring - Canon
In the 2nd configuration I get a sensible well collimated image but with the symmetrical but typical SCT edge of field astigmatism distortions

The key thing is that the distortion rotates with the flattener.  
http://www.markshelley.co.uk/webdisk/c11flattener2nd.jpg
http://www.markshelley.co.uk/webdisk/c11flattener2nd_rotated180.jpg

Let us suppose, for the sake of argument that the flattener is working correctly and can produce a perfectly flat field as the publicity states.  Then in order to produce the amount of defocus seen in the corners of that image, there would have to be a tilt of around 1mm from edge to edge of the CCD.  This would be obvious to the eye and even more obvious to my ruler.

I really want this flattener to work but I'm becoming despondent now.

Mark  

[mickw]

The top left then bottom right are awful, similar to your original.

This is after the flattener was supposedly tested 

[MarkS]

If it's clear tonight I might perform a Roddier test using WinRoddier. 
If you speak French you can read about WinRoddier here:
http://astrosurf.com/tests/roddier/projet.html

Such a test should indicate exactly what is wrong with the wavefront emerging from the corrector and might just give a clue as to the underlying problem.

Mark

[MarkS]

I came across a site yesterday with a nifty two piece collar that fits in the optical train and allows you to collimate the camera/chip using three screws. It would take up to three kilos before they recommended you use the lockdown grub screws. If I could only remember the company name...

Was it the following?
http://www.teleskop-express.de/shop/product_info.php/info/p4744_TS-M48-tilter---compensation-for-field-tipping-in-astro-photogra.html
Or this?
http://www.teleskop-express.de/shop/product_info.php/info/p4745_TS-T2-tilter---compensation-for-field-tipping-in-astro-photograp.html

[The Thing]

Similar sort of thing, but it was in american or in english and claimed to address design issues with other available units.

[MarkS]

Similar sort of thing, but it was in american or in english and claimed to address design issues with other available units.

Is it this one?
http://www.gerdneumann.net/v2/english/eng_ctu_camera_tilting_unit.html

[The Thing]

That's the beasty.

[MarkS]

That's the beasty.

It looks easier to use than the TS one.

[MarkS]

ASA also produce a Keller corrector for an SCT:
http://www.astrosysteme.at/images/Corr_2Zoll_Schmidt-Cassegrain-Red.pdf

As far as I can see, the design is similar (probably identical?) to the Teleskop Service version, including the corrector to CCD spacing of 97.5mm

Mark

[MarkS]

Finally got round to having another quick go at using the TSSCKorr2 flattener - this time with the tilt device between the flattener and the scope.  Result below shows the centre and the extreme corners of the Canon 350D image.



The image shows the same star but shifted around in the FOV.

To achieve this I had to apply 1.5deg tilt to the flattener.  I reckon with a bit more tweaking (e.g. adjusting the CCD to flattener spacing) I might get this almost perfect.  But Sunday night I wasn't really in the mood for it.  And there was too much moon.

But it is not good to have to buy a tilter in order to correct the flattener ...

Mark

[Rocket Pooch]

Mark,

Remember the primar mirror in that scope moves around hence the tilt which will be exaggerated at shorter f ratio's.  What you will probably find is if you get it bang on at the start of a session then by the end of a session the mirror will move during RA movement and the stars will be out again a bit. 

Having said that these star images do look pretty good, not perfect, but the scope is not a 10" RC, I think thats going to be usable.

One thing on both the Edge and ACF scopes they really have focused on stopping primary slop.  The ultimate answer is a proper RC I guess.

Chris

[MarkS]

Remember the primar mirror in that scope moves around hence the tilt which will be exaggerated at shorter f ratio's.  What you will probably find is if you get it bang on at the start of a session then by the end of a session the mirror will move during RA movement and the stars will be out again a bit. 

Yes it is quite possible that the primary mirror slop may be a problem.  When I previously used the C11 regularly, I had to collimate it for the area of sky I was imaging, before starting the imaging.   It is quite possible that these effects could be magnified by a highly optimised flattener.  I'll do some further testing on this issue at some point.


Mark

[The Thing]

You could fix the mirror and add a Crayford on the back. It's commonly done with (older) Meades as the mirror slop can be atrocious. You have to decide on a focus range as the Crayford can't replace the full movement of the mirror then set the mirror at that point. Does the C11 have a mirror lock? If not I have heard of jubilee clips being put around the draw tube, but then you are stuck with the imaging train that mirror position + Crayford suits.

Also re-greasing the draw tube may minimize movement, even moving the mirror from limit to limit to redistribute the grease periodically helps a lot. I find if the scope has got hot the issue is worse, possibly the grease flows a bit. This is common on Saturdays at DSC when the scope is out in the open all day .