C11 Flattener

[MarkS]

I had another go at this last night.  The scope is collimated; the T-ring extensions are all square but something somewhere is way out of kilter.
http://www.markshelley.co.uk/webdisk/c11flattener.jpg

I'll do some more experiments tonight to narrow down the cause.

Mark

[Carole]

That's very odd Mark.  It's all the little stars that are affected and not the big one.
I am confident with your skills and determination you will get to the bottom of it.

Carole

[mickw]

I seem to remember you having concerns about the spacing of the canon chip after modding it.

Could the flattener combined with the long focal length be highlighting this ?

[The Thing]

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...

Probably a lot more sensible to shim the CCD with some very thin washers (of the sort you can find in old cassette decks and VHS machines). I put three identical washers under my CCD mount points to compensate for the extra thickness of Edmunds optics glass when I did the Ha mod.

Mind you if it  is only the small stars that are affected you will need some very peculiarly selective collimation!

[MarkS]

It's not the Canon's CCD - I have proved this to be square using both the Bananascope and the C11 (without the new flattener).

I've done some more experiments.

I'm using a set of extension tubes to obtain the correct distance (97.5mm) from the back of the flattener to the CCD.  If I remove only the flattener from this optical train then I can get an image with symmetrical distortions - the usual C11 distortions.  As soon as I put the flattener back into the optical train everything goes squiffy - note I am using exactly the same set of extension tubes in both cases.   As I rotate the flattener, the "squiffiness" rotates with it.  One possible explanation is that the flattener is not seating properly on the extension tubes - however, the tubes would have to be 1mm out of square to generate such distortion.  Visually I see no seating problems and measuring it shows nothing wrong.  I therefore conclude that the problem is likely to be with the flattener itself.

I'll give the flattener a careful visual inspection in daylight tomorrow but I think it might have to go back.

Mark

[MarkS]

I may have worked out the problem.

There's a lot of curvature in the image I posted.  So I went back to first principles and checked everything again.  The 97.5mm should be measured from the base of the m48 thread on the rear of the corrector but I've been measuring it from the very rearmost plane i.e. the "top" of the thread.  The height of the thread is 4mm so this means my spacing is probably 4mm wrong.  This might account for the curvature

There are no more clear nights for a while so the 4 spires of St Mildred's at Tenterden may have to suffice as a target.

[RobertM]

Could be that the collimation was a tad out and it's been amplified by the flattener being at the wrong distance - that distance is supposed to be very critical.  It would explain why the optical centre appears to be upper right of chip centre.

Just a thought
Robert

[MarkS]

Could be that the collimation was a tad out and it's been amplified by the flattener being at the wrong distance - that distance is supposed to be very critical.  It would explain why the optical centre appears to be upper right of chip centre.

I had come to the same conclusion - I really hope that's what the problem is - I have a lot planned for the C11 this Winter!

Mark

[MarkS]

No good  

The corrector to CCD distance is now correct at 97.5mm and the collimation is more or less spot on.  But the results are almost identical to the earlier posted image.  So the earlier 4mm error wasn't the cause of the problem.

I'll send a couple of pics to Teleskop Service to see if they have any other suggestions.

Mark

[RobertM]

Hmmm, looks like a duff un then.

I also have a lot planned for the C11 but it was reliant on that flattener coming up trumps !

[MarkS]

Teleskop Service have been very helpful.  They agree that the cause is likely to be the corrector itself but they've asked me to return the corrector with the adapters so they can check out the whole optical chain.

Mark

[RobertM]

That's good, I've always found them very helpful so I'm sure they'll get you sorted.

[MarkS]

They've tested it and decided to replace it.  The replacement has been tested in advance

[RobertM]

Which is what they should have done in the first place what do they take us for ... and yes I know there are plenty of answers to that one  

[MarkS]

I received the replacement flattener today and luckily the sky was clear tonight to enable me to give it a test.

The results speak for themselves:
http://www.markshelley.co.uk/webdisk/c11flattener2nd.jpg

As before, if the flattener is rotated, the distortion rotates with the flattener.  This is so extremely disappointing 

Mark