Celestron C11 with Starizona LF Corrector

[MarkS]

As you may be aware, I've recently started experiments with my large sensor (36x24mm) Sony A7S camera on my Celestron C11.  The only corrector suitable for a full size sensor is the Starizona LF SCT Corrector (LF stands for large format) which flattens the field and reduces the image by 0.72x

My Cocoon image is an example using that corrector but it had really squiffy stars on the right hand side:  http://www.markshelley.co.uk/Astronomy/2018/Cocoon20180718_full.jpg

Further experiments with adjusting collimation (using the adjusters on the secondary mirror) reached the best compromise where one side of the image had stars with horizontal coma and the other side had stars extended vertically.  Not at all satisfactory!

To cut a long story short I found that the front corrector plate together with the secondary mirror did not line up with the principal axis defined by the baffle and the primary mirror.  The corrector plate was at least 2mm off centre.  This in turn was caused by the body of the scope not being aliggned with this principal axis.  So I had to unscrew the body from the rear assembly of the scope which gave me sufficient leeway (once I had enlarged the screw holes) to bolt it back into a position where it was central on the axis defined by the baffle tube and primary.  This was done very carefully using a combination of lasers and line of sight.

The difference is astonishing.  Now the corrector plate is centralised on the main axis of the scope the stars are much closer to being perfect

Looking back, this same tilt (caused by the off-centre corrector plate) is probably why I never had much success with my earlier Telescope Service SCT Corrector.  TS recommended me to buy a tilt adjuster but that never really cured the problem.  It's now obvious why.

The only website I could find that gave any help performing this optical alignment was this one:
http://www.wilmslowastro.com/tips/c14_optics_alignment.html

Mark

[MarkS]

As you may be aware, I've recently started experiments with my large sensor (36x24mm) Sony A7S camera on my Celestron C11.  The only corrector suitable for a full size sensor is the Starizona LF SCT Corrector (LF stands for large format) which flattens the field and reduces the image by 0.72x

My Cocoon image is an example using that corrector but it had really squiffy stars on the right hand side:  http://www.markshelley.co.uk/Astronomy/2018/Cocoon20180718_full.jpg

Further experiments with adjusting collimation (using the adjusters on the secondary mirror) reached the best compromise where one side of the image had stars with horizontal coma and the other side had stars extended vertically.  Not at all satisfactory!

To cut a long story short I found that the front corrector plate together with the secondary mirror did not line up with the principal axis defined by the baffle and the primary mirror.  The corrector plate was at least 2mm off centre.  This in turn was caused by the body of the scope not being aligned with this principal axis.  So I had to unscrew the body from the rear assembly of the scope which gave me sufficient leeway (once I had enlarged the screw holes) to bolt it back into a position where it was central on the axis defined by the baffle tube and primary.  This was done very carefully using a combination of lasers and line of sight.

The difference is astonishing.  Now the corrector plate is centralised on the main axis of the scope the stars are much closer to being perfect

Looking back, this same tilt (caused by the off-centre corrector plate) is probably why I never had much success with my earlier Telescope Service SCT Corrector.  TS recommended me to buy a tilt adjuster but that never really cured the problem.  It's now obvious why.

The only website I could find that gave any help performing this optical alignment was this one:
http://www.wilmslowastro.com/tips/c14_optics_alignment.html

Mark

[Carole]

You are clever Mark.  I would not have a clue about this sort of thing. 

Carole

[MarkS]

You are clever Mark.  I would not have a clue about this sort of thing. 

Maybe clever, maybe just a gambler.  I fear that one day I'll take something apart and never be able to reassemble it!

Mark

[JohnP]

Nice Mark - Well done - look forward to seeing the results in your next images.

I fear that one day I'll take something apart and never be able to reassemble it!

... I still can't believe you dismantled a brand new £2000 camera..... brave man...

[Carole]

... I still can't believe you dismantled a brand new £2000 camera..... brave man...

I thought it was the scope he took apart.

But having said that Mark did take apart the camera when it was new when he modded it.

I'll take something apart and never be able to reassemble it!

If I tried it it would be a dead cert, which is why I never tamper with anything.  Tried that once with a car repair when I was in my early 20s, and because I did not re-assemble the part correctly, I ended up with a much more expensive repair afterwards.  Never messed about with anything since.

Carole

[MarkS]

Now that I've done the scope once and seen a massive improvement, I'm trying to think of a way of making the alignment even more accurate.

The key issue is how to direct a laser beam straight down the exact centre of the cylindrical baffle tube.  I made a circular template with a pinhole in the middle to position over the end of the baffle which is inside the scope.  The beam that emerges through the pinhole will be centred, at least to the accuracy of the pinhole position.

But is there a better way?

Mark

[JohnP]

But having said that Mark did take apart the camera when it was new when he modded it.

Carole - that's was I was referring to....

[Carole]

Ok John l did wonder.

[The Thing]

Well done Mark! Seemingly not an uncommon thing to get the best out of a mass produced SCT.

I've had to shim the secondary on my LX90 SCT to get it to remain centred. I've also read a very interesting piece on making sure your corrector plate is actually oriented correctly. Apparently the factory marking if any weren't always quite right. Here's a YouTube video showing how to check. I think Metaguide can provide a suitable star profile diagram or probably the star profile in PHD2 would do. Don't know what software he's using in the video. The second video is interesting too!  https://www.youtube.com/watch?v=WQyMk914nfM

[MarkS]

Well done Mark! Seemingly not an uncommon thing to get the best out of a mass produced SCT.

I've had to shim the secondary on my LX90 SCT to get it to remain centred. I've also read a very interesting piece on making sure your corrector plate is actually oriented correctly. Apparently the factory marking if any weren't always quite right. Here's a YouTube video showing how to check. I think Metaguide can provide a suitable star profile diagram or probably the star profile in PHD2 would do. Don't know what software he's using in the video. The second video is interesting too!  https://www.youtube.com/watch?v=WQyMk914nfM

That's very interesting.  I like the use of the webcam - I got similar results from a Cheshire eyepiece as a second check.  So he diagnosed the central hole in his corrector plate was off centre - I hadn't even considered that as a possibility.  Now I want to go back and check 

I can't believe how badly these things are thrown together!  They must be doing a better job with assembling the Celestron Edge HD variety (with integral corrector) otherwise they'll have terrible aberrations all over the place when imaging.

Mark

[The Thing]

I suspect that its SCTs of a certain age that really suffer from this sort of shoddy workmanship. The Chinese have made strides since the scope manufacturers first outsourced the manufacture of their products.

[MarkS]

I have now checked the circular hole in the corrector plate.  It was slightly off-centre - by approx. 0.5mm.  This is in addition to my original problem and means that my corrector plate is still very slightly decentred relative to the axis through the baffle tube and primary. There is sufficient lateral movement in the retaining ring of the secondary to be able to allow adjustment for this and make both the retaining ring and the corrector plate properly centred.  I may or may not do this at some later time.

Mark

[RobertM]

But is there a better way?

Mark

Yes there is.

Use a Howie Glatter laser collimator with adaptor that projects a cross hatch pattern.  Use it on the focuser to shine up the baffle tube on to a piece of white paper at the corrector.  This does assume that the focuser is dead central and that there is minimal mirror slop but you'll see that in the projected image.  I have these if you want to borrow them.

Robert