Focal Reducer/Adoptive Optics

[jabberwock48 (John)]

I am trying to use a 0.8 focal reducer/corrector that recquires a precise separation of 97.5mm between it and the CCD sensor in conjunction with a Starlight Xpress AO unit. The latter has a multi-coated AR bloomed plane-parallel optical window with a thickness of 12mm and a diameter of 40mm. This requires the separation to be increased by 4.1mm to compensate for its optical thickness. With this done I am still getting distorted stars in the corners of the image after repeatedly adjusting the separation by small amounts to optimise the image. This is using an APS sized sensor which the reducer is supposed to be suitable for.
So the question for any optical gurus is this - is there a fundamental conflict in using these two items together (telescope is a Meade LX200GPS) or do I need to persevere?

[richardo (Rich)]

Hi John,
I'm no guru, but speak from experience having gone down this road with correctors and SX ccds.
Using any reducer/ corrector that needs a specific distance can be a little tedious at times to say the least!

You're saying that there is still distortion in the corners, if it is an even amount to each corner, then it will definitely be your spacing distance.
Just measure from your CCD window to the reducer at the specs (without the added 4.1mm) and see how things look.
If it is better in the corners than your previous spacing with the 4.1mm added, obviously you'd be working around this. Probably a little less spacing will help. I believe that there is generally a +5/-5 tolerance to most reducer/ correctors.
If it is worse, then unfortunately, you'll have to keep persevering with adding a little more at a time. I found just adding in made up washers around 2mm to the train helped me get to an exact spacing.
If the degree of distortion to the corners is not even, then I'd be going back to the basics.

#1.Check your collimation first off.
#2.Check your imaging train for sag, poor connections.

If the above things are good, then I'd be checking the following.
#3. If you're using an SXVF/R- m25c or m26c, (as stated aps size) these being the newer camera enclosures, have the adjustable face plates.. I'd be checking that the CCD chip plane is orthogonal with the incoming light cone. (if you're using a DSLR, then don't worry about this..)

Hope this helps maybe a little. Any way, it's good to have someone to use as a sounding board after battling an issue on your own.

Rich

[jabberwock48 (John)]

Hello Rich
Thanks for the reply and advice. I think the reducer/corrector that I am using has a very small tolerance so I will have to perservere knowing that it is possible to combine the two. My CCD is a QHY8Pro which also has tilt adjustment albeit using a different method and I have CCD Inspector2 to check the collimation.
Just need some decent weather to have another go
Regards
John

[richardo (Rich)]

No worries John!
It's a pain when you need to test and the weather is crap! (excuse my French)
When your weather improves, go back to the imaging train without the reducer, then crank up CCD Inspector and see how the collimation looks... that is if you're finding there is some non evenness to the distortion to the corners. But better to get things good before tracking down other issues.

I used some plastic washers made from old Tupperware containers to give me a couple of mm's of spacing to add to the train while testing... this then got me to a ball park distance. I then could work out what I needed in the way of extension rings that would fit the bill.

Don't worry, you'll get it. I've read quite a few post around the place with folk having the same issues with reducer/ corrector.
All ended well!

Keep us posted on how you get on.

All the best
Rich

[gregbradley]

2 concepts here have to be clearly understood to get results;

1. Metal Back Distance - this is the distance between the outside wall/flange of the corrector to the CCD chip not the CCD window which would be 10mm or more in front of the chip.

See this photo to get it totally clear:

http://rainerehlert.com/Takahashi/MetalBack-00.jpg

2. Corrected circle:

This is the diameter of the circle of light the corrector has where the aberrations are corrected.

Outside that circle you will start getting aberrations.

For example if you use a large chipped camera with a small opening focuser with a corrector you will not be able to correct to the corners as typically a large chip will require 3.5 inches of opening or greater including the size of the corrector to give a large enough corrected circle.

What Rich says is correct. If you have distortions equally in each corner it is either wrong metal back distance (spacing is wrong) or your corrector has too small a corrected circle - one or the other.

If one or more corners are distorted but not all then it means you have tilt and you need to pack out the offending corners to make level and square.

The amount you may need to pack out may be very very small depending on the size of the chip - smaller chips are easier than larger ones.

For example I was recently chasing down a bit of tilt with a 16803 chip (very large) it was 2 cigarette paper thicknesses out in one corner! That largely corrected it. That is a fraction of 1mm in thickness. A smaller chip camera won't be that fussy but is still likely to be a fraction of a mm.
Spark Plug gap spacers are good for that. you can get them at SuperCheap auto etc.

Greg.