OAG setup trouble (C11 EdgeHD + reducer)

[Wiu-Wiu]

I am having 2 issues with the setup of my OAG. I have no idea how to solve them. 

Equipment: 

- C11 Edge HD

- Celestron 'deluxe' OAG

- ZWO Asi 224MC guidecam

- ZWO ASI 071 MC PRO image cam

- Reducers: either the Astro Physics CCD Telecompressor or the Explore Scientific 3" reducer/corrector (I know, it's for my apo, but the retailer's website states it gives good result on edge HD scopes too... (haven't tried it yet though)

First: My guidecam can't reach focus. 

When my main cam reaches focus, I need to focus my guide cam further inward. But I am at the limit, and still get donuts on my stars.

So: how do I make sure my guide cam gets into focus - AND keep the spacing of my imaging cam at correct distance... (see other problem though...)

Problem 1.2: The view in my guide cam are HALF donuts. No matter where I place my star in my field of view (by moving the scope), I only get stars that are cut in 2. Is this due to the position of the camera's sensor? or is the pick-off prism in the wrong position?

 

 

Problem 2: Spacing. 

This might be better off in a separate topic, but here goes: I've been trying to determine the correct distance for my Astrophysics CCD compressor. It was supplied with the scope and the Celestron reducer is WAY overpriced. 

Quote

Optimal back focus: 85 mm - if you change the distance, the kompressor factor will change. You can change the distance between M48 connection thread to the sensor from 70mm to 90mm with good results.

The focal length of the CDD Telecompressor is 12 inch (305 mm).

Requires 5 inch Back Focus

 

Quote

The CCD Telecompressor (CCDT67) The focal length of the CCDT67 CCD Telecompressor is 305 mm or 12.0” Again, using the formula introduced above, a distance of 101 mm or 4.0 inches gives the stated 0.67X compression. (305 - 101) / 305 = 0.67 Because the CCDT67’s focal length is much shorter than the 27TVPH’s, an equal change in distance will have a greater effect on the compression. If we shorten the distance by 35 mm as we did above for the 27TVPH, the compression factor becomes 0.78X. (305 - 66) / 305 = 0.78 Changing from 0.67X to 0.78X is certainly more drastic than changing from 0.75X to 0.80X as was the case for the 27TVPH. Please note also that the CCDT67’s more aggressive compression makes it less suitable for faster instruments. We strongly recommend using the CCDT67 only on instruments with focal ratios of f/9 and higher. The CCDT67 takes up 16 mm of the distance in your calculations. The rest of the distance is in the nosepiece, filter wheel, CCD camera and any extensions you may use. Note also that, unlike the above example with the 27TVPH, the entire nosepiece length is a part of the light path with the CCDT67 since the nosepiece does not get buried inside another adapter relative to the telecompressor. The CCDT67 has a clear aperture of 44 mm or 1.75”. At 0.67X compression, it will fully illuminate a 29 mm circle - 44 mm x 0.67 = 29 mm. To give a real world example, let’s assume the following: We will set up a system with the CCDT67 @ 16 mm; our AP16T - 2”/T2 nosepiece @ 29 mm; a color filter wheel @ 25 mm; and a CCD camera @ 25 mm. 16+29+25+25 = 95 mm. (305 - 95) / 305 = 0.69X 44 mm x 0.69 = 30 mm - the fully illuminated circle. Please review adapter connection options on the Exploded placement diagram

 

I had it set up like this, total distance between compressor optics and sensor at approx. 100mm 

(compressor - OAG - filter drawer - camera. This is the MINIMUM spacing I can possibly achieve. I do have extension pieces available)

 

Since Guiding didn't work due to the problem in 1.2, I could only do unguided subs of 30s - you can see the stars in the corners are not perfect. 

[Davey-T]

Not sure if it's of any help to you but I'm using the APCCDT.67 on my Meade 10" flat field SCT and have it fitted inside the baffle tube.

Dave

Article here by a forum member, for Meade scope but may be of some help.

https://astrojolo.com/gears/acf-10-and-ccdt67-telecompressor/

[Wiu-Wiu]

Not sure what you mean by 'fitted inside the baffle tube'. 

I have a connector to the back of the OTA which accepts 2" nosepieces and since the Telecompressor is also 2", I inserted it as instructed by some of the documentation). I'd say this is what you mean?

Should I have inserted it more or less in order to get rid of the longer stars in the corners? (and how silly they don't provide proper spacers or rings to make sure the compressor is inserted correctly)

If I read the article correct, the spacing between telecompressor and sensor is not "critical" for image quality, but for the reducing factor.

[Davey-T]

Apologies "inside the baffle tube was wrong" here's mine setup, the reducer fits inside the Meade motor focuser.

I didn't worry about spacing just screwed it all together and it is what it is, haven't done any maths to work out the actual focal length.

Stars are round into corners but I'm only using the Atik314L+ so probably not the whole FOV.

Dave

[Davey-T]

Comparing pic's my reducer looks further from the OAG, maybe a spacer between your reducer and the OAG would help ?

Dave

[Wiu-Wiu]

Thanks, that helped  

 

I could add more spacing between things, but I was/am worried that adding more distance to the image train will decrease the reducing factor (but as long as I can keep it under control by guiding I'd be ok with that)

Not sure if that would make the star shapes better though. (but what would...??)