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jabberwock48

Starlight Xpress AO unit and focal reducer

5 posts in this topic

I wonder if anybody is using this combination. Not surprisingly I was finding autoguiding a 14in Meade SCT via a guidescope, while working was not giving very tight stars. So I bought a (second-hand) SX AO unit and used with a LodeStar OAG this works very well. I then bought a TSSCKorr2 0.8 focal reducer/flattener but cannot seem to get the combination to work optically as can be seen in the image of M27 attached. The centre of the image (using an APS size sensor - QHY8Pro- which the reducer is supposed to be OK with) is alright but the outermost stars are very distorted. I have increased the reducer to CCD spacing by 4mm to account for the optical thickness of the SX AO deviator plate ( stated physical length of SX AO 68mm, optical length 64mm)

Anyhow I guess the question is this - does anybody know of a fundamental incompatility between an SX AO (i.e. the 12 mm thick glass deviator plate) and a focal reducer with a tight tolerance on the reducer to CCD sensor. If not more work to get the spacing right and eliminate any tilt. Thanks for any help.

post-17111-133877637964_thumb.jpg

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Perhaps I can ask a simpler question. Somebody somewhere must be using an SX AO with some sort of focal reducer. If so did you leave the reducer to sensor distance as specified or did you add 4mm to allow for the optical length of the SX AO being 4mm less than the physical length.

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I've left a reply to your other similar posting on ukai. Basically, I'm using the same reducer on my 10"lx200, and dslr, and am getting similar distorted stars, even with proper spacing. It may be something to do with pinching of the reducer optics in the microfocuser, but I need to test it further.

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Thanks for the reply. With the AO I have altered the spacing and adjusted the tilt without making much difference. Next time will try without AO and with variable spacer to see if that gives any improvement. I also have a 2in extender so can clamp on that rather than the reducer to see if that helps.

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I was making progress last night until a sudden power cut. Basically I think the separation has to within a very close tolerance such that trying to measure it is not good enough. Yesterday I thought of a good way of doing it.

Focus a star in the centre of sensor, then taking continuous images move the scope a little so the star is in one corner of the sensor (the one with the worst distortion). Then adjust the separation until the star is round. Repeat as necessary. In my case I had a good distance of thread between the CCD and the SX AO to allow 2 or 3mm of adjustment. When I resume (hopefully tonight) I will repeat and then find a spacer to take up the unused thread so the all is firm. There may also be a bit of tilt to correct for but I think spacing is the major issue. I do not believe clamping the corrector in the focuser is an issue.

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