OAG and connection/spacing questions

[Helen]

I've got a Brightstar OAG Accessories @ Modern Astronomy

which has worked well with my SXH9 and guidehead on my Megrez 90 on the last 2 nights

I'm now looking to expand the use to include a FR and/or a filter wheel.

I've got the WO111 FR Reducers/Flatteners - William Optics 0.8x Reducer / Flattener III

I know I'm tired after two nights with not much sleep, but I'm struggling to know where to put it! My understanding is that the 55mm spacing to CCD chip is important, so would that mean it needs to go after the OAG?? If so, how do I attach it, as its got a 2inch nosepiece on it .And the same with the filter wheel, before or after OAG, and before or after FR?

I'm off to make a strong cup of coffee and then head to the Obs for experimentation (after carefully noting the current spacing which works very well!!).

Any help/guidance gratefully received

Helen

[SteveP]

Helen

The easy one to deal with is the filter wheel. It goes on the scope side of the OAG. This is to avoid having to guide through a filter which will limit the number of guidestars especially with narrow band filters.

The more problematic one if the FR. This should ideally be scope side of the OAG so long as its backfocus distance (55mm?) is sufficient for the OAG/adapter/image camera combo. You'll need to measure this carefully. The main problem with putting it after the OAG is that the guide camera's CCD needs to be parfocal with that of the main camera and will therefore need to stick out of the OAG by quite a distance

HTH

Steve

[Tim]

Think you mean the other way round Steve FW on the rear of the OAG.

If you can Helen, get everything behind the flattener. This can be tricky with only 55mm to play with though. What is the width of the OAG, the FW, and the distance to chip from front of camera?

[Tim]

The filters buy you a mm or so as well Helen.

[Helen]

Thanks both. My OAG is about 45mm, filter wheel about 26mm, chip is probably another 10mm inside casing. So not good! Will the outcome be a different reduction? or am I not going to get focus?

Thanks

Helen

[SteveP]

Think you mean the other way round Steve FW on the rear of the OAG.

Oops, yes I do!

[SteveP]

Helen

Apologies for the earlier confusion!

As a minimum, you're comparing components taking up about 100mm after you've made allowance for very slim adapters, against a BF of 55mm. There is usually some lenience with FF/FR's. I've used a Celestron and an optec in the past and they didn't appear too sensitive to being say 5mm out. But that's not the case here. As a minimum you'll find that the field just wouldn't be flat. It's difficult to say how intrusive that would be. Equally I'm not familiar with your refractor and wouldn't want to guess whether you'd reach focus.

Have you thought about contacting whoever sold you the FF and ask for their advice?

Steve

[Helen]

Thanks Steve. It does look much too great a distance doesn't it... (my measurements were taken with its all put together including the adapters, so its closer to 85mm than 100mm, but even so).

I'm wondering whether if I'm using a smallish chip the FF might not be so essential, and if I want a bigger chip my QHY8 is colour so I won't need the filter wheel.

I think some experiments are called for!!

Helen

[Tim]

This is one of the reasons I use the 9mm TS OAG.

Your best bet would be to try with the OAG in front of the reducer (scope side) and the filter wheel+spacer+camera behind it. Are you using the Atik? There should be a data sheet somewhere giving you the distance from front of camera to chip. Try to achieve 56mm if you can.

I'd be interested to hear how you get on with that, I've never tried with the OAG on the wrong side of the flattener

[Tim]

If you are using the SXV, (like it says you are, silly me) the distance to chip is 17mm as I recall.

[Tim]

Ooops, half a millimetre out, its 17.5mm

http://www.starlight-xpress.co.uk/Handbooks/SXVF-H9%20handbook.pdf

[Helen]

Thanks for the info TJ. Cloudy, so can't experiment tonight

Helen

[Tim]

You can try to achieve dual focus during the daytime Helen, in fact that will likely be easier for you, if you have a distant object to focus on.

[Helen]

Yes, I initially got it parfocal in daylight so should do that again. Would I be able to check if the spacing is causing distortion that way too I wonder?

I've just got maxim to talk to the filter wheel, so that's progress 9even on a cloudy night!)

Helen

[Tim]

I think you would struggle to tell the difference with distortions, unless you happened to have a prepared board some distance away with some kind of markings on it that would highlight any discrepancies.....

You can pretty much take it as red that unless you get the 55-56mm right, then the stars wont be as good as they can be. I can honestly say I spend more time adjusting spacings between cameras and filters than any other aspect of the hobby I really should invest in another OAG and T thread spacing adapters to alleviate that!

FWIW, the baader adjustable spacers are pretty invaluable, if not intuitive to use. They allow you to pretty much get where you want to be by having in effect a variable length.

I cant even get Maxim talking to ME, let alone the accessories At least, not in a language I am comfortable with

[Helen]

Thanks TJ. Maxim did just throw a complete wobbly with the filter wheel, refused to recognise is and initialise it. So I showed it who was boss - yes pulled the plug out! - and when plugged back in it worked fine.

Let's hope for some clear skies for proper testing...

Helen