Tips and Tricks for Using an Off Axis Guider?

sheeny (Al) · #1 16-03-2006, 05:05 PM

Does anyone have any tips and tricks for using an off axis guider?

In particular, how to get the guiding eyepiece and the camera both in focus at the same time?

I have an OAG which I used with my OM-1 at prime focus, but really only used it as a camera adapter. I've read a bit about the difficulties in finding bright enough guide stars, etc but given I have the OAG and don't have a guidescope I'm prepared to persevere with the OAG a bit for a while.

I haven't used the ToUcam with it yet, but I'm sure I bought the T thread adaptor so I can. I'd like to be able to guide some DSO stuff...

hmmm... I was about to commit myself to spending some time on the weekend (daytime) playing with the ToUcam and off axis guider but just remembered I'm helping friends move...

Thanks,

Al.

GrampianStars (Rob) · #2 16-03-2006, 10:29 PM

I used an illuminated reticle a must IMO back in the
35mm film days
just rotate the reticle with the dec axis and make sure
whatever is in the crosshair stays there
hard work on the eyes bye the way
and don't bump the scope
or touch the eyepiece

[1ponders] (Paul) · #3 16-03-2006, 10:56 PM

I'd offer you some advice if I could Al but my sanity isn't all it used to be after trying to guide with an OAG and a ToUcam.

Using a reticle I could get away with, but most of the time I just couldn't get a star that the toucam could guide on. That's why I went the guidescope way. If I can think of anything that helped in anyway I'll let you know.

Good luck with it.

btw what sort/brand of OAG do you have?

sheeny (Al) · #4 17-03-2006, 08:48 AM

Its a Celestron OAG. I can rotate the eyepiece though about 120 degrees and adjust the mirror position a bit. At this stage I'm only planning to manually guide and use the ToUcam for image capture so I will be using a 10mm guiding eyepiece with an illuminated reticle.

I did have a little play with it when I first got it and found that with the OM-1 focussed I couldn't get focus in the eyepiece - so any dims stars just disappeared, and so reduced the number of candidate stars to guide on.

Al.

[1ponders] (Paul) · #5 17-03-2006, 11:33 AM

Yeah it's a bit of a problem. To focus your guiding eyepiece you'll need to pull it out of the eyepiece holder until the stars focus. Pretty clumsy way of doing it.

Try Here

sheeny (Al) · #6 17-03-2006, 01:42 PM

Thanks Paul.

Yep, that's the beast I have. I have tried moving the eyepiece out to try get focus, which as you say is a clumsy way to do it, but I couldn't achieve it. I can't remember now whether I had to come further out or get the eyepiece closer... I didn't perservere with it for very long anyway!

But I will have to have another play with it! Hopefully the focal plane of the ToUcam is different enough to the OM-1 that I can get close to focus.

Al.

sheeny (Al) · #7 17-03-2006, 09:12 PM

Hmmm...

I've just had a play with the OAG again. I set it up in the garage and then focussed on a brick wall about 50 to 100m away. With the ToUcam focussed on the wall, I can't get anything resembling focus in the eyepiece, no matter what eyepiece, or combination of eyepiece, barlow and projection tube I try...

Then I had dinner and a couple of glasses of red wine...

So I've gone out to the garage again after dinner and set about measuring the length of the light path to the sensor in the toucam, and to the reticle in the eyepiece. By my calculations, the light path to the ToUcam sensor is 32 to 35mm shorter than to the reticle of my eyepiece. Just looking at the OAG is enough to confirm the light path should be shorter to the camera... all that's in doubt as a result of my couple of glasses of wine is the precision of my measurements and maths!

So what I need to get my OAG into reasonably usable shape is a 30 to 35mm long T thread extension tube (of course I'll check that dimension again!).

Just editted this... I've found a set of T thread extension tubes that should do. It should get me from 0 to 31mm in roughly 6mm steps depending on the combinations.

Thanks,

Al.

[1ponders] (Paul) · #8 17-03-2006, 10:44 PM

I think I see your problem now. I use the meade oag and it has a longer eyepiece tube than the celestron. I have np achieving focus. If you want to borrow mine your welcome to use it to work out the length of your extension tube.

sheeny (Al) · #9 18-03-2006, 04:59 PM

Thanks for the offer Paul, but my problem is the other way (if I'm reading your post right)... the light path to the guiding eyepiece is 32mm longer than to the focal plane (sensor) of the ToUcam. So when the ToUcam is in focus, there's no way I can get the eyepiece closer to get it in focus...

Never mind, I'll have another play when my T thread extension tubes arrive!

Thanks,

Al.

[1ponders] (Paul) · #10 18-03-2006, 07:33 PM

Do you have an old barlow anywhere Al? rip out the lens elements and use it as an extension tube for the ToUcam.

sheeny (Al) · #11 19-03-2006, 09:49 AM

Nah mate, sorry, I don't... but I also don't have a T thread to eyepiece tube adapter either. I'm using a T thread adapter for the TouCam straight onto the back of the OAG.

I suppose in hindsight I might have been better off buying a T thread to eyepiece adapter and using the standard ToUcam eyepiece adapter in that... that would probably give me about the right length light path (or at least closer) and some adjustment (albeit crude) on focal plane position...

Isn't hindsight a great thing?!

I might still do that depending on how the T thread extension tubes work out... lemme see... that'll be about "plan C" by my count...