Off-axis guiding

Lee · #1 15-10-2006, 07:03 PM

As an imaging grasshopper (who was incidentally stepped on last night).... I realise the need for guiding eventually, although I'm not going to try it yet - I have enough variables to contend with presently.....

Since I have started using the Newt, I'm thinking of going for an off-axis guider eventually. Have found some info on the Lumicon easy guider....

http://lumicon.telescopes.com/produc...der-26489.html

What do people think of -
1. Off-axis guiding in general.... Will I be able to pop a camera of sorts on this an auto-off-axis-guide....
2. This guider in particular
3. This merchant.... (important)

My brain is hurting, and my wallet cowering in the corner waiting for another beating.....!

Cheers

jase (Jason) · #2 15-10-2006, 08:52 PM

Off-axis guiders are great for telescopes prone to mirror movement such as SCT and some other Cassegrains as the guider is on the same optical axis as the camera. Should a mirror move the guider will pick it up and in some cases attempt to correct it (depending on how much movement is experienced).

Personally, I think if you're using a fixed mirror design or one that does not experience focusing/mirror flop such as the secondary focusing, you're better off with a dedicated guide scope. Make sure it is firmly fixed to the main telescope to minimise flexure.

Having a dedicated guide scope also allows you to guide on bright stars. With the off-axis guiders I've used, the light sent to the guider is dim making guide star selection difficult. The light is dim because a) it traverses a pick off prism to divert some of the primary optical path light to the guider b) the pick of prism is at the edge of the field of view. If your Newt has a well corrected flat field to the edge, then you might be ok. Most SCTs are very very disappoint off axis views. Stars are usually elongated.

The other items to be aware of are vignetting and parfocal. If the guider has a small aperture, you will be certain to experience some vignetting. This is not really bad as post processing with flats can sort most situations out, but try and avoid it if you can. In addition make sure the guider and main imaging camera are parfocal. If the main imaging camera reaches a different critical focus point (cfp) to the guider, you'll have have to match it (to make the guider reach cfp.

Finally, make sure you've got enough back focus to accomodate the off axis guider. Your focal length may change significantly.

Sorry, I can't provide any specific models or manufacture for the Newt OAG's. If you had an SCT, I would say you can't go past the Lumicon GEG. If you need to use one, the GEG is undoubtedly one of the best around.

Dennis · #3 16-10-2006, 07:18 AM

Hi Lee

Are you going to guide manually, (by eye), or via a ccd?

A Newtonian generally has a small amount of back focus, making it impossible to use an OAG unless you have a low profile focuser and low profile OAG.

An OAG generally produces distorted guide stars as they are often located at the edge of the FOV and are highly magnified by a Barlow.

If I did not have any problems with mirror movement, differential flexure, weight and balance, I would favour the separate guide scope. If the separate guide scope is also adjustable, it can make finding a suitable guide star that much easier.

With 35mm film I have used an OAG (Taurus Tracker II) and whilst it made OAG as easy as it can probably get, it was occasionally frustrating finding suitable guide stars.

I had none of that frustration when using a separate guide scope.

If you are going to use accessories such as reducer/correctors, then sometimes inserting these into the optical train with an OAG may require some tweaking to get things back into focus. A separate guide scope will not require any such tweaking.

One of the biggest challenges with a separate guide scope is ensuring that there is no differential flexure between imaging scope and guide scope. If there is, this will cause trailed stars.

Hope that helps a little.

Cheers

Dennis

PS - Here is a link to the newer Taurus Tracker III http://www.taurus-tech.com/tracker.htm

Garyh · #4 16-10-2006, 08:36 AM

Hi Lee,
Good info from Jase and Dennis there....

The problem is that with nearly all newts is there is not enough backfocus to put a OAG in the image train and if there was I say you would experience vignetting. You would have to move your mirror up another 50-60mm to get focus. I have only started the last few months doing guided shots with the new newt and am using a 70mm guidescope. My only problem is flexure which I am slowly sorting out. But is the only way to go for you.
A good set of adjustable rings mounted on your 8" OTA to suit your ed80 and you`l be ready to give it a go...
Lots of work getting a setup spot on hey...

Now that my mirror no move onto reinforcing those tube rings!!!
Cheers Gary

Lee · #5 16-10-2006, 08:51 AM

Back to the drawing board I think!

Lots of work getting a setup spot on - lots of work getting a setup functional more to the point here!.... I wish I was going for spot on at the moment!
On this subject - the rings for my 8" dont have mounting gear on top, ie just a smooth belt of metal (Aluminium I believe).
Should I try and attach something to these, or just get some rings with a mounting bracket integral?

Merlin66 (Ken) · #6 16-10-2006, 08:52 AM

I used the Lumicon short body OAG for many years... Highly recommend it. It has an adjustable prism which can make life a bit easier.
Only 40mm overall length in the optical path and will accept 48MM filters.

Garyh · #7 16-10-2006, 09:57 AM

Had a look at the Lumicon guider and looks very nicely made, merlin66 says its around 40mm thick, you will have to move that mirror 40mm up or less with a lower profile focuser...mmm....more work. I had a taurus tracker last year which was ok but was much deeper than the Lumicon. The problem with it I found was problems getting to focus with a 3x barlow and 12mm eyepiece. found it frustrating finding bright enough guidestars (don`t have the sharpest eyesight) ended up selling it and getting a guidescope.. Don`t have room on the newt for one as there is only 12mm from t adapter to focuser and I don`t feel like drilling more holes in her..

I am trying a side by side arrangement at the moment. Guidescope sits on a alluminium bar bolted to the dovetail between the rings. Adjustable in both axis to hunt down that guidestar. Maybe a losmandy side by side plate DSBS if wide enough for 8" newt and ed80 or losmandy rings on a dovetail plate? would be good for you..your wallet will go into withdrawals but....I would go for this if money was more abundant...
Some other IIS guys should be able to point you in the right dirrection with there setups with the g-11.
Cheers

gbeal · #8 16-10-2006, 10:29 AM

Lee,
never been done the OAG trail, but most that have either swear by it, or at it, most at it.
I have always used a separate guidescope, and on my newt it sits on top, but in your case, you have to muck about with things, so another option could be better. On top, you will need a couple of radius blocks, and a mount the guidescope rings on these, or get rings with a curved base that fit your existing tube rings. Either way the guidescope rings need to allow plenaty of adjustment for locating a suitable guidestar.
With some of my other scopes I use a side by side arrangement, home made and Losmandy call it a "DSBS". Both have their merits, and drawbacks. Need some pictures, get me at g.beal@xtra.co.nz
Gary

Lee · #9 16-10-2006, 01:46 PM

I'm thinking about getting some aluminium plate and making something.... I can foresee hassles in making it easily adjustable for maintaining balance..... Might as well bite it and get the Losmandy plate! I see my set-up/tear-down time increasing!