Off Axis or Guide Scope

[RD400C (Garth)]

I am in the process of building up my viewing & photography equipment, and am now looking at options for guiding during photography. Does anyone have some advice on the merits of a guide scope, compared to off axis guiding? I would imagine the variables would be to do with the photographic equipment, but I would be keen to hear from any experienced members, as I haven't purchased the guiding nor photographic equipment as yet.

[astronobob (Bob)]

Hi Garth, It all really depends on your set-up & experience, , I have used a guidescope for last 5-6yrs with good effect, but not perfect ? Now im using a 10" newt with 1200mm Focal length on a Neq6 mount which requires more accurate guiding and having a seperate guidescope from the main imaging scope will often have 'flexture' between the two, This will cause small but annoying star trailing in images as the imaging scope will move/lean a smidge differenty than the guide scope! So Im looking at going Off axis soon as this will erradicate 95% if not 100% of flex issues, , Though there are some other considerations, eg, OAG need more sensitve guide camera, have less field for choosing stars, and need perfect space fittings to have both imaging camera & guide camera to both reach very sharp focus ? ?

If you can tell us what scope/s and mount you are using, this will go long way to have further advice !
Rgrds !

[RD400C (Garth)]

Thanks for the reply Bob, I am very new to the photographic side of astronomy, the set up is a 130mm refractor, on a robust but non go-to mount. I have a reducer / flattener that gets the FL down to ~750mm. From what you have said, I think that a guide scope may be best for my current experience [next to nil], given this will also reduce the load on the main image train. But I am interested in any other insights, very much appreciated. Garth

[Bart]

I'd go the guide scope route for now, considering the short focal length. Try to make sure the your guide scope is solidly mounted and pointing at the same field as your imaging scope. Use high quality, strong mounting bars like Losmandy style over Vixen and try to avoid using cheap mounting rings with plastic screws for the guide scope. If you can, use solid rings and spend some time shimming and adjusting the guide scope to get good same field coverage as your main scope.

Oh, and remember, you get what you pay for. Buy quality gear.

[RD400C (Garth)]

Cheers Bart, I think you are right, and I am making sure I buy quality equipment, I'm too old to muck around with stuff that doesn't work. Garth

[5ash (Philip)]

I have heard the spiel about heavy duty bars many times . Yes it's true for heavy setups but with a regular guide scope , for example, an Orion 80 mm that's quite light, and a short focal length main scope you should have no problems with a vixen bar. I run a skywatcher ED120 with an aluminium bar attached to the tube rings , ontop of which is a gso vixen dovetail clamp ( nice and cheap) into which the vixen rail of my Orion short tube guide scope attaches. This is mounted on an NEQ6 pro. I use an Orion starshoot autoguider and have made multiple exposures of up to 15 minutes over several hours with nice round stars in all my pictures. With my C9.25" I use an off axis guider . This is not due to flexure problems but the size of the opening in my observatory.However with its longer focal length I think I would stick to OAG any way
Hope this helps regards philip
Ps forgot to mention I replaced the adjustment screws in my guide scope rings with metal ones

[RD400C (Garth)]

Thanks again for the insights, I will now plan for a guide scope set up, go for quality, reasonably light and good mounting. Next will be imaging equipment, but that will need to wait for now. All good, and a long week end at last. Garth

[gregbradley]

SBIG STi guide camera and lens mounting kit. I have the guide kit which I intend to use on my large CDK scope and I expect it to work well.

No need for a guide scope with that unit per the marketing and I am expecting it to work well.

I use a Vixen VMC95 scope for a guide scope for my TEC110 imaging scope. It works well. There can be flexure at some angles from the focuser mainly because the camera I use is so heavy not because the focuser is faulty. But it works plenty well and I get round stars routinely
in 10 minute exposures.

I really like this little SBIG STi guide camera. I highly recommend it.

Alternately if you are planning to buy a dedicated CCD camera for astrophotography the SBIG self guiding camera range or the QSI with built in filter wheel and off axis guider are hard to go past.

Greg.

[johnnyt123 (John)]

Hi Garth..

Just putting in my 2 cents worth...

I have been doing astrohphotgraphy for just under a year now...so my experience is also limited.

I have always used a separate guide scope and made sure that its focal length was always the same as or longer than the main imaging scope as this will result in more accurate tracking.

I have recently purchased a QSIwsg-8 which has an off axis guider. I only used it once and as was mentioned before you do need more sensitive guide cam like a lodestar to pick guide stars. I have returned for a separate guide scope for now....until i purchase a Lodestar.

hope that helps

[RD400C (Garth)]

Thanks John,

The best thing about IIS is the feed back, all of this very salient information, the FL of the guide scope is good to know, and should have be obvious to me but wasn't. The final budget for the CCD is not yet decided, so I attempting to cover the range of eventualities! So I will do some future state maths, on self guiding CCD, vs separate guide scope, vs OAG, as the technology is changing all the time, but the quality equipment is not cheap. All 2c worth[s] are much Appreciated. Cheers Garth

[gregbradley]

I think the important point about a guide scope is that it is rigidly mounted and has a rigid focuser with no slop.

Roland Christen from Astrophysics also recommends the guide scope rings be attached to the OTA it is going to guide.

When you think about it this would get rid of potential flexure.

I find the Vixen VMC95 great bang for buck as a guide scope. It has a focal length of about 1 metre, is small and light, has mounting holes in the body to screw it on to the main OTA and the focuser has no slop or mirror shift. Its also very cheap.

Greg.

[g__day (Matthew)]

Would you consider On axis guiding - using a cold mirror from say Foresight Innovations?

If your ever graduate into long focal length guiding I'd say an ONAG, OAG or self guiding CCD are your best bets

[Paul Haese]

Either OAG or ONAG are the best solution and in the long run the most cost effective too.

[RD400C (Garth)]

Greg, Matthew & Paul, thanks for the insight, and advice,
My synopsis of the answers so far;-

Guide Scope
Desc;-
Independent image path from objective lens to separate CCD, mechanically linked to main scope. This is used to guide main imaging equipment
Pros:-
Separate field of view able to select any tracking source
Separate focusing mechanism than main imaging equipment making set up easier
Cons:-
Separate scope and mounting equipment to buy
Flexure between Guide Scope and Imaging Scope degrades tracking accuracy
Generally a smaller scope with a different focal length than the main scope, making guide source selections and guiding more difficult

Off Axis Guiding
Desc;-
Shared image path from common objective lens to a separate CCD, sample of main field of view is taken by an off axis diagonal mirror
Pros:-
Only one scope, less to buy and less to set up
No flexure issues between guiding and imaging equipment
Cons:-
Sample of main FoV may not contain suitable object to guide, or require rotation of imaging equipment
Image taken off axis is limited by the small mirror and may make source selections more difficult
Separate focusing path still required to focus guiding image, not really an issue if equipment is well designed and made

On Axis Guiding
Desc;-
Shared image path from common objective lens to a separate CCD, the guiding image is at IR wave lengths filtered by a 45 deg ‘cold’ mirror, which diverts the main image at non IR wavelengths to 90 deg off of it’s original path
Pros:-
Full image area available for review and guiding during the imaging process
Probably don’t want the IR in the main image anyway, as well as other filters would be in use
Wider range of objects available for guiding
No flexure issues
Cons:-
More glass between the image and the imaging equipment

So what will I do, probably go the guide scope option, as I am a very amateur amateur, and the OAG an OnAG seem to be geared to those very experienced in astrophotography an even for the remote operation of their equipment. I might even be able to get most of the equipment 2nd hand.

But to receive the advice I have from those in the forum who are experts in the field has been great.
The only further question will show my ignorance on how a modern CCD works, but I assume that you cannot sample the image for self guiding during capture as this sampling disrupts the capture process?

An article I found very useful
http://www.cloudynights.com/item.php?item_id=2706

[g__day (Matthew)]

With your intented path I would say give it a go - if it works for you great! If you find like me and others you simply can't nail the levels of differential flexure and mirror shift / flop issues simply add and OAG and your problems will be solved - plus you will have a second scope for some visual - before you image! Don't try looking in the sceond scope when you're imaging - you are bound to touch the scope's eyepiece and ruin your shot!

[RD400C (Garth)]

Thanks Matthew, I think I am getting the general hint from the experienced members that OAG or OnAG is the better way to go, regardless of experience level. I am open to this path also, as there are a lot of hours that you and others have put in to arrive at this conclusion, I have had a look at some of the fantastic images, so I think that I would be foolish to ignore this. Cheers Garth

[lazjen (Chris)]

Beginner here - based on the info/advice here I went the OAG route - don't regret it at all as it's kept setup relatively simple and reduced the set of errors/issues/problems that I would have to deal with.

[Logieberra (Logan)]

I lost 10+ kg off my imaging setup when I went with OAG. OAG just makes sense, on so many fronts

[RD400C (Garth)]

Thanks All,
Now I will explore the options for a quality OAG and CCD set up. Off to the money tree in the back yard to see what I can shake out

[g__day (Matthew)]

Lumicon aren't bad OAG -several have focal reducers if you want faster imaging