In a world of OAGs, why use a guidescope?

[Jbro1985]

When I started guiding, it was recommended that I use and OAG rather than guidescope.  My imaging train now involves ZWO OAG, ZWO EFW followed by the ASI1600mm-c.  The recommendation was made with a view to avoiding flexure.

I have an 8" SCT and an Evo 80ED DS Pro (both with their respective focal reducers).  I achieve back focus on both quite easily and although first time around the OAG was tricky to set up, it isn't once you've done it once.  I've been able to set up the prism so that it's effect on the chip is minimal - no noticeable vignetting in a corner.

I appreciate that there may be circumstances with DSLR photography that make it hard to achieve back focus, but there's some really narrow OAGs about now that make focus easier. 

Anyway, I see some really advanced imagers still using the guidescope setup and it makes me wonder, given that guidescopes are heavier and introduce flexure, if you can use an OAG, why would you use a guidescope?  

[ChrisEll]

My experience is very similar to yours, even down to the same telescopes. I have a Celestron OAG and I've always got better guiding with it than with my Orion 50mm GS. The OAG was very tricky to use at first but you get there in the end. I have books on AP where the authors simply dismiss the OAG... quite odd really.

[Tim]

The only reasons I can think of, are these:

1. Masochism (see No.2)

2. Technophobia/ Luddism (see No.1)

3. Spacing issues on certain optics

4. Needing to guide on the object you are shooting, eg. a comet or asteroid if your mount doesn't have the ability to track them

5. Guiding at extreme focal lengths using a less than optimal sensitivity guide camera, with a small chip imaging camera, which might reduce the chances of finding a guide star.

[johnrt]

mainly because (even by your own admission) they are tricky to set up and if my current guide scope set up works perfectly without flex why would I change it? ?

[Jbro1985]

38 minutes ago, Tim said:

The only reasons I can think of, are these:

1. Masochism (see No.2)

2. Technophobia/ Luddism (see No.1)

3. Spacing issues on certain optics

4. Needing to guide on the object you are shooting, eg. a comet or asteroid if your mount doesn't have the ability to track them

5. Guiding at extreme focal lengths using a less than optimal sensitivity guide camera, with a small chip imaging camera, which might reduce the chances of finding a guide star.

No. 3 I get.  If you can't then you can't.

No. 4 feels sensible.

No. 5 is a bit chicken and egg to me.  If you're at a long focal length, and presumably guiding with a much lower focal length guidescope, the advantages (subject to being able to do d a suitable star) are more pronounced from the OAG by my understanding.

 

1 minute ago, johnrt said:

mainly because (even by your own admission) they are tricky to set up and if my current guide scope set up works perfectly without flex why would I change it? ?

Ha!  Indeed - why do imagers change anything in their setup that works fine as it is... 

If you're setup is lighter, you may see a little benefit guiding I guess even without flexure.

[Zakalwe]

OAGs can be a royal pain in the bum to set up, especially if you are using a focal reducer with a small backfocus and a filterwheel.

OAGs have a tiny FoV and can make finding a suitable guidestar difficult.

The pick-up prism, by definition, has to intrude into the light-cone. This can cause shadowing on the imaging sensor. In a fast imaging rig even more-so.

OAGs come into their own when using a 'scope with a moveable mirror as they can account for mirror shift.

In comparison, a guidescope can be much simpler to setup. For imaging rigs with short focal lengths then a guidescope may be the simplest solution to the problem of guiding. An OAG can be an unnecessary complication.

[ollypenrice]

Because it ain't broke...

Olly

[alexbb]

And it's easier/faster to switch the imaging camera on another scope and use the same separate guiding scope.

But now I don't plan to use my lens for a while, so I use the OAG even on my rather short 130pds and I can also use it on my new small Esprit. Good thing is that the Esprit flattener / SW coma corrector backfocus distances are the same and I can switch the whole bundle. It doesn't involve too much thinking and looks nice.

[Shibby]

Also: price. Originally, my finderguider + webcam setup was far cheaper than OAG + decent guide camera. (The webcam was unlikely to be sensitive enough for an OAG setup).

With my current OAG + optics, I don't have the back-focus

[Jbro1985]

Some interesting points. 

It feels like the main limiting factors are price (webcam sensitivity), back focus, the pain in the ass factor, and shadowing. 

Not much can be done about back focus - it either exists or it doesn't.

So eliminating that from the list of issues there are three and the pain is the ass factor, well I'm not going to lie, it was a pain to set up the OAG, just like the first time we set out mounts up.  I've never had to worry about it since though and I'm not sure it's an extra complication.  I might argue otherwise; only have to keep one scope in focus.  It's not the big deal it's made out to be and anyone that enjoys tinkering won't care.

I haven't myself had much of an issue with shadowing but I know it's out there.  Can it not be processed out via the normal flats though (seems to be for me)?  

Olly makes a good point, but I wonder if you had the money again, what would you do?  

Guidescope still or OAG?

I'm not convinced, unless money is a real problem or you just can't get back focus, there's an advantage of the older guidescope setup in most situations.  Situationally, there'll always be a circumstance that's better suited to guidescope but is that the exception of the rule?

[AKB]

1 hour ago, Jbro1985 said:

I'm not convinced, unless money is a real problem or you just can't get back focus, there's an advantage of the older guidescope setup in most situations.

Sorry, is that a typo?  Should this not read:

"there's NO advantage of the older guidescope setup in most situations.”

[Tim]

When I started imaging with an OAG, the guiding was done manually with a crosshair eyepiece on the OAG, now THAT was a pain!

I use my camera and OAG setup with all my scopes, including any that come for Sky at Night review. Even with a full frame camera and fast optics as f3.8 I haven't had an issue.

Guiding at 2.5m+ used to sometimes require rotating the OAG a bit to find a star, but then I got a Lodestar camera which helped.

I do however own at least three OAG's with a combined cost that would be way more than a decent guidescope. I always reckon that the less you put on a mount the better though.

For narrowband imaging I like to take long exposures, usually 30 minutes, regularly 60 minutes, and sometimes up to 3 hours. This is  no problem even at 2m+ with an OAG, not sure a guidescope would work as well there.

Horses for courses as they say

[DaveS]

There is, of course, another way. Ditch all the guide cam nonsense and go to encoder guiding with a sky model. I've not used a guide cam for 18 montha at least.

[alacant]

OAG all the way for me. Everything is difficult the first time you do it. Think back to polar aligning. Flex is present whether you like it or not and is inversely proportional to the cost of your setup. OAGs deliver, are light and easy to use.

Just my €0.02. HTH and clear skies.

[alacant]

Just now, DaveS said:

encoder guiding with a sky model

Ooo, that sounds nice. Can you post a link?

Cheers.

[DaveS]

It's not cheap .

I think 10Micron mounts can guide on their encoders with a model, @Davey-T has one, he will know. The king of encoder guiding is ASA but their DDM mounts are seriously hazardous to the bank balance. Best bet would be a S/H DDM60 from someone upgrading to the 85.

[Aidan]

As relatively new user of OAG, as mentioned, the biggest problem I had was spacing.  I was right in the absolute limit with my Flattener. The setup and getting it to work was a challenge, but I did so in the end.  When I introduced the flattener everything  got rather difficult and frustrating.

For the time being I’ve gone back to guidescope,  but will revisit OAG sometime soon as my new camera will give me an additional 4mm of backspacing to play with.

[swag72]

I use both an OAG and a guide scope..... The OAG is used on the long focal length scope and works a treat. The guide scope is used on a dual rig where it is not possible to use an OAG and guiding..... despite me having a camera that has a built in OAG I can't use it ..... 

[ollypenrice]

4 hours ago, Jbro1985 said:

 

Olly makes a good point, but I wonder if you had the money again, what would you do?  

Guidescope still or OAG?

 

Same again. What would I be trying to improve? I'm inundated with possible guide stars right across the chip. I don't get any flexure. The only time I've had it was with a wobbly SX OAG turret for which I had to make a supporting strut. I can change from 11000 CCD to 490 CCD without affecting the guider. Quite literally I haven't touched the guidescopes in maybe three or four years. Not focus, not anything. In this game there is one rule which over-rides all others. When it works, leave it alone.

I used an OAG on a scope which needed it. I recommend their use on such scopes. 

Olly

[ollypenrice]

1 hour ago, alacant said:

 Flex is present whether you like it or not and is inversely proportional to the cost of your setup.

It's a safe assumption that some flexure is present, just as it's a safe assumption that some image degradation occurs when the light from the edge of the light cone is passed through a prism the size of the head of a match. The truth is that both systems work and fighting a holy war on the matter is as silly as all other holy wars.

Anyway, let's ask Edwin...

lly

[carastro]

All the reasons above. 

In particular (various quotes from above):

Technophobia/ Luddism (see No.1)

Because it ain't broke...

OAGs have a tiny FoV and can make finding a suitable guidestar difficult.

back focus, the pain in the ass factor (not from personal experience, but have read too many of other people's problems which has put me off even trying).

Lastly, I have multiple scopes which I switch around.

Carole 

 

 

 

 

[Stub Mandrel]

12 hours ago, Jbro1985 said:

if you can use an OAG, why would you use a guidescope?

Because it's cheaper?

[Jbro1985]

Hahaha - no declarations of war are being made here!

I think you have made a great point though - Guidescopes have a clear advantage where interchanging cameras or scopes are concerned (unless back focus is consistent between cams or, like Tim, you've got at least 3 OAGs!). 

I don't have that issue myself; I'm pretty monogamous to my single scope and camera setup...  To my wife's dismay.

 

[Anne S]

My reason for using a finderguider comes under point 3. My SX694 and filterwheel don’t leave me enough space to include an OAG when using a reducer/flattener. I could use one with my 6 inch RC but as my guiding is working fine...

Also some of the time I use a GPCam instead of a guidehead.(My second ccd is an Atik) I suspect it might be difficult to achieve focus and maybe  problem with guidestars as it’s nowhere near as sensitive.

Anne

 

[Stub Mandrel]

I had to use a video compressor yesterday to help my daughter make an assignment small enough to upload.

The compression settings go from 'Low Quality' at one end to 'Placebo Quality' at the other.

I suspect the same applies to guiding; if your accuracy is better than your pixel scale further improvement is illusory...