OAG or separate guide scope?

[opticalpath]

Adrian: ok, so that's interesting - how does this work then?

The Lodestar has an ST-4 output on it as well as a USB connector - I had (perhaps, naively) assumed that meant it could send signals directly to the mount rather than the computer directing the mount- is this not the case then? And if the computer has to send the ST-4 signal to the mount what is the purpose of the ST-4 port on the Lodestar?

New to this so happy to be advised.

John

I think others may have already answered this, John but fwiw I see it this way: an auto-guiding system needs 3 things:

1. a guide camera (and guide scope or OAG) to take successive images of your guide star

2. a software program running that drives the guide camera. Its job is to

  - trigger the successive exposures and download the guide camera images

  - calculate how far the guide star has moved since the previous image and issues corrective impulses.

3. a connection to the mount that the software uses to communicate the corrections.

With the Lodestar, the mini-USB socket must be connected to the PC so the guiding software can do (1) and (2). What can be confusing is the several ways that the guide program can do (3) i.e. connect to the mount in order to send the correction signals. You can cable connect the ST4 socket on the mount to the mini socket on the Lodestar. Alternatively, if your main imaging camera has an autoguider ST4 socket you may be able to cable that to the mount instead (that's what I do). Or if your mount is already connected to the PC with EQMOD/ EQDir or similar, you can use pulse guiding and dispense with the ST4 connections altogether. The point is: it's not the guide camera itself that has to communicate with the mount; it's the guide program running on the PC that has to do it, so the physical data connection can come via another device (imaging camera or EQDir adapter .... or Lodestar) that is connected to the PC and that the guide program can talk through.

...... or did I just make it sound more complicated

 

Adrian

[Physicist13]

Why's that then Olly?  I don't doubt you're right but I am curious as to why!  I swap my OAG off of my SCT and happily on to my FSQ85; it guides just fine and with the shorter focal length I can always find a suitable guide star.  No worries about differential flexure so what's the big advantage of a separate guide scope?

you mean disadvantage?? non-OAG guiding suffers from two main problems (which may or may not matter in any specific application, hence often it doesnt matter).

1: differential flexure between guide scope and imaging scope causes errors, 2) the guide star is usually (not always!) further away from the centre of the imaging FOV which also causes errors. Counter this with some loss of light to imager, backfocus room required ,difficulty in finding a bright enough guidestar.

[x6gas]

you mean disadvantage?? non-OAG guiding suffers from two main problems (which may or may not matter in any specific application, hence often it doesnt matter).

1: differential flexure between guide scope and imaging scope causes errors, 2) the guide star is usually (not always!) further away from the centre of the imaging FOV which also causes errors. Counter this with some loss of light to imager, backfocus room required ,difficulty in finding a bright enough guidestar.

Well quite, but Olly was suggesting an advantage to using a separate guide scope over an OAG for shorter focal length refractors.

To your points: I might have missed something here - in which case apologies - but there is no loss of light to the imaging cam if the pick off prism is out of the way of the main imaging sensor (as it is with most set ups), back focus would be a problem with Newts, but not with 'fracs I don't think?  The final point is a fair cop, but you have a wider field of view to play with on a shorter f/l frac and I've never had a problem finding a guide star using a 500-600mm f/l 'frac.

I don't really understand why OAGs are not more popular!

[kirkster501]

I don't think you have much alternative at that focal length - OAG.  I can get away with guidescope on my RC8 scope.  Just about I think.  What puts me off about OAG is the fiddling with spacers to get the distance between sensor and reducer correct.  It a right, royal PITA and a problem I can do without.

[opticalpath]

I don't think you have much alternative at that focal length - OAG.  I can get away with guidescope on my RC8 scope.  Just about I think.  What puts me off about OAG is the fiddling with spacers to get the distance between sensor and reducer correct.  It a right, royal PITA and a problem I can do without.

I might not have understood the issue you describe, but if you're using a focal reducer, isn't there always some fiddling required to get the correct spacing to the sensor, whether or not there's an OAG in the imaging path? If you added a filter wheel for example, you'd have to change the spacer ... but isn't it just a one time job?

Adrian

[kirkster501]

I might not have understood the issue you describe, but if you're using a focal reducer, isn't there always some fiddling required to get the correct spacing to the sensor, whether or not there's an OAG in the imaging path? If you added a filter wheel for example, you'd have to change the spacer ... but isn't it just a one time job?

Adrian

No, not with the CCDT67 telecompressor.  There is an optimal compression of 0.67.  But if your spacing is not exact then all it will do is effect your reduction ratio, so in that regard I could indeed insert a OAG to pick off some light and it will then put the compression either higher or lower than 0.67 by virtue of the change in spacing.  I use the Guidscope on both my imaging scopes though.  It is also a real PITA to get the FSQ-85 spaced properly with the 0.73 reducer and a OAG without resorting to custom made adapters etc.  Hence me not wanting to go the OAG route.  If it works for you then go for it!

[opticalpath]

The filter wheel and OAG port is built into my camera, so I suppose I'm biased .  I have to regard the camera, filter wheel and OAG as the imaging 'unit' with a fixed back-focus .... which actually simplifies things in a way. 

I can see it could be different matter, though, if I was swapping various elements in and out of the imaging train on more than one scope.

Adrian