In a world of OAGs, why use a guidescope?

[IanL]

I've never had any luck with OAGs. I have one cheapo model sitting in my 'astro junk' box and another better one that I acquired (after going down the guidescope route) that the previous owner modified to fit on an EFW2 with the minimum effect possible on backfocus.  Experiences included:

- Hard to focus - could only do so with the prism intruding significantly over the chip.

- On one, prism/camera stalk welding itself to the OAG body (same sort of metal-on-metal problems as with welded adaptors and extensions). Could only be moved with excessive mechanical force damaging the stalk.

- On the other, light leaking in around the stalk aperture due to poor fit (but didn't weld itself - swings and roundabouts, but in both cases not fit for purpose).

- On the rare occasions I felt it was set up right, no star could be found with my guide camera. Now I could have gone out and bought a more sensitive (and much more expensive) guide camera, but it was cheaper to buy and mount an ST80 rather than carry on throwing good money after bad.

Don't get me wrong, this isn't a religious thing. I'd dearly love to get an OAG working so I could use my SCT for small galaxies, but given the paucity of clear nights in the UK, I've concentrated on the ED80 in the interests of actually getting images out of my equipment rather than frustration.

The guidescope does not suffer flexure that is sufficient to have a measurable effect over the course of a typical exposure, of that I'm confident. It's not that difficult to set up a mechanically sound guidescope at 500-800mm focal length, but I accept it would be more likely to be an issue at longer lengths. For me, the PE and other mechanical gremlins of my fairly average NEQ6 are a much more significant issue as far as I can see.

I'm with those who say 'If it ain't broke, don't fix it', having made that mistake a few times already!

[Zakalwe]

Why limit the conversation to OAG vs guidescopes? Why not use an ONAG? No pesky prism intruding into the lightcone. The guiding is done using the IR part of the spectrum, so less susceptible to seeing effects.

Or you could consider adaptive optics. Then the guiding isn't reliant on having the mount to make the corrections, which makes the guiding far more responsive.

 

 

[ChrisEll]

2 hours ago, IanL said:

I've never had any luck with OAGs. I have one cheapo model sitting in my 'astro junk' box

I can see how a duff OAG can put you off for good. Avoid the ones where the camera sits on a stalk with the prism... they will be wobbly. As for sensitivity move the prism as far into the tube as possible for the best illumination so the whole prism is visible from the front - M48 tubes may be needed. Choose one with a large prism. My guide cam is a lowly QHY5-II Mono, is only 50% QE but it works fine with my OAG and provides a 30'x38' FOV.

[laser_jock99]

I use both OAG and guide scopes. OAG on long focal lengths or long subs and guide scope for anything else. Simples.

[Jbro1985]

7 hours ago, Zakalwe said:

Why limit the conversation to OAG vs guidescopes? Why not use an ONAG? No pesky prism intruding into the lightcone. The guiding is done using the IR part of the spectrum, so less susceptible to seeing effects.

Or you could consider adaptive optics. Then the guiding isn't reliant on having the mount to make the corrections, which makes the guiding far more responsive.

 

 

These are both interesting.  I haven't heard much about them myself.  I'm not in the early adopter category (though sometimes I'd like to be) but I'd like to give them both a go once a bit more mainstream. 

I'm glad that news of "it" not being "broke" hasn't reached the companies we rely on to innovate and apply technologies allowing us, should we wish to, alternative ways to maximize our enjoyment of this little hobby of ours  ;-p

[Zakalwe]

6 minutes ago, Jbro1985 said:

I'm glad that news of "it" not being "broke" hasn't reached the companies we rely on to innovate and apply technologies allowing us, should we wish to, alternative ways to maximize our enjoyment of this little hobby of ours  ;-p

It's just horses for courses.

Guidescopes are cheap, dead easy to use and work absolutely fine for the focal lengths that many (most?) DSO imagers use.

OAGs are really the only solution when a moveable mirror is involved or where the focal length gets long.

Both can be made to work in either scenario, but why over complicate things? Efficiency is something to be desired...anyone can make a job overcomplicated.

 

[Jbro1985]

23 minutes ago, Zakalwe said:

It's just horses for courses.

Guidescopes are cheap, dead easy to use and work absolutely fine for the focal lengths that many (most?) DSO imagers use.

OAGs are really the only solution when a moveable mirror is involved or where the focal length gets long.

Both can be made to work in either scenario, but why over complicate things? Efficiency is something to be desired...anyone can make a job overcomplicated.

 

Agree on efficiency being something to be desired.

[Stub Mandrel]

The ideal would be a dichroic mirror allowing full-spectrum imaging and routing either long IR or UV to a suitably sensitive camera.

[IanL]

30 minutes ago, Stub Mandrel said:

The ideal would be a dichroic mirror allowing full-spectrum imaging and routing either long IR or UV to a suitably sensitive camera.

Yes that's what an ONAG does, if you have the £££ to pay for one.

[feilimb]

If you use an OAG on eg. an SCT how would you go about setting it up in guiding software (eg. PHD, Ekos, Linguider) ? Usually one feeds in the focal length and aperature of the guidescope, but if you feed in the focal length and aperature of the SCT (with or without focal reducer) would the software have issues with the fact that the FOV available to the camera is not the same as if it was natively placed at the back of the SCT with those parameters?

If so is there some workaround in PHD etc. to 'tell' the software that you are working with a very restricted FOV?

[IanL]

15 minutes ago, feilimb said:

If you use an OAG on eg. an SCT how would you go about setting it up in guiding software (eg. PHD, Ekos, Linguider) ? Usually one feeds in the focal length and aperature of the guidescope, but if you feed in the focal length and aperature of the SCT (with or without focal reducer) would the software have issues with the fact that the FOV available to the camera is not the same as if it was natively placed at the back of the SCT with those parameters?

If so is there some workaround in PHD etc. to 'tell' the software that you are working with a very restricted FOV?

PHD doesn't care about the field of view.

The calibration process empirically determines the relationship between pixels and guiding pulse durations, plus the orientation of the RA and Dec axes relative to the X and Y axes of the sensor.

Entering the pixel size and focal length enables PHD to:

- Determine the correct calibration step size (in conjunction with the Dec of the calibration star).

- Display graphs and stats in arcseconds rather than pixels.

Aperture has no bearing on PHD setup.

[DaveS]

21 hours ago, Zakalwe said:

Or you could consider adaptive optics. Then the guiding isn't reliant on having the mount to make the corrections, which makes the guiding far more responsive.

Adaptive optics are something I've been thinking about, not for guiding (Encoders do that better than anything else) but for cutting through the seeing. Direct Drive and Encoders will sort out all normal guiding issues, including wind gusts (With 100 Hz feedback, it's very quick) but cannot "see" how a star is moving due to the seeing.

So ... Encoders to do the main guiding and AO to take care of the seeing ...

Possible?

[Stub Mandrel]

2 hours ago, feilimb said:

If you use an OAG on eg. an SCT how would you go about setting it up in guiding software (eg. PHD, Ekos, Linguider) ? Usually one feeds in the focal length and aperature of the guidescope, but if you feed in the focal length and aperature of the SCT (with or without focal reducer) would the software have issues with the fact that the FOV available to the camera is not the same as if it was natively placed at the back of the SCT with those parameters?

If so is there some workaround in PHD etc. to 'tell' the software that you are working with a very restricted FOV?

It only affects the FOV if the OAG doesn't have an image circle big enough to cover the whole sensor. As most guiders have small sensors, I doubt it is an issue.

[JohnSadlerAstro]

Its simple. A guidescope setup looks better. It's more impressive.