Guiding telescope size and cameras

[Rob6542 (Rob)]

A few websites I have bounced off in my research recommend a guide scope should be 1/3 the length of the imaging scope as a rule of thumb. I have a 10” (254mm) RC OTA and its focal length is 2000mm therefore going on the 1/3rd rule the length the guide scope I need would have to be 650mm long.

I did read one or two articles that challenged the 1/3rd rule as modern laptops, guiding software and cameras allow you to get away with smaller guide scopes. This makes sense to me, plus it would reduce the additional weight on the mount.

Going on that I am looking at two guide scopes

• Zwo 60/280mm f/4.6 (0.64 kg)
• Sky-watcher 80/400mm f/5 (1.5 kg)

and I am looking at two cameras, all of which are colour as I would use them to do some planetary photography (DSO imaging is done with a DSLR)

• Zwo ASI124MC
• ProStar LP-Guide Auto guiding camera

I therefore have a few questions for those of you gurus.

• Is it okay to use a short guide scope (i.e. disregarding the 1/3rd rule of thumb),
• If question 1. Is ‘yes’, then of the two guide scopes which would be the better option, and lastley
  
• Which of the guide cameras would be better option.

Thanks for you advice much appreciated

[Retrograde (Pete)]

Hi Rob,

As no-one else more knowledgeable has responded yet I thought I'd jump in and ask the obvious question - have you considered going for an off-axis guider?

Imaging through an RC at a focal length of 2000mm might not be well suited to guiding through a guidescope given the risk of differential flexure (no matter what guidescope you choose). Some people make a guidescope work when imaging at long focal lengths but most with setups similar to yours would go the OAG option I believe

It might difficult to know whether the guidescope option will work or not till you try it. Perhaps a small ED/Apo refractor that could also be put to use as a wide-field imaging scope &/or grab & go visual scope might be an option in case the guidescope path doesn't give satisfactory results?

[kencas (Ken)]

Take this with a grain of salt as I'm an absolute newbie, but I was under the impression that a guide scope of nominally 30/120 is only effective up to approximately 1000mm focal length, and up to 50/150mm or 60/240 and longer up to approximately 1500mm focal length. But these would also be dependent upon the pixel pitch of the guide camera, and also at their "best" performance on a completely rigid mount.

For longer focal lengths Off Axis Guiders (OAG) are sourcing their targets through the same image chain as the primary optical image pathway, This means that any movement within the scope (such as SCT mirror slip) can be compensated for, whereas an external rigid guide scope can't "see" any changes to the primary optical path and can't compensate.


But also, an OAG is also using the focal length (give or take a bit) of the primary scope itself, and so a 1500mm SCT would act somewhat like a 1500mm guide scope by diverting a "very small" part of the primary optical path to the guide camera, whereas a guidescope is only around the 150-700mm focal length.


But again, I am only too happy to be corrected by those who know a hell of a lot more about it than I do!!!

[Rob6542 (Rob)]

Originally Posted by Retrograde

Hi Rob,

As no-one else more knowledgeable has responded yet I thought I'd jump in and ask the obvious question - have you considered going for an off-axis guider?

Imaging through an RC at a focal length of 2000mm might not be well suited to guiding through a guidescope given the risk of differential flexure (no matter what guidescope you choose). Some people make a guidescope work when imaging at long focal lengths but most with setups similar to yours would go the OAG option I believe

It might difficult to know whether the guidescope option will work or not till you try it. Perhaps a small ED/Apo refractor that could also be put to use as a wide-field imaging scope &/or grab & go visual scope might be an option in case the guidescope path doesn't give satisfactory results?

Originally Posted by kencas

Take this with a grain of salt as I'm an absolute newbie, but I was under the impression that a guide scope of nominally 30/120 is only effective up to approximately 1000mm focal length, and up to 50/150mm or 60/240 and longer up to approximately 1500mm focal length. But these would also be dependent upon the pixel pitch of the guide camera, and also at their "best" performance on a completely rigid mount.

For longer focal lengths Off Axis Guiders (OAG) are sourcing their targets through the same image chain as the primary optical image pathway, This means that any movement within the scope (such as SCT mirror slip) can be compensated for, whereas an external rigid guide scope can't "see" any changes to the primary optical path and can't compensate.


But also, an OAG is also using the focal length (give or take a bit) of the primary scope itself, and so a 1500mm SCT would act somewhat like a 1500mm guide scope by diverting a "very small" part of the primary optical path to the guide camera, whereas a guidescope is only around the 150-700mm focal length.


But again, I am only too happy to be corrected by those who know a hell of a lot more about it than I do!!!

Thanks for the replies.

Not sure about using a OAG. My thinking is, and I am happy to be corrected. I would have thought using a guide scope of the focal length and aperture I am looking at would give a wider field of stars to lock onto with the guiding software (e.g. PHD), whereas the OAG would only give you the view the OTA is seeing or part there of. Would that be wide enough?
In regard to flexure, my OAT has a Losmandy "D" style (3inch) dovetail plate on the bottom and the top, thus the guide scope would be secured on top of the OTA, would this be rigid enough to reduce flexure



Cheers and thanks

[jwoody (Jeremy)]

Gday Rob
I use an OAG with my RC10 and a QHY5IIL guide camera and always have guide stars.
As long as your guide cam is sensitive enough, the FOV will be fine. Mono would be better than colour.
Jeremy

[Rob6542 (Rob)]

Thanks Jeremy, can I ask what is your OAG.
Ta

[jwoody (Jeremy)]

Gday Rob
I have the QHY medium sized OAG.
http://www.qhyccd.com/index.php?m=co...atid=153&id=39

[doppler (Rick)]

Originally Posted by Rob6542

Thanks for the replies.

In regard to flexure, my OAT has a Losmandy "D" style (3inch) dovetail plate on the bottom and the top, thus the guide scope would be secured on top of the OTA, would this be rigid enough to reduce flexure

Cheers and thanks

Flexure can be anywhere in your system, the slightest movement will put the two cameras out of sinc. When I was using a guide scope I found that the flexure was in the guide scope focuser tube.

[Rob6542 (Rob)]

Thanks, will have a look

[DJT (David)]

Am with Woody on this. I use an OAG with the RC10, very rare that you cant pick up a star with a sensitive guide camera.