Off axis guiding threshold

[StuartT]

I currently don't do any guiding at all. I wanted to master the basics first, so I have been keeping my exposures to < 30sec.

However, I think it might be beneficial to think about guiding, as - even with an excellent camera - 30 sec may be too short for some targets.

My question is, at what FL does differential flexure make OAG a necessity? My main imaging scope is FL=1080mm (or 808mm with reducer). Would it be necessary to use OAG? Or is that more for FL towards 2000mm plus?

Thanks

[Clarkey]

I am using a guide scope at 1600mm which is not causing problems. However it will depend on rig a set up. I also image at 1000mm and less with a scope. I do have an OAG but it was often struggling to find a decent guide star so I went back to the trusty ST80 to guide.

At 800mm both would be OK, especially with a refractor. Personally I find the OAG a right faff!

[StuartT]

3 minutes ago, Clarkey said:

I am using a guide scope at 1600mm which is not causing problems. However it will depend on rig a set up. I also image at 1000mm and less with a scope. I do have an OAG but it was often struggling to find a decent guide star so I went back to the trusty ST80 to guide.

At 800mm both would be OK, especially with a refractor. Personally I find the OAG a right faff!

thank you. V helpful. What guide scope would you recommend? Is there a relationship btw the FL of the imaging scope and the guidescope? 

[Same old newbie alert]

What scope are you using Stuart?

[StuartT]

4 minutes ago, Same old newbie alert said:

What scope are you using Stuart?

Esprit 150 ED APO. At full FL it is 1050mm and with the reducer it is 808mm

[Same old newbie alert]

Reason I asked was because if you was using a SCT with mirror flop issues no doubt I'd recommend a oag... I think you have a choice... I know plenty that use oag with big fracs and swear by them.. don't think it's of any benefit under 100mm aperture thou

[Clarkey]

55 minutes ago, StuartT said:

What guide scope would you recommend? Is there a relationship btw the FL of the imaging scope and the guidescope? 

I use an ST80 with a ASI120mm along with the RC8 and an ASI1600mm. The pixel scale ratio is in the region of 3 to 1 as although the FL is 4 to 1 the 120mm has smaller pixels. I think the general advice is keep it less than a 4 to 1 pixel ratio. For an 800mm most 50 or 60mm guide scopes would be OK. 

Edited February 27, 2022 by Clarkey

[Elp]

A general rule of thumb for guidescope FL is for it to be at least a third to a half of your main scope focal length.

[Clarkey]

4 minutes ago, Elp said:

general rule of thumb for guidescope FL is for it to be at least a third to a half of your main scope focal length

I think half is a bit excessive. I have guided without issue at 1000mm FL with a finder at 180mm and it was fine. I accept it was pushing the boundaries a bit, but it was at about 4 to 1 pixel scale which is not too unreasonable in my opinion.

[Elp]

Just in case user decides to upgrade main scope later. Your experience makes me want to try my 120mm with my Newtonian and see how it guides.

[StuartT]

1 hour ago, Clarkey said:

I use an ST80 with a ASI120mm along with the RC8 and an ASI1600mm. The pixel scale ratio is in the region of 3 to 1 as although the FL is 4 to 1 the 120mm has smaller pixels. I think the general advice is keep it less than a 4 to 1 pixel ratio. For an 800mm most 50 or 60mm guide scopes would be OK. 

ah, so it's actually pixel scale ratio (rather than FL ratio) that counts then? That's useful to know. Thanks.

I plan to use a Meade LPI-G mono camera (which actually has the same pixel size as my main imaging camera - which makes life easier). So I should prob go for a guide scope with FL of 300mm then (for both my main scope FLs)

Thanks so much

[michael8554]

To guide a long-focus OTA at a reasonable image scale ratio, a long-focus guidescope is often required.

And the problem is flex in the relatively long, heavy, guidescope, often mounted in adjustable guide rings, with a guide camera held by lock-screws instead of threaded connections.

We're talking flex of only microns.

Hence the need for OAG's.

You can add SCT mirror flop to the list of Diff Flex generators.

Michael

Edited March 1, 2022 by michael8554

[Clarkey]

43 minutes ago, michael8554 said:

And the problem is flex in the relatively long, heavy, guidescope, often mounted in adjustable guide rings, with a guide camera held by lock-screws instead of threaded connections.

I definitely agree with the need for strong attachment. I use this set up for mine which seems to work well:

Guide Scope Bundle - Suitable for Piggy Backing | First Light Optics

Also gives me a nice grab-and-go scope for camping!

[michael8554]

Yes Clarkey, decent rings and The purpose-made focus extender enables the camera to achieve focus without over-extending the focuser draw-tube. 

Hopefully corrects the ST80's floppy focuser.

Michael

[StuartT]

Ok, thanks for all the advice. I have ordered the Primaluce Lab 60mm guidescope. FL=240mm and nice and bright at f/4

https://www.widescreen-centre.co.uk/primaluce-lab-60mm-guide-scope-with-plus-80mm-guide-rings.html

[rickwayne]

Note that the rules of thumb were developed in the eyeball-guiding days. Since current software guides to sub-pixel resolution, you can be a lot more relaxed and still get good performance.

However, do pay attention to the advice about mounting the guidescope. The amount of differential flexure that will lead to guide problems is absurdly tiny -- after all, we are talking about fractions of an arcsecond here.

[alacant]

On 27/02/2022 at 20:21, StuartT said:

at what FL does differential flexure make OAG a necessity

Hi

I don't think we have a fixed answer, rather the use of an OAG is determined by the combination of mount, camera and telescope you have.

With a refractor at 800mm, I'm sure you're gonna be OK with a separate guide telescope. But, if you're near weight limit on your mount, then go with an OAG. A properly mounted guide telescope wiil add around (at least?) 1.0 kg to your telescope, an OAG around 0.3 kg.

We prefer OAGs for most telescopes; they're lighter and not only give nice stars, they give nice stars for every frame. even the one on the other side of the meridian flip, and even when using an out-of-the-box-low-end Newtonian.

Having had to sort out many problematic guiding setups with separate guide telescopes, most if not all could be improved by mounting the guide telescope adequately. Preferably directly bolted onto a rigid rail joining the top of conventional tube rings.

10 hours ago, rickwayne said:

current software guides to sub-pixel resolution

+1. 

To which we would add that modern astro processing software is also capable of rescuing images which in the past would have been discarded. 

Cheers and HTH

[StuartT]

I have now purchased a guide scope with a helical focuser.

It suddenly occurs to me - what do you do about focussing with guidescopes? My two imaging scopes have motor focusers, but I assume people don't generally attach those to their guidescopes? Do guidescopes drift out of focus over a session? Or is pin sharp focussing of less importance to guiding scopes?

Edited March 5, 2022 by StuartT

[david_taurus83]

I use one with a Redcat lol. Had issues before with a guidescope with some pretty bad flexure. Instead of wasting time trying to sort it I just went OAG as its pretty necessary for me with long subs on my rusty old CCD.

For subs below 300s though, I'll be using a mini guidescope on the Redcat with my DSLR.

 

[vlaiv]

15 minutes ago, StuartT said:

I have now purchased a guide scope with a helical focuser.

It suddenly occurs to me - what do you do about focussing with guidescopes? My two imaging scopes have motor focusers, but I assume people don't generally attach those to their guidescopes? Do guidescopes drift out of focus over a session? Or is pin sharp focussing of less importance to guiding scopes?

Precise focus is really not important for guiding.

Most guide scopes are fast achromats and stars will be bloated anyway. Some defocus even helps with seeing effects. Only time when precise focus is beneficial for guiding is when one is guiding on a single faint star and good focus provides good SNR.

With modern PHD2 and multi star guiding - this is really not issue and it is enough to set decent focus at the beginning of the session. Any focus drift will not have impact on guiding.

[StuartT]

39 minutes ago, vlaiv said:

Precise focus is really not important for guiding.

Most guide scopes are fast achromats and stars will be bloated anyway. Some defocus even helps with seeing effects. Only time when precise focus is beneficial for guiding is when one is guiding on a single faint star and good focus provides good SNR.

With modern PHD2 and multi star guiding - this is really not issue and it is enough to set decent focus at the beginning of the session. Any focus drift will not have impact on guiding.

thanks. Yes indeed, the guidescope I have bought is a f/4 achromat. Useful to know focus isn't a big deal. 

(now I just have to learn PHD2!)