Advice on guide scopes for longer focal lengths

[Sarek]

I've recently been lucky enough to aquire 2 scopes (Starfield 102 and SW Skymax 150) that have longer focal lengths than I've been used to. To this point I've been using a basic guide scope (Astro Essentials 30mm). In conjunction with my SW 72ED/ASI Air this has worked pretty well but I'm wondering if it will still be sufficient for these larger scopes with their longer focal lengths?

I've read some advice that refers to the 1/3rd focal length rule for guide scopes but I'd be interested to hear if folks agree with that and whether there are recommendations for particular guide scopes. 

[AKB]

The first question is really: “how good is your current guiding?”

Assuming that your new scopes are not significantly heavier than your existing one, then your guide performance should be similar.

The second question is: “what image scale will your new scope/camera combination yield?”

As long as your guiding performance is around twice as good as your image scale in arc-seconds per pixel, then all should be well.

Tony

[Sarek]

5 hours ago, AKB said:

The first question is really: “how good is your current guiding?”

Assuming that your new scopes are not significantly heavier than your existing one, then your guide performance should be similar.

The second question is: “what image scale will your new scope/camera combination yield?”

As long as your guiding performance is around twice as good as your image scale in arc-seconds per pixel, then all should be well.

Tony

Current guiding varies quite a lot but on average I'd say it's usually between 0.6 and 1.0 rms

Both scopes are around 5.5 Kgs v 2.0 Kgs with the SW 72ED

If I've used the in-line calculator at Astronomy Tools correctly the the Starfield, combined with the ASI 533 MC Pro  and my 32mm/f4 guidescope would yield a imaging/guiding ratio of 1: 473 and my existing set up 1: 2.78. Changing to a guidescope with a focal length of 200 brings the latter down 1: 2.85, so virtually the same.

Does that suggest the current guiding scope might struggle a bit with the Starfield?

I probably won't be using the Mak for DSO's, just planetary imaging so guiding is probably not needed.

 

Edited December 12, 2023 by Sarek

[WolfieGlos]

I use my Zwo 120mm guide scope with asi 120mm mini for my Starfield 102, on a HEQ5. Works fine for me @Sarek . Guiding does naturally vary, generally I’m around 0.8rms but I’ve had it as low as 0.4 and worst (producing usable results) at 1.8 rms. That’s with 5 or 6 minute subs. 

Generally I’m imaging with an APSC DSLR , but it also worked well with my 585mc - albeit only once with 30s subs.

Looks like we’ve taken a similar upgrade route from a 72ed! Good luck, you’ll love the Starfield 102, it’s a quality scope. I’ve even used it for a bit of planetary with a 2x Barlow on the 585mc. I did think about a Mak 127 sometime next year, so I’d be interested in seeing how you get on with the 150. And no, guiding not needed for planetary. Just very good GoTo with the small FOV. 

Edited December 12, 2023 by WolfieGlos

[Sarek]

44 minutes ago, WolfieGlos said:

I use my Zwo 120mm guide scope with asi 120mm mini for my Starfield 102, on a HEQ5. Works fine for me @Sarek . Guiding does naturally vary, generally I’m around 0.8rms but I’ve had it as low as 0.4 and worst (producing usable results) at 1.8 rms. That’s with 5 or 6 minute subs. 

Generally I’m imaging with an APSC DSLR , but it also worked well with my 585mc - albeit only once with 30s subs.

Looks like we’ve taken a similar upgrade route from a 72ed! Good luck, you’ll love the Starfield 102, it’s a quality scope. I’ve even used it for a bit of planetary with a 2x Barlow on the 585mc. I did think about a Mak 127 sometime next year, so I’d be interested in seeing how you get on with the 150. And no, guiding not needed for planetary. Just very good GoTo with the small FOV. 

Very similar upgrade route indeed! I'll give my current guide scope a go and see how I get on but very encouraging to know you've had success with the Starfield.

I wasn't planning on getting a planetary imaging scope so soon but a second hand option came up on the 150 that was too good to miss. I'll concentrate on the Starfield for now.

 

[bosun21]

2 hours ago, Sarek said:

Very similar upgrade route indeed! I'll give my current guide scope a go and see how I get on but very encouraging to know you've had success with the Starfield.

I wasn't planning on getting a planetary imaging scope so soon but a second hand option came up on the 150 that was too good to miss. I'll concentrate on the Starfield for now.

 

Hope you get a lot of use and pleasure from both the scopes. The 150 served me well when I started planetary imaging. All the best.

[Sarek]

7 hours ago, bosun21 said:

Hope you get a lot of use and pleasure from both the scopes. The 150 served me well when I started planetary imaging. All the best.

Thanks Ian. I'm chuffed to bits with both of them and it was really nice to meet you.

I can't wait to get started with them now!

[ollypenrice]

On 12/12/2023 at 19:14, Sarek said:

Current guiding varies quite a lot but on average I'd say it's usually between 0.6 and 1.0 rms

Both scopes are around 5.5 Kgs v 2.0 Kgs with the SW 72ED

If I've used the in-line calculator at Astronomy Tools correctly the the Starfield, combined with the ASI 533 MC Pro  and my 32mm/f4 guidescope would yield a imaging/guiding ratio of 1: 473 and my existing set up 1: 2.78. Changing to a guidescope with a focal length of 200 brings the latter down 1: 2.85, so virtually the same.

Does that suggest the current guiding scope might struggle a bit with the Starfield?

I probably won't be using the Mak for DSO's, just planetary imaging so guiding is probably not needed.

 

Don't worry about the guidescope / imaging scope ratio. As Tony said earlier, your guide RMS in arcseconds should ideally be no more than half your image scale in the imaging scope. If your guide RMS is 0.6 arcsecs you are good to image at 1.2 arcsecs per pixel. If it's an RMS of 1 arcsec then you're good to guide at 2 arcsecs per pixel.

What is the image scale of your camera in the new refractor? That's what you need to be looking at and comparing it with the guide RMS.

Olly

[Sarek]

6 hours ago, ollypenrice said:

Don't worry about the guidescope / imaging scope ratio. As Tony said earlier, your guide RMS in arcseconds should ideally be no more than half your image scale in the imaging scope. If your guide RMS is 0.6 arcsecs you are good to image at 1.2 arcsecs per pixel. If it's an RMS of 1 arcsec then you're good to guide at 2 arcsecs per pixel.

What is the image scale of your camera in the new refractor? That's what you need to be looking at and comparing it with the guide RMS.

Olly

Thanks.  Camera and new refractor image scale is 1.36. I can occasionally get under 0.68 guiding RMS but its often a bit higher than that - I'd guess a a modal average of 0.8 or 0.9

Maybe its too close to worry about at this stage!

[ollypenrice]

15 hours ago, Sarek said:

Thanks.  Camera and new refractor image scale is 1.36. I can occasionally get under 0.68 guiding RMS but its often a bit higher than that - I'd guess a a modal average of 0.8 or 0.9

Maybe its too close to worry about at this stage!

It will give you a decent result even if not an absolutely optimal one. There are so many other things that come between the imager and the perfect image that this won't be the biggest deal on the table.

I would say that the best regularly attained RMS for the EQ5/EQ6 will be about 0.5 arcsecs. Someone will have got below that, I don't doubt, but 0.5" is very good for these mounts. How much improvement, if any, would come from a longer FL guidescope? Who knows? It won't be much and it might be nothing.

Olly

[Sarek]

Thanks - that's good enough for me.