Optimal Imaging to guiding ratio

[neilallison]

I've just used the Guidescope Suitability Calculator on https://astronomy.tools/ to gather information about the imaging setup I currently have.

It says that the imaging / guiding ratio is 1:7.94.

I'm guessing that ratio is too high.

Is there an optimal ratio that I should be aiming for?

[Startrek]

I image with 8” f5 and 10” f5 Newts with 60mm guide scopes ( not OAG ) and my image scale ratios are 4.2:1 and 4.0:1 respectively
By the way my OTA’s are carbon fibre which have minimal flexure and COE
Guiding performance is fine with both rigs
7:1 seems a tad high IMO depending on your rig’s mechanical set up , your mounts performance and local seeing
From my experiences aim for 5:1 or better ( if possible) Some folk have guided successfully with image scale ratios of 10:1 but I wouldn’t subscribe to that scenario
Cheers
Martin

[neilallison]

Thanks Martin
My scope is also a carbon fibre tube (Vixen R200SS) f4 with a Parracorr II on a Losmandy G11 mount.

So do you think I would be best trying a different guide scope or guide camera (or possibly both) ?

[Startrek]

You can evaluate your own image scale ratio with the following formula rather than on line calculators -

A) Main Imaging Telescope Image or Pixel Scale
Main Camera Pixel size in microns x 206.3 / Telescope focal length in mm

B) Guide Scope Image Scale or Pixel Scale
Guide Camera Pixel size in microns x 206.3 / Guide scope focal length in mm

Image Scale Ratio is B divided by A

Either use a longer focal length Guide Scope or smaller camera pixels

You can experiment with various combos to achieve a tighter image scale ratio
Ideally 4:1 is a good number which is what I image at with my f5 longer tube Newts

Cheers
Martin