Matching guidescope FL to main scope FL

[LewisM]

Is it wise to have the GS FL as near the main scope FL?

For instance, with my f/4 Pentax, would an f/3 guidescope (converted finder) be a good match? With my f/8 main, would an old f/9 converted Toya 50mm refractor be suitable?

I have use the f/9 GS on an f/6.3 main before with great results as well as the f/3 GS on the same scope. The f/4 Pentax surely should use the short FL GS?

[mithrandir]

Lewis, it's the focal lengths and pixels sizes that matter. Aim for the ASP (arcsec per pixel) of the guide camera to be no more than 10 times that of the imaging camera.
EG:
Imaging - Celestron C8+Atik 314L+ -> 0.65 ASP
Guiding1 - Orion ST80+ASI120MM -> 1.93 ASP
Ratio: 2.969
Guiding2 - Orion ST80+Atik 16ic -> 3.18 ASP
Ratio: 4.892

Ron Wodaski's CCD Calculator (http://www.newastro.com/book_new/camera_app.html) makes doing the math simple.

[Paul Haese]

Sub pixel guiding rules out a lot of the old rules regarding guiding.

That said, you should where possible use an OAG for guiding. Flexure will always be present and will limit the length of your subs.

[Merlin66]

I agree with the rule of thumb of guiding being within x10 the imaging scale (arc sec/ pixel).
PHD2 and other guide software can do an excellent job at sub pixel guiding.
The usual caveats of flexure and slop in the guide system still apply.

[bert]

I think you guys maybe looking at guiding from the wrong perspective. Good guiding is good guiding at any focal length. Bad guiding can sometimes be hidden by imaging at a shorter focal length, so why not just go for good guiding?

[Shiraz]

I'm with Brett on this one - angular guiding errors are independent of imager focal length, so they just need to be good enough to hide below the seeing.

for example, 3.75 micron guide pixels (ASI120/QHY5L2) with a 180mm fl guidescope yield 4 arcsec pixels. With 10x sub pixel resolution in the guide software you can resolve 0.4 arcsec errors, which with 2 arc sec seeing and a good mount, should keep stars within about 10% of the seeing limited FWHM with any imaging system.

[Merlin66]

The 10:1 rule seems to work......
But, getting those 0.4 arc sec error signals to the mount and then getting the mount to make the corrections is an interesting engineering/ electronic challenge.

[Shiraz]

ah yes, but that is a different question :)

[mymoon]

Quite a lot of Urban legends on autoguiding floating around , some created by SBIG themselves.

Read here (http://www.sbig.com/site/assets/files/1359/autoguidingversion3_mb.pdf)

Go to page 5 if in a hurry.

Share your experience.

Cheers