Guidescope choice - I don't understand the logic of the masses

rogerg (Roger) · #1 23-04-2007, 03:23 PM

I have a friend who is considering a guidescope for her 12" LX200. It appears the general consensus amongst the masses is that an Orion 80mm ED is the best choice.

I don't understand that choice for several reasons:

Only 600mm focal length vs minimum 1900mm of the LX200.
No focus lock (only focus tension, which from my use of an Orion 80mm ED is not enough to hold a SBIG, large eyepiece, or similar.
The Orion costs about $675 when a 70x700, 70x900 or 9x900 Skywatcher costs significantly less.
The Orion weighs more than a longer focal length 70x700 or 70x900.
A wide field refractors just doesn't make sense to me, for guiding. If choosing purely for guiding why not long focal length? F/10 or F/12 won't matter with a SIBG or long exp modified ToUCam for guiding.

Skywatchers have longer focal length, focus lock and are lighter.

The choice seems plain to me. You don't need the optical quality of the ED (who cares about false colour in a guide image) and you do need long focal length and light weight.

Yet people get the Orion ED. Is that just because they want to photograph through it also and are happy to sacrifice guiding accuracy for that?

Thanks,
Roger.

bluescope · #2 23-04-2007, 03:39 PM

I would think that it is the dual purpose aspect of it that appeals to people Roger. However being a Newt imager I am heading down the OAG path.

[1ponders] (Paul) · #3 23-04-2007, 03:42 PM

Your right Roger, the longer focal length is the norm, but with the Orion you get a great widefield scope to boot. You can increase the carrying capacity of the focuser (I've never had a problem with mine with any cameras even pointing vertically) by doing this MOD

The old rule of thumb for guiding was to have a guidescope focal length at least half as long again as your imaging scope. But this was for visual/manual guiding. With autoguiding and subpixel guiding, theoretically I can guide my 2000mm fl SCT with a 400mm telephoto lense. (Well if my mount is working to the best of it's abilities and the seeing isn't too jumpy

)

I find guiding the 8"LX200 ota not a drama with the Orion. Even at f/10 which isn't very often, except on really bright objects.

btw you can always whack a barlow or powermate into the Orion if it is a problem.

Dennis · #4 23-04-2007, 04:13 PM

I think that if all that you require is a pure guide scope, then you are spot on Roger.

However, if you also want it to perform a multi-purpose role such as a wide field visual 'scope, a bird spotting 'scope and a fast f ratio DSLR imaging 'scope, then the ED 80's come into their own.

Cheers

Dennis

[1ponders] (Paul) · #5 23-04-2007, 04:17 PM

Quote:

Originally Posted by Dennis

DSLR imaging 'scope

You're not wrong there Dennis, especially for those just starting out on this slippery slope, a match made in heaven.

rogerg (Roger) · #6 23-04-2007, 04:38 PM

Agreed on all accounts.

In this case the Orion is barely a scratch on the wide field refractor this person already has (but is too heavy to mount as a guidescope on the LX), so I suspect wide field photography is not a consideration.

74tuc · #7 23-04-2007, 10:10 PM

Hi all,

About the selection of the focal length of a guide scope.

There seems to be a rule of thumb that states that the guide scope focal length should be a third to a half of the main scope. Can anyone point me to any link that actually shows how this rule of thumb was arrived at.

Jerry.

iceman (Mike) · #8 24-04-2007, 06:12 AM

Most people I see tend to swap the roles, and use the ED80 for widefield imaging and guide through the LX200!

jase (Jason) · #9 24-04-2007, 02:05 PM

Quote:

Originally Posted by 74tuc

Hi all,

About the selection of the focal length of a guide scope.

There seems to be a rule of thumb that states that the guide scope focal length should be a third to a half of the main scope. Can anyone point me to any link that actually shows how this rule of thumb was arrived at.

Jerry.

Understand where you're coming from Jerry.

There is quite a lot of conflicting information out there. Seriously, everyone provides a guide because in reality it is not possible to set a fixed rule. There are simply too many combinations - different telescope focal lengths, illuminated recticle eyepeice magnification, guider pixel sizes... it goes on and on.

What people don't take into consideration is seeing conditions, guiding tolarances and mount accuracy.

Perhaps its time for a reality check people. I refer to the following link - http://www.aozc64.dsl.pipex.com/astro/guiding.htm