Looking for a guide scope

[Carl]

Hi folks
Forget the off axis guider stuff, I cant see a thing. Everyone suggests a guider scope instead.
I have an 8" SCT on CG5 mount (dont laugh) on a permamant peir in my Observatory. So what is a practcal sized guide scope for that mount and what should i spend (lets be realistic folks, I have a mortgage, kids at Uni and John Howard isn't handing money out in my direction).
Keep in mind, believe it or not, that i found an old Tasco 60mm 700mm f/L refractor in a rubbish bin. The focuser is very solid metal with very little wobble,not like the crap on the current skywatcher scopes and when i cranked up the magnification i got clear separation with Alpha Centuri
Regards
Carl

[[1ponders] (Paul)]

Your Tasco would be fine for auto guiding Carl. If you are manually guiding then your guidescope should ideally be at least 1.5 times the focal length of your imaging scope. If you were to get a 2.5 or 3x barlow (you will need a .925" to 1.25" adapter I suppose.) and a .63 FR for your SCT, you would be fine manually guide with an illuminated reticle. I imagine you will be pushing the weight limit of your mount though.

[allan gould]

1ponders is right. A focal reducer for the 8", a parrallel port or USB adapter from Shoestring astronomy, a webcam and laptop running guidedog,guidemaster or phdguiding will see you there. If not then 2x barlow on your guidescope and you can manually guide. The latter can be a real pain when fatigue sets in or an aching neck and you hit the wrong button on your handpaddle and ruin a 15min-1 hr guided shot.

[DavidH]

Well, don't have a great deal of experience yet, but I'm trying my old Tasco 50 x 600 refractor (35 yrs old!) as a guidescope on my ed80. Seems to work OK, but the ed80 focal length is the same as the Tasco. In your case the focal lengths of the guide scope and imaging scope may have to be more evenly matched by using barlow/focal reducer. I am using a Toucam as the guide camera, and have found that the focuser needs to be wound nearly fully in to focus. This has the added benefit of reducing the focuser play a lot. I bought a Toucam 0.965" adapter for the guidescope from Stephen Mogg, and made a pair of guide rings from ply to mount the guidescope on the ed80.

In short, I believe that the 60 x 700 is certainly worth having a go with.

Regards,
David.

[mark curtis]

I always wondered if that might be a good job for somthing like a Meade ETX60,70,90. God knows there should be a good job somewhere for an ETX.

[jase (Jason)]

May get shot down in flames here, but your off-axis guider (OAG) may still be the optimal way to go for the 8" SCT. The OAG can make it a little more difficult to find a suitable guide star as the pick-off prism location at the field of view edge produces faint and elongated stars. (The latter is actually not a fault of the OAG, it’s the off axis performs of the SCT).

Assuming, the SCT has no moving mirrors I wouldn't hesitate in suggesting a guide scope. However this is not the case. The primary mirror, unless locked down, will move around. If your auto guider is attached to a guide scope and is not on the same optical path as the primary mirror, there is no way of compensating for slight mirror movements.

This is only trade off I see using a guide scope. You may be lucky in that there is no primary mirror movements exhibited - a well greased baffle tube perhaps. In some cases, when you are using narrow band filters a guide scope is the only practical way of guiding.

There has been plenty of discussion on forums regarding guide scope focal length and the importance of aligning this with the main telescope focal length. You can use a shorter focal length for the guide scope, say 80mm. However if your main scope is imaging at 2000mm, your auto guiding through the 80mm objective may not be aggressive enough to ensure round stars at 2000mm. Admittedly, see conditions and a stable equatorial mount do play a key factor at these focal lengths so its unlikely you'd go this high.

This is a case of trail and error, what works for some, may not work for others.

[gbeal]

I use (or did until I got a self guiding camera) an 80mm Nextstar refractor, same as the Short-Tube 80. It has a 400mm focal length, and I use it with the ToUcam, non modified. Scopes that I image with are either 800mm focal length (the Tak), or 1250mm (the newt). At no stage have I found the discrepancy between guidescope focal length and imaging scope focal length to be an issue. One thing I did find however was the need to get decent guidescope rings, and ended up with some DR125 Losmandy ones, well worth the money. This is more the limit of the ToUcam than anything, but worth considering.
Suggestion is to try the 60mm, but be careful with the barlow concept, as it will no doubt make the already dim image more so. Jase may be right about the flexure, and hence the reason for an OAG, so take care with this as well.

[Outbackmanyep]

Originally Posted by Carl

Hi folks
Forget the off axis guider stuff, I cant see a thing. Everyone suggests a guider scope instead.
I have an 8" SCT on CG5 mount (dont laugh) on a permamant peir in my Observatory.
Carl

Have you tried using the radial guider with an f6.3 focal reducer???

I use a radial guider with all my astrophotography at prime focus, (except planets and moon and sun), its usually recommended by the manufacturer to use the "off-axis/radial guider" with a focal reducer to give you pinpoint sharp stars at the edge of the field suitable for guiding!
I also bought an eyepatch from the chemist to use on the eye im not using to help minimise eyestrain during long exposures and help concentrate on the guide-star.