Off Axis Guiding

[halli]

Hi

I am considering getting a OAG and using my ASI 120MC colour camera as the guide camera.

Has anyone any experience of this combination - will I be ok in finding guide stars ?

Thanks in anticipation

Ian

[james_screech]

Not quite the same combination but I've used an ASI120MM, C8 and OAG and have never had a problem finding a guide star. The ASI120MC won't be as sensitive due to the Bayer matrix but I would think it would still work.

James

[halli]

Thanks for the response James - I might well give it a go !  

Its a toss up between buying some guide rings for a new scope to re-use a ST80 as a guide scope or buy a OAG.  The OAG will save me having to share the ST80 between two scopes.    

Looks like there is not much difference in the cost between the OAG and the guide rings.

Cheers

Ian

[ollypenrice]

Don't buy any guide rings. They are not only useless, they are worse than useless. If you use an ST80 guide scope you'll find guide stars, simple as that. Well, in ten years I've never failed to find several. The guide rings introduce flexure, especially on short bodied, fast F ratio guide scopes. You don't need them and they are not rigid. How many reasons do you need for not buying them??

lly 

[halli]

Thanks for your response Olly.   I understand your misgivings about guide rings !

I have recently bought a GSO RC8 scope and am looking at setting up the guiding system.  The scope has Losmandy plates top and bottom so i am looking at fixing the ST80 to an inverted Losmandy plate.    

I have only been able to find guide rings with Losmandy clamps to do this as  a standard offering.

 I currently use a rigid system for the ST80 without guide rings on my 130 pds which works quite well and have never not been able to see a guide star.   The ST80 is bolted to a vixen dovetail and then to the tube rings on my 130 pds.

The only reason I was considering the guide rings was because they seemed easy to attach to the losmandy plate not because I felt I needed the adjustment they offered.  !  

I am tempted to use an OAG because of the long focal length of the RC8 which is F8 so the guiding would be more accurate however I have no experience in using OAGs - hence the question about guide star availability.    

The alternative is to share the ST80 between  the 130pds and the RC8 but I need to work out how to fix the ST80 to the Losmandy plate.  I guess i can drill a few more holes to bolt the ST80 tube rings to it but was looking for an easy way out !

Further opinions and comments welcome !

Cheers

Ian

[r3i]

Off axis guiding is the way to go with your RC. I've been successfully using one with mine for a few years now.

[halli]

Thanks Mike I think I will take the plunge.  Astromarket do an OAG for 109 Euros !

[halli]

Incidentally do you use a focal reducer with your RC ,  Mike  ?

[r3i]

Incidentally do you use a focal reducer with your RC ,  Mike  ?

Yes I use the Astro Physics CCDT67, which is a popular choice for these scopes.

[swag72]

I used an OAG with my 8" RC and also a C9.25. I never failed to find a guide star. But I am using a Lodestar, which is pretty sensitive..... I have no idea how sensitive your camera is in comparison.

I think OAG is the way to go at longer focal lengths. I guess worse case scenario is that you need to replace your guide cam..... Best case scenario is that you don't!

[r3i]

I think OAG is the way to go at longer focal lengths. I guess worse case scenario is that you need to replace your guide cam..... Best case scenario is that you don't!

I've been using my trusty old Meade DSI 1 colour as my guide cam - probably not the most sensitive instrument in the world, but I've always managed to find a guide star with it, though admittedly I've had to rotate the pick off prism from time to time to locate one when the target object is not in the Milky Way.

[halli]

Ok i'm convinced and have ordered an OAG from Astromarket - I think the ASI 120MC is quite sensitve

Thanks  Ian

[ollypenrice]

Good decision. Far better to use an OAG in these circumstances. Both the focal length and the possibility of mirror movement say so.

Olly

[Tinker1947]

Don't buy any guide rings. They are not only useless, they are worse than useless. If you use an ST80 guide scope you'll find guide stars, simple as that. Well, in ten years I've never failed to find several. The guide rings introduce flexure, especially on short bodied, fast F ratio guide scopes. You don't need them and they are not rigid. How many reasons do you need for not buying them??

lly 

Is this a old kit list.....

http://www.astrophotography.ie/equipment.htm