Is a 90mm guidescope good enough?

John K · #1 23-09-2007, 07:35 PM

Just wanting to get some advice from the experienced deep sky imaging people on the forum.

Will a 90 mm Mak Cass guidescope with a focal length of 1250mm be good enough to do deep sky imaging "manually/visually" with a 12.5" f/5 scope with a focal length of 1650mm? I have a 12.5" illuminated reticle and 2 x barlow.

I have been trying an off axis guider which is not bad but wanting to see if a guidescope is better than an off axis guider. Will images be brighter in the 90mm guidescope compared to the off axis guider? (i.e. 10% of the light of a 12.5" is equivelant 2.5" right?)

Also plan to go auto guider within 6 months.

If you were in my position would you save your money and go with an autoguider and keep the off axis guider or go to a guidescope as it will be eaier in the long run?

Weight limitations mean a 90mm guidescope is the max I can go with and even that is pushing it.

Look forward to advice.

John K.

h0ughy (David) · #2 23-09-2007, 07:53 PM

overkill the guidescope is f13 or thereabouts and the main scope is f5, you should have well guided shots - mind you you will need a good guide camera and good tracking

[1ponders] (Paul) · #3 23-09-2007, 08:38 PM

Careful of using a Mak/Cass, it will be a fine guidescope as long as it doesn't have too much mirror shift . I tried to use an OAG for a while, but then sanity prevailed

and I went for the guidescope.

But the time you get to autoguiding you could do it with an ED80 if yo wanted to

John K · #4 23-09-2007, 08:43 PM

Thanks guys, that unit I am looking at due to weight is a Sky Watcher 3.5"

http://www.myastroshop.com.au/products/details.asp?id=MAS-252

This unit does not appear to have a mirror lock. Can a mirror lock be fitter to scopes like this in any way after purchase?

Dennis · #5 23-09-2007, 08:51 PM

Might be worth checking the size of the FOV of the F14 Mak Cass, as ideally you would like a nice, wide FOV to choose a suitable guide star, without the need to fiddle with the rings that hold the guide scope.

I personally would avoid any guide scope that has a moveable mirror, as potentially it is another source of flexure in the overall system.

Cheers

Dennis

[1ponders] (Paul) · #6 23-09-2007, 08:54 PM

What mount are you working with John?

John K · #7 23-09-2007, 08:59 PM

Quote:

Originally Posted by [1ponders]

What mount are you working with John?

It's on a split ring horse shoe mount with a 36" RA drive.

http://www.flickr.com/photos/johnkaz...7594183210993/

John K · #8 23-09-2007, 09:00 PM

Quote:

Originally Posted by Dennis

Might be worth checking the size of the FOV of the F14 Mak Cass, as ideally you would like a nice, wide FOV to choose a suitable guide star, without the need to fiddle with the rings that hold the guide scope.

I personally would avoid any guide scope that has a moveable mirror, as potentially it is another source of flexure in the overall system.

Cheers

Dennis

Thanks Dennis, great advice, and getting realy concerned about the mirror shift issue!

[1ponders] (Paul) · #9 23-09-2007, 09:26 PM

If you are concerned with the weight issue you may want to have a look at Tornado33's custom built OAG. It certainly seems to do the job. I think you'd want to mount a fairly substantial refractor for your setup, though if you could mount it more towards the lower end of the newt that may help alleviate some of your weight concerns.

Alchemy (Clive) · #10 23-09-2007, 09:39 PM

with todays autoguiding programs you dont need a log focal length guidescope, 600mm is fine. try keeping f ratio down as it will give you more opportunitys for guidestars, i tried guiding with f11 and found it a pain. went for ed80

ballaratdragons (Ken) · #11 23-09-2007, 09:44 PM

I concur. A short-tube 80mm Refractor is great. With a Focal reducer is even better.

It doesn't need to be an ED or an Apo. An achro is fine for guiding as the autoguiding programs follow a centroid, not a nicely focussed crisp star. Even slightly out of focus stars work fine.

mostschaedel (Gerald) · #12 24-09-2007, 09:53 PM

Hi John!
Yes, you can use a 90mm guidescope.
I am doing this with an ETX90 with a STV guider connected. I guide a C14 with 2700mm focal length with a Caon 10D DSLR on it.
Works nice as long you have a bright guiding star within 0-3 degree off the object you want to expose.

To see results have a look at:
http://www.werbeagentur.org/oldwexi/...y_deepsky.html

I stopped the off axis guiding as i could very seldom find a guiding star.

John K · #13 25-09-2007, 09:01 AM

Quote:

Originally Posted by mostschaedel

Hi John!
Yes, you can use a 90mm guidescope.
I am doing this with an ETX90 with a STV guider connected. I guide a C14 with 2700mm focal length with a Caon 10D DSLR on it.
Works nice as long you have a bright guiding star within 0-3 degree off the object you want to expose.

To see results have a look at:
http://www.werbeagentur.org/oldwexi/...y_deepsky.html

I stopped the off axis guiding as i could very seldom find a guiding star.

Thanks Gerald. That's some very fine images you have taken and the level I would like to try and get to one day.

You have not found any problems with not having a mirror lock on the ETX90? or does this scope have a mirror lock?

mostschaedel (Gerald) · #14 25-09-2007, 09:34 AM

Hi John!
The ETX90 which i use does not have a mirrorlock. But thats not a real problem. The mirror, if it moves, moves once during meridian passage, so usually i am doing many 6 minute exposures - in 3 hours i get 30 images and maybe 2 of them are bad because of mirror movement.
The mirror movement is only a problem for the focus on the C14 and on the ETX90. After meridian passage i check and readjust the focus of both telescopes. This are the only "non mirror lock" issues i have.

So, i dont see the missing mirror lock as a real showstopper.