Is a 90mm guidescope good enough?

[John K]

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]

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]]

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]

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 (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]

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]]

What mount are you working with John?

[John K]

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

http://www.flickr.com/photos/johnkazanas/sets/72157594183210993/

[John K]

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

[[1ponders]]

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]

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]

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]

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/gallery_deepsky.html

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

[John K]

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]

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.