small refractor and other items for imaging. advice required

cheers for the info Dave.

I understand your point, and believe me, if I could afford a CGE I would have one. Also, I dont have my own place ( I rent in Dublin) so i have nowhere to store a giant mount and tripod. the CG5 is the best compromise i can find based on portability, cost and sturdiness.

A dedicated CCD is not an option either due to cost. The Canon will have to do for the time being.


With regards to some DSO's looking small in a small refractor, I can simply swap the guide camera to the refractor, and image through the SCT. although it will increase exposure time, it will give me more magnification.

With regards to periodic error, I thought that autoguiding was designed to remove periodic error. This is the information if have found on the web. I mean, if you have perfect polar allignment, why would you need auto guiding except to account for physical defects in your setup, such as periodic error?

The whole point of this setup is to try my hand at some DSO photography without having to spend thousands and thousands on gear. A 2k camera and a 5k mount would be lovely, but not fesiable for me currently or for the forseeable future.

Hi guys,

PE on a CG5GT is quite large. I think it's in the region of about 30 which is a bit much for a mount that cost €1295.

That said, I can get 5 minute exposures easily when autoguiding but I often have to throw out quite a few frames as bright stars often have a barely noticeable mirror image that I guess is caused by the PE & autoguider dragging the star back to it's proper location.

I wouldn't consider an achro at all as you will have false colour due to the fact that different wavelengths in the visual spectrum will not focus to a single point on your imager.

William-Optics have a nice range of affordable APOs, while not as good as the traditional vendors they are still excellent.

Trev

Fintan - autoguiding will not get rid of PE. It might help a bit but won't eliminate it - particularly if its 30 arcsec as Trevor notes. However, you may still end up with acceptable results but that will likely involve discarding quite a few poor images prior to stacking.

It is not possible to get perfect polar alignment with amateur equipment. Besides, even if you did, objects across the sky still move at differing rates depending on their location. Atmospheric refraction will vary depending on air mass - this leads to tracking errors. Some vendors are trying to minimise this through the use of software but that would be on equipment that is probably out of your fiscal reach.

I'm not so sure that the SCT will lead to exposures longer than the refractor - as well as focal length, you should consider the light gathering power of the SCT compared to the refractor.

As regards the price of good quality equipment for what you want, you might be surprised. The secondhand market regularly pops out good equipment for low prices. The Meade DSI is very reasonably priced, even new. There *are* bargains out there...

If you follow the three steps I pointed out, that should at least result in you knowing what you can afford for a given result...

By the way, it took me the guts of a year to figure out what I needed to purchase to give me the results I need...

Cheers
Dave

cool. thanks for the info guys.

Trev, I got the CG5GoTo for 700 quid from AI to replace my standard CG5.

with regards mounts, whats the minimum mount you guys would recommend that has very little periodic error, or at least is below the threshold that can be autoguided out? Everyone seems to have either a CG5 or equivalent, or else a huge jump up to CGE and other similar models. is there a midrange mount there that fits the bill?

700! Got mine there too but that was something like 18 months ago - that's some price drop! The UK vendors refused to to sell me any Celestron stuff as there was a dealer in Ireland already which is a pity as the local price was the same as buying a mount + ota from the uk :evil:
I guess Skywatcher bringing out a cheaper goto mount has caused some price drop.

Anyway, I'd stick with the mount you have. The better mounts are usually incredibly expensive & very heavy so are better suited to a permanent setup + you may find you don't need a better on at all.

The mounts I would like would be the Losmandy Gm8, Gm11 or Vixen Sphinx II, but that will probably be a couple of years down the line. Gm8 and Sphinx would be considered mid-level.

with regards mounts, whats the minimum mount you guys would recommend that has very little periodic error, or at least is below the threshold that can be autoguided out?

Fintan - you can't guide out periodic error.

I'd stongly you suggest you read Ron Wodaski's book or similar. In order to avoid severe frustration and bad purchases you really need to understand what you what and what is out there that will achieve it. If you can't affor the equipment that will take the images you want, you will need to alter your expectations downwards.

You need to match up the camera and the telsope - bearing in mind the minimum undistorted image your mount will give. If you use Wodaski's calculator, this will help you choose the right telescope.

You will also need to match the SCT and the guide camera you wish to purchase to ensure it will work with the Canon/refractor combo.

I can't stress enough that you really need to do the research before you take the plunge or whatever you buy will soon be in the for sale section...