Difference in focal length between imaging scope and guider

Gem (Grant) · #1 28-05-2012, 07:42 AM

Hi all,

I am guiding with 400mm FL and imaging at 1480mm FL. I am want to improve the guiding I am doing. How long does the guider need to be in relation to the main imaging scope? I hear sometimes 1:3 ratio. Is this right? Does any use barlow lens with their guider?

joecool (Mark) · #2 28-05-2012, 09:41 AM

Craig Stark reckons you can guide with 200mm focal length. The software used for guiding can calculate the airy disk of the guide star even if it only falls across a couple of pixels, hence finding the center of that star. More important to you may be to have a bright, wider field of view with a sensitive guide camera so you can always find suitable guide stars with good SNR. (I use a 127mm f/10 SCT with a f/3.3 focal reducer giving 412mm focal length, and a DSI II camera. This gives 0.1sec between updates for guiding. 0.2sec on nights of poor seeing to smooth the seeing out so it is not chasing the seeing).

[1ponders] (Paul) · #3 28-05-2012, 10:23 AM

I'm using a 50mm finder to guide and image @ 1800mm FL with np at all... apart from a little bit of flexure occassionally. I used to guide using a 420mm refractor but found I got better results using the finder as it seemed to eliminate much of the flexure issue.

Gem (Grant) · #4 28-05-2012, 06:46 PM

Quote:

Originally Posted by joecool

More important to you may be to have a bright, wider field of view with a sensitive guide camera so you can always find suitable guide stars with good SNR.

How sensitive do you mean? I use a QHY5. Does that count as sensitive enough?

Thanks all for the replies. I found when I imaged at 400mm and guided at 2350mm my tracking was great. Now I am guiding at 400mm and imaging at 1480mm.. it just isn't as good... many factors I guess.

joecool (Mark) · #5 28-05-2012, 07:02 PM

I've had a QHY5. They're not real good. Very bad actually. Went to the DSi II colour and it's way way better. A friend of mine has the DSI II Pro (monochrome) and it works noticeably better again. Have a look at this http://www.stark-labs.com/craig/arti...oundup_API.pdf
Have a quick look at page 4 - have a look at the Orion (QHY5). Yuk. So much noise.

No flexure with my system. I've got a rail on the top and bottom of the LX200 with SOLID connectors to the 127mm hanging on top and the guide camera is screwed to the back via the screwed in focal reducer. No focuser to flex here either...

mill (Martin) · #6 28-05-2012, 07:43 PM

Quote:

Originally Posted by joecool

I've had a QHY5. They're not real good. Very bad actually. Went to the DSi II colour and it's way way better. A friend of mine has the DSI II Pro (monochrome) and it works noticeably better again. Have a look at this http://www.stark-labs.com/craig/arti...oundup_API.pdf
Have a quick look at page 4 - have a look at the Orion (QHY5). Yuk. So much noise.

No flexure with my system. I've got a rail on the top and bottom of the LX200 with SOLID connectors to the 127mm hanging on top and the guide camera is screwed to the back via the screwed in focal reducer. No focuser to flex here either...

When you have a dark frame with the QHY5 then there is not much noise at all.

Terry B · #7 28-05-2012, 10:10 PM

My QHY guider has lots of noise but it doesn't much matter for guideing.
I have swapped it for an STi but this is because of sensitivity for spectroscopy work rather than the guider not working well.
I have only used 2 different scopes for guiding and did find the longer focal length one was better. I used a 600mm and a 950mm whilst imaging at 1800mm. The 950mm seemed better but this might also be because it is bigger in diameter so collected more light and hence had brighter star images for the same exposure.