Do fast f-ratios complicate OAG guiding?

[Jessun]

I'm sat here with a brand new and excellent looking FLO OVL OAG. (Hehe we better know our acronyms...)

It will soon end up behind a SW reducer on my on my ED80 and it will be my first venture into using an OAG.

At some point the SW may well be replace by something faster - and that's what brings this question to mind, as put in the title.

I'm thinking reflexes, shadows due the steeper angles of the light cone, and possibly star deformaties on the guide chip. Or is it in fact the opposite - that you're simply more likely to pick up faint stars?

It's just out of curiosity really at this stage. Any input welcome!

Cheers,

/Jesper

[MartinB]

If your scope doesn't have a flat field then you may be using distorted stars to guide. Most guiding software calculate a centroid for guiding so distorted stars with work ok within reason. I sometimes use an SX active optics unit. This needs a very bright guide star for it to be effective and I have sometimes had to go way off axis to find one. I've successfully used some very wonky stars.

[Freddie]

I use OAG on my MN190 (F5.3) no problems. Guide cam is a lodestar and I always have a handful of guide stars to choose from.

[steppenwolf]

I don't think this will be a problem - as Martin says, modern guiding software is pretty resilient when it comes to misshapen guide stars and you should be able to keep the prism outside the light cone falling on the imaging camera's sensor.

[orion69]

Actually, guiding should be easier because guiding star will be brighter, star shape doesn't matter much.

[Jessun]

Thanks everyone for your input. Good to know, for some future day.

But one step at a time with this OAG malarky, got work to do now getting spacing sorted for the OVL with a filter wheel in the train as well that may look familiar to someone here .

/Jesper

[sloz1664]

Hi jesper,

Setting up your OAG can be very tricky, getting the spacings right for both the Lodestar and your M25C.

Setting it all up during daylight and focusing on a distant object will save you a lot of frustration trying to set it up

prior to imaging and if you are using the OAG on various combinations ensure you take note of all your settings

otherwise you could have a frustrating nights imaging.

Wouldn't be without my OAG now, much better guiding. All the best.

Steve

[Jessun]

Thanks Steve!

/Jesper