OAG and focusing

[RayD]

I was looking at my guide stars the other day (night) and they seemed a bit larger than normal, and I got to wondering whether the guide camera focus can be drastically affected by which filter I am using, assuming my Ha filter focuses at a different point at which I set the focus on my guide camera?

[newbie alert]

Ha is at a different focus point for sure but..id of thought that the oag prism picks off white light before it either goes through to the filter wheel/ dslr sensor..and ha makes stars smaller and they be red , so not too sure on that..maybe the focuser on the guide cam has changed position?

[RayD]

2 minutes ago, newbie alert said:

Ha is at a different focus point for sure but..id of thought that the oag prism picks off white light before it either goes through to the filter wheel/ dslr sensor..and ha makes stars smaller and they be red , so not too sure on that..maybe the focuser on the guide cam has changed position?

Yes I see what you mean, but my thought is that if I focus my main camera with my Lum filter, and then the OAG at that point, which is, say, 45mm on my focus tube, then I move to my Ha filter and I re-focus to a point of say 45.5 or 46mm, this surely upsets the focus on the OAG?

My brain hurts doing this hobby 

[newbie alert]

I see what you mean..but as I've not used my oag system yet I can't answer that one...

But interested in the answer!

[ollypenrice]

Craig Stark, the author of PHD, says that precise focus for guiding is actually a disadvantage. Software doesn't guide on 'as star' but on a calculated centroid for the chosen star, which is why we can guide to sub pixel accuracy.

So far as I know, nobody refocuses an OAG between filters. I certainly never gave this a thought when using one.

Olly

[RayD]

22 minutes ago, ollypenrice said:

Craig Stark, the author of PHD, says that precise focus for guiding is actually a disadvantage. Software doesn't guide on 'as star' but on a calculated centroid for the chosen star, which is why we can guide to sub pixel accuracy.

So far as I know, nobody refocuses an OAG between filters. I certainly never gave this a thought when using one.

Olly

Thanks Olly.  No I wasn't considering refocussing, I just wondered at what point it could be problematic, or if it would be prudent to measure the offset between the longest and shortest focus point and focus my OAG central of those points.  I haven't found it to be a problem, but the stars seemed a fair big larger than normal, so I assume the Ha offset coupled with very cold weather meant the focus point must have been a fair bit out from where I initially focused the OAG.

[ollypenrice]

9 hours ago, RayD said:

Thanks Olly.  No I wasn't considering refocussing, I just wondered at what point it could be problematic, or if it would be prudent to measure the offset between the longest and shortest focus point and focus my OAG central of those points.  I haven't found it to be a problem, but the stars seemed a fair big larger than normal, so I assume the Ha offset coupled with very cold weather meant the focus point must have been a fair bit out from where I initially focused the OAG.

If using a Crayford, could it be slippage? Counting steps at the focuser shaft only works if the drive doesn't slip. It can be sensitive to temperature as well.

It might just be the seeing as well. It hasn't been good here because of the cold northerly airflow.

Olly

[swag72]

Yes Ray..... when I use my narrowband filters the guide stars are very much larger than when I use LRGB filters. Because the focus point has changed in my system with the different filters, the OAG stars have changed too.

I have never found this to be an issue, thankfully PHD seems equally at home with larger unfocused stars! Does it work?.... if it ain't broke don't fix it, as you can be as sure as eggs are eggs that in trying to get it sorted you will mess up something else ?

[RayD]

3 minutes ago, swag72 said:

Yes Ray..... when I use my narrowband filters the guide stars are very much larger than when I use LRGB filters. Because the focus point has changed in my system with the different filters, the OAG stars have changed too.

Ah ok thanks Sara. I’m not using a Crayford so slippage is out, but I’d never really noticed it before so It must have been the cold pushing the focus point further out than normal. 

As Olly notes, the out of focus stars are not likely to cause an issue, but it’s amazing the new things you notice as you go along in this hobby.