Question regarding focus range with GSI OAG.

[ollypenrice]

One of my robotic clients has a Baby Q working at just over F5 and a QSI583 with OAG. He has a mixed set of filters, so Baader LRGB and Astrodon Ha. The OAG is scope-side of the filters. He's finding that he can't guide with the Astrodon filter selected in the imaging camera. My guess is that there is too much difference in focus position between the Astrodon and the other filters, throwing the guide camera too far out of focus. Can anyone confirm that this can be a problem or should we be looking elsewhere? I've never used a QSI myself.

Thanks,

Olly

[swag72]

I've got a mixed bag of filters in my QSI...... Baader RGB, Hutech IDAS for luminance and Astrodon narrowband. I can see the difference in focus when I change over filters but it's not so bad that it can't guide. I *think* I probably focussed the OAG with the IDAS filter and was perhaps lucky that the focus isn't too far out for the other filters.

I would think the difference in the focus point is the issue..... should be easy enough to get a middle focus point on the OAG that will satisfy PHD.

[vlaiv]

At F/5 critical focus zone is about 60um (or 1/16th of a millimeter), so no doubt that different filters will have different focus, filters add about 1/3rd of their thickness to the optical light path, so difference in thickness of more than 1/5 mm will throw the focus off.

Having said that, I've found that PHD2 is very resilient to poor OAG focus. My OAG is "focused" by manually moving T2 connector up/down the stalk (sliding motion so it can't be precise) and I've never had any issues with guiding, even at F/4.5. To add to that, stars at the edge of the field are very distorted, but centroid algorithm seems not to care (it just needs to calculate "center" position of whatever star shape and monitor it's movement frame to frame, so it does not care if it is true star center). Slight guide star defocus can help with seeing, so it is sometimes recommended for brighter guide stars.

[steppenwolf]

I have the 683 and a mixed bag of filters including Astrodon, Baader, Astronomik and an IDAS LP filter. I calibrated focus between guide and imaging camera using the IDAS and have never had any guide focus issue,

[Davey-T]

I use my QSI683 with integrated filter wheel with Baader LRGB and Astrodon NB filters focussing is different for imaging but not enough to affect the OAG indeed PHD guides on some very strange looking stars.

Dave

[ollypenrice]

OK, thanks folks. That's a big help. It looks as if the problem will be lying elsewhere. We'll take a look.

Olly

[Jkulin]

Can I be thick?

I have just switched over to OAG guiding and as my prism is in front of the filters, the type of filter has no affect on the guiding, the focus changes a tiny bit but that's it.

I was using my G2-8300 MkII with a Celestron OAG and that was working fine, but should have back my new Moravian MKII OAG that again goes in front of the filters, does the QSI go before them then?

[Davey-T]

Yes the QSI prism is in front of the filters but the whole focus point moves slightly when focusing different filters so moves the prism at the same time, not enough to affect the guide camera focus.

Dave

[Jkulin]

Thanks Dave, so yes you get a slight change in focus, but I've found PHD2 copes with it admirably, in fact it almost helps, if it was out much, then I would pause and refocus the OAG a touch.