Filter thickness and OAGs

[opticalpath]

I have just added an Astrodon Ha filter to the (Astronomik) RGB filters in my QSI camera. I knew that the thicker Astrodon filter would mean a change of focus position from the RGBs, but no big deal to adjust that as required. What I hadn't considered, though, was how big the shift is and that the focus would also shift on my OAG camera ..... which is NOT convenient to refocus back and forth! The difference in focus is such that when I refocus for the Ha filter, OAG stars become very bloated with dark central shadows. It's still possible to guide on those out of focus blobs but might not be so easy with fainter guide stars. I think my best option will be to adjust the OAG focus to a compromise defocused position between the two extremes.

It should work out OK but not ideal, and something I just had not thought of when I added the different brand filter. May have to start saving for the Astrodon RGBs too

Adrian

[swag72]

Oh my Adrian - What a total pain in the rear end. We sort out one problem or make a small change and a whole host of other issues raise their ugly head. Hope you get it sorted.

[opticalpath]

I don't think it's going to be a show-stopper, Sara .... at least I hope not.  It seems quite feasible to guide on somewhat out-of-focus stars.  In fact it's sometimes recommended since the fuzzier target produces a steadier calculated centroid to guide on, so I'm hoping that a midway defocused setup of the OAG will work for both focus extremes.  If not, I guess I'll have to raid the bank for the full set of Astrodons. 

The new Ha filter (I opted for the 5nm) is great - a huge improvement over my old one so I'll be sticking with that whatever happens! 

Adrian

[ollypenrice]

Pain! The other thing about the Astrodon is that it is almost impossible to focus on anything but a bright star.

I think you should be able to devise a quick refocus guide for your OAG camera. I can't remember how it focuses on the QSI but couldn't you have a couple of depth-feeler guages to hand for a quick refocus? Given the difficulty of focusing the Astrodon I doubt you'll want to do much filter chopping in one night. While you can be out of focus for guiding for the reasons you say, the problem is that OAG stars can be faint and if way out of focus they get fainter. With holes in the middle I think you'd be done for.

If it's of any interest I don't find a radical focus shift between the Baader LRGB set and the Astrodon 3. They are not parfocal but the problem (for me) is nothing like yours. What does Sara find?

Olly

[nmoushon]

Is it possible with your setup to move the OAG infront of the filters? I know some set ups have this ability and some dont.

[opticalpath]

Fortunately the problem is not as bad as that, Olly.  In the QSI camera, the integral OAG pick-off is in front of the internal filter wheel, so the OAG gets the full unfiltered light, undiminished by the NB filter downstream.  Last night trying out the new filter for the first time, I managed to guide just as well as usual on a very out-of-focus star .... I mean really bloated!  So I think I'm going to be OK if I just set the OAG focus permanently at a midway point - a little out of focus for both Astronomik and Astrodon filters .... but not as bad as last night.
 
The Astronomiks are 1mm thick compared to 3mm for the Astrodon; that's why it's such a big focus shift.
 
If I really did have to adjust the OAG camera focus constantly, that would be a pain!  It's a threaded adjustment with a locking grub screw, but since I made a nice Arduino stepper motor drive for the focuser that I can operate remotely (bliss!), I wouldn't want to have to make that sort of manual adjustment all the time.  I hope it won't come to that.
 
Adrian