Focusing between different filters?

[kirkster501]

Folks, what's the best way of doing this if you are on an object that does not have a bright star to focus on in the FoV? For example. Lets say we are imaging the North American nebula. Would you focus on Deneb say with Red, grab your lights with Red then slew back to Deneb, refocus with Blue and go back to NA etc etc????

Interested in the workflow. I do not think my blue filter is parfocal. Very, very close. But not quite. Is this usual with blue? I am too new to mono to really know at this stage.

Rgds, Steve

[James]

Where that's happened to me (rarely luckily) I've just slewed to one side (west usually) until I found a bright enough star, focused and slewed back. That way by finishing slewing east I take up any backlash.

A slight difference in focus for blue is not unheard off. I use a set of Baader filters and find that the blue usually needs a very very slight adjustment - it's happened in all the refractors I've imaged through

James

[JamesF]

I know it's said that the reflective Baader RGB filters are supposed to be parfocal whereas the absorbtive ones aren't, but I've noticed my blue subs (with reflective filters) sometimes look a bit off. At the moment I'm cycling the filters for each sub, so I can't really put it down to cloud or anything like that.

In fact I have a set of blue subs of M15 from two nights ago that will not stack in DSS at all, despite the red and green ones stacking without a problem. I don't usually have problems, but this set just won't stack at all. The only other possible reason I've come up with thus far is that perhaps the blue end of the spectrum was suffering more from atmospheric distortion.

James

[orion69]

Filters can be parfocal, but usually problem is in refractor. There is no perfect APO, so there should always be difference in focus points for different colors. Better APO, less difference.

[laser_jock99]

I have a matching set of Baader 2" narrowband filters- they are supposed to be parfocal but I don't think they are either. I usually refocus at filter changes or at the very least check the first sub carefully in case something shifted.

[Freddie]

I assume you are using Artemis for capture. If so, just click on one of the stars when using a short looping exposure. This will bring up the FWHM info that you can use to adjust focus.

[ollypenrice]

It isn't the Baader filters that aren't parfocal, it's the lens that isn't fully corrected. Blue scatters the most and is the hardest to control. Even down at F3.9 the Takahashi Baby Q was parfocal in the Baaders. You do get something or your WHAAAAT DID THAT COST?????

It seems you don't get it in the Borg! But, hey, there is a difference in price.

There is no need for me to slew away to focus usng the short FL refractors because the Artemis FWHM works perfectly. After an hour or so I can use FWHM on any old small star and refocus and restart. If using FWHM you should NOT slew to a bright star because in the optimal 3 to 4 sec focusing subs anything like a bright star will be saturated. Your star must not reach 65000 counts. Well, that was true till I bought the 3Nm Astrodn Ha and there you could hit Vega with 3 minutes and still barely see the darned thing!! Slewing time...

In the big scope FWHM is useless and a Bahtinov is the only hope. This does need a slew. What a darned nuisance!!

Olly

[JamesF]

In the big scope FWHM is useless and a Bahtinov is the only hope.

Have you worked out why that is, Olly? Naively I'd have expected a long focal length scope to produce stars that worked nicely for FWHM.

James