Out of focus some times, on some colours

[rogerg]

Getting back in to astro photography after a bit of a break, I'm using my ST7 + CFW8 with a new adaptor that lets the optics of the F/6.3 reducer fit inside the 2" draw tube. So my optical train has changed since last I used it.

Something bizzar is hapenning that I can't explain...

I focus using FocusMax with the clear filter (no filter), then take a colour series. Clear is always in focus, the others - Red, Green and Blue, vary from shot to shot (even for the one colour) being in focus or slightly out of focus to different degrees.

I really can't pick why the focus is changing slightly. The robofocus sure isn't moving, neither is the NGFC slipping from what I can tell (and that adds up considering the clear shots are in focus). But why the colours would go in and out of focus.... I'm confused...

Ideas?

[Gama]

OK, first, i'll assume that the filters are not Parfocal. You need to get parfocal filters, as they all will focus at the same point after the filter. They do this by either layers or thickness of the glass. This is why you buy them in sets.
Secondly, i'll assume again that by "clear filter (no filter)" means there is no filter and just the ccd in line with the light path. Again, the light focuses at different points, and because there is air in the path, it will focus at a different point as it will when there is glass in its light path. So by placing any glass, clear, colored etc, dont make a hoot what type, even plastic, it will refract (Bend) light differently with each wavelength focusing at different points thru different glass. I could go on and on, oh wait i did. thats enough.. !
One last thing, make sure your filters have I.R block, as this will also give you blurred images.

[rogerg]

I think you have found my problem..

The filter set is all parfocal (astronomik LRGB set) and I have had good success with it a couple of months back, but I'm using clear where as before (a couple of months ago when I was doing LRGB imaging) I was using the L instead of Clear.

The rest of the variations might be explained by viewing once I get it in focus for the L instead of Clear.

Thanks

One question - IR blocking... I don't understand why that is required? One of the things I've liked about the ST7 is it's sensitivity in the IR....

[iceman]

I've got the Astronomik LRGB set as well, but find that each channel still focuses at a different point, even though they claim to be parfocal.

I believe it's due to the atmospheric dispertion, and the different wavelengths are being scattered by different amounts and so they'll focus at different points.

It appears to be more noticable at long focal lengths for planetary imaging, perhaps less noticeable at shorter focal lengths for deep-space/wide-field imaging.

I always [have to] refocus between each colour channel. Only a very slight amount, but it's definitely noticeable.

[Merlin66]

When a filter or any other flat lens is put in the optical path between the objective and the image, the focus will move by approx 1/3 of the plate thickness.

[bird]

If you're using the Astronomik RGB filters then ad additional IR blocking filter is not necessary, the Astronomik filters will also be blocking IR (and UV).

The clear (no filter) should probably be fitted with an Astronomik visual bandpass filter, I think they call it an L filter. This will help block IR/UV.

You want to block these things because they focus at different points to the visual spectrum, and will make the images more fuzzy.

cheers, Bird

[rogerg]

Interesting.

That's disappointing that you have to re-focus for each colour Mike. I have to admit I didn't do that a couple of months back when I took my images of NGC 1566 and NGC 1232. I'm now interested to take a look carefully and see if there is a difference in new shots and in those old ones.

After Gamma's reply last night I did take 2 of each colour, including L instead of clear (no filter). At a quick glance (sort of had to hurry off to get some sleep) the images all looked much the same with respect to focus, but viewing was quite unstable so there were variations even between shots of the same colour.

I'm working at about 2100mm at the moment, so if there is a problem at longer focal lengths I'm sure I'll notice it.

The L in the set is IR blocked so is probably the one you're meaning Bird.

I have to admit I was thinking I'd be able to gain some more exposure detail by retaining the IR sensitivity on the L/clear channel by not using the filter. Not the case it seems.

Thanks all.