Dumb question about large achro + narrowband

[g__day]

Hi all,

Just idly pondering and wondered what happens if you take a large aperture achromatic refractor and do narrowband shots through filters into a sensitive mono camera?

We all know achromatics won't focus all primary colours simultaneously, but I wondered if you use filters one at a time that only let red, blue or green light pass - and focus for each colour - does it matter that the other two colours aren't focused if you are simply filtering them out anyway?

Would happen if you took a say 150mm achro and mated it to a sensitive mono camera with good filters on a good mount and then integrated each shot?

How much better will an apo be then an achro if you are focusing one colour at a time through it?

Many thanks,

Matthew

[MattT]

There is this thread from CN to trawl through...http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/5696032/page/12/view/collapsed/sb/5/o/all/fpart/1
Hope it is of some use.
Matt

[Merlin66]

I basically do that when imaging the spectrum produced by a achromat collimating lens...
Yes is the answer.
Each narrowband "image" will end up at slightly different focus point ( depends on the lens design) but will definately be in focus.
(I use a non-rotating Borg helical focuser which can be calibrated within 5 micron. Once you have the various focus settings you can then just dial them up.)
Hope this helps.

[Poita]

Which focuser model is that Ken?

[gregbradley]

I believe that achromats should work fine as APO simply means the various colours focus at the same time. If you focus each channel individually then it should be sharp.

Greg.

[g__day]

Wow Greg thanks - seems like a really cost effective way of doing colour processing on a mono camera using the right filters. Given a 6" achro costs very little - ponder if more folk might be interested to try this!

[PRejto]

Seems to me the luminance might be a problem. Yes, you could add RGB to get a synthetic Lum channel, but it seems to me (in my limited experience) that the best images have very long luminance exposures and even use binned 2x2 for colour. So, yes it seems possible to get an in focus colour image from an achromat, but would it look as good, or as deep, as an APO considering the lack of in focus luminance?

Peter

[Merlin66]

Peter,
It's the Borg/ Hutech #7315...
I use it with a "special" T thread nosepiece on the spectroscopes.
Works 100%

[Lee]

You can do narrowband no problem - ie Ha/O-III etc; I don't think R/G/B is considered narrowband, given you are still talking about 100-150nm band of wavelengths, which are all going to focus differently.

[multiweb]

+1 . as other said you'll have no trouble at all for NB or if you collect colors separately. The only time you'll want an apo is when you shoot with an OSC. My QHY8 with the ED80 for example of Pentax lens had always blue channel blurred. No problem otherwise.

[gregbradley]

The only reservation I would have is that lenses can vary in their performance on various colour channels. So it may be sharp in green but poor in red. Dr Rohr tests lots of scopes and you can see that in his tests. Often scopes will be good with little aberrations in one colour yet lousy in blue for example.

APOs tend to be good in all channels but even then they are often weak in one.

As I recall my FS152 Tak was quite weak in blue so blue ringed stars often occurred. I can't remember now if I did an image where I focused each channel. I think at one stage I did that and I still got some blue ringed stars.

Why?

Because the blue subexposures had much larger stars as the lens did not perform as well in the blue spectrum.

So that is still likely to occur. But I have been advised for example with the Pentax 67 300mm F4 lens that it would be good for NB but would shot chromatic aberration in colour shots.

So it might be a bit of trial and error to find out. NB though are so narrow that it may work fine. No guarantee though.

Greg.

[DavidU]

Here is a couple of narrow band images done with a 120mm achromat at f/5
Quite pleasing really...
http://polarisb.blogspot.com.au/2010_09_01_archive.html