Using a non-APO for Mono imaging with Filters....

[RichieJarvis]

Hi All,

I am considering buying a 6" non-APO refractor. Now, my logic tells me that an APO using filters (LRGB or Ha/OIII) shouldn't be a problem with chromatic aberation, but I am wondering whether I am right?

I mean, surely I can refocus slightly between each filter to get round the non-APO-ness?

Does anyone have any experience they can share on this subject??

Thanks,

Richie

[Xlracer]

Richie

Monochromatic light via the rfilter shoukd prevent any colour finging, but the focus position to the blue will be a little bit different and may require a refocus, however the amount will always be the same if the same filyers are used so could perhaps be allowed for quite easily.

Dave

[rich_t]

Hi Richie,

I'm guessing the narrow filters would be OK but not sure about the 'L' part of LRGB that you mention? That's passing the whole visible spectrum?

Rich.

[ngc2403]

actually they work quiet well as narrowband imaging scopes, however not sure you can grab L?

as i understand it the red and blue are focussed at the same point but the green is at a different point to both further out i think, hence why some people talk about the purple halo's around the stars as it is a mix of red and blue, because our eye's are most sencitive to green

[Ben Ritchie]

From a technical point of view you'll get no 'false colour' with narrowband filters, although you may (and probably will) have to refocus between OIII (green) and Ha/SII (red). You may get some softness with RGB broadband filters (especially the R and B filters, as the colour correction is diverging quite rapidly at the ends of the spectrum).

As i've said before, I think that spherochromatism (change in spherical aberration with wavelength) WILL be an issue even with narrowband filters, so the far-red Ha and SII narrowband lines won't be as sharp as with an apochromat because the lens is likely to be optimised for the green. However, I'm not sure how much of an effect this will have in practice.

[Merlin66]

Ben,

When used with these narrow band filters (<10nm) there's no perceived chromatic abberations, it's the rate of change of abberation across the imaging wavelength which could cause the problem; this is why a cheap refractor doublet can work so well for solar imaging in Ha.

[roundycat]

just a thought, if you have a scope that has lateral chromatic aberration you may find you get a different sized image in different colours so you will need to do something with them before combining. Longitudinal CA may also present the same problem but not to the same degree. Nothing to do with focus.

Dennis

[RichieJarvis]

Thanks for the feedback and advice guys.

I've spoken to a couple of imagers as well - the most telling comment was 'if it was possible, dont you think the rest of the imaging community would save money instead of buying expensive large aperture refractors'?

I won't be investing in the 6" frac therefore.

Cheers,

Richie

[Merlin66]

Richie,

You've raised a good point in this thread...

Has anyone got examples of narrow band images with a Non-Apo which show the "problems"; or is it the issue of possibly having to re-focussing between exposures? For a OSC camera I'd say the expensive Apo would be good, but very few of the current Ooooh Aaaaaah images are one shot?????!!!!

[ngc2403]

Richie,

For a OSC camera I'd say the expensive Apo would be good, but very few of the current Ooooh Aaaaaah images are one shot?????!!!!

true but it would be nice to try it out

[Ben Ritchie]

Has anyone got examples of narrow band images with a Non-Apo which show the "problems"; or is it the issue of possibly having to re-focussing between exposures?

There are some H-alpha images online taken by Matt Taylor with a Meade AR6 achromat which look pretty darn good, which suggests that the theoretical problems aren't so much of an issue in practice - saying that the AR6 is a f/8, and i'm guessing that's (significantly?) less likely to show problems than, say, a f/5. But it may well be the case that the optical issues exist at sub-arcsecond image scales, and if you're imaging at 1.5"/pixel under 2.5" seeing then who cares anyway?

The other issue seems to me that the large achromats are getting big and heavy and therefore have significant mount requirements, Taylor's "cheap" 'scope is taking 15-minute exposures on a $10k mount - you're not going to bung one on a EQ5 and start taking long subs. So from a purely practical standpoint you may be more mount-limited than optics-limited in this area anyway.

[Ben Ritchie]

e.g. this one, which was taken with the AR6 and a DSI Pro II

M16 in HAlpha Using the Meade AR6 Achromatic Refractor photo - Matt photos at pbase.com

[ngc2403]

I will be very happy if i get that sort of result with my 5'' refrac guided through a narrowband filter.

at least i see that the resullts can be very good so heres hoping that i get a good clear summer to narrowband image or at least that is the plan at the moment see how it goes i feel with the observatory build first

[RichieJarvis]

Interesting - the main objection did seem to be LRGB, not Ha/OIII

Thanks for the info guys... Maybe I should ponder some more....

[Merlin66]

That would make more sense to me!