View Full Version here: : APO performance with achromat + mono + LRGB

07-12-2017, 05:29 PM
I've been hearing this awhile back, that you are able to get APO or near-APO performance when you're doing imaging with an achromat scope coupled with mono camera + LRGB filters.

Of course, you will need to refocus each time you're imaging in different LRGB filter.

So the question is, will this work? After you focus perfectly in each LRGB filter and combine into single color image, is the resultant image CA free?

07-12-2017, 05:42 PM
I've done this with SHO narrowband imaging and it works well.
I would think that with LRGB the RGB images might be alright but surely the L, as it covers most of the visible spectrum, would be soft.
HaRGB might be worth a try?

07-12-2017, 06:02 PM
I have imaged with a Bresser 152 f5 Achro (with a rear petzval corrector) for several years. Strictly narrowband but it works very well. The thing about achros is that you will need an image capable focuser. I would not suggest trying broadband, unless you want to try a very long focal length which reduces CA considetably - however, long focal lengths means long sub times, good guiding, etc.


07-12-2017, 06:13 PM
As Hugh suggested you'll get bloated stars in the L filter. You can do RGB imaging without L and it should work well for bright objects, but it will be slow for dim targets.


Peter Ward
07-12-2017, 06:17 PM
Sorry there is no free ride to be had here. The main problem from the RGB chain is you end up with 3 different spot sizes at three different focal lengths, and slightly different field curvatures.

Trying to register them is nigh impossible....

That said, a single narrow band channel, eg. H-alpha could be tack sharp provided the optics have no spherical or similar errors.

07-12-2017, 07:46 PM
Have you examples showing your concerns?

07-12-2017, 08:15 PM
Spot diagrams :)

07-12-2017, 10:51 PM
For a good achromatic the spot diagrams for red/green/ blue will all show tight focused results - when the chromatic aberration curve is taken into account.
I can cleanly focus target stars from CaK (393nm) through to Ha (656nm)with the achromat in the spectroscope. Yes, there has to be re-focusing between the wavelengths, but when this is done the image and result is very sharp.
(Spherical aberrations could be an issue......)

08-12-2017, 06:28 AM
That works well when you’re looking at 0.5-1 angstrom passband but when shooting with a green filter you are actually going from almost red to almost blue. It gets worse when you are going from effectively IR to green with the red filter and green to UV with the blue.

08-12-2017, 08:03 AM
And you will get large halos around brighter stars, even with best narrowband filters - no filter blocks 100% of the out-of-band light.

I feel that this effect also applies to some degree to extended objects (nebulae, galaxies) leading to smearing out of the signal and ultimately less crisp detail in the DSOs. I'm talking pretty pictures, as I have no experience with spectroscopy.

08-12-2017, 08:55 AM
I see you posted the same question on the SGL forum.
They have given you some very good replies.
Bottom line: Try it and see.

08-12-2017, 08:59 AM
I suspect that the wings of the PSF are a much greater factor in this than out-of-band light.


08-12-2017, 10:12 AM
Easy answer:
Try it and post the RBG images for comment/ discussion.....
(with the spectroscope and a 600 l/mm grating, the spectral range coverage is 70nm - the loss of detail across this range is minimal)
The RBG coverage (assuming a good UV-IR also) would be roughly blue=400-500nm, green=500-600nm, red=600-700nm.
The amount of "out-of-focus" would vary for each filter depending on the chromatic curve for the achromat being used.

08-12-2017, 11:55 AM
True, nonetheless I had noticeably but not vastly larger halos with my ED doublet than with the current triplet (same user :-), camera, same 3nm n LRGB filters, location, aperture and similar f-ratio). So I suspect an achromat would give inferior RGB/narrowband data relative to an ED doublet at same f-ratio, not to mention a well corrected triplet :question:

08-12-2017, 02:29 PM
Interesting comparison, Suavi.

08-12-2017, 04:27 PM
Thank you Rick. Just sharing my experience with two similarly sized scopes. I think your suggestion of RGB imaging and combining them into a Luminance will give the tightest stars with an achromat. And I would also suggest that undersampling might help too.

08-12-2017, 11:57 PM
Yea, I have an ST-80 now and thinking to upgrade my scope.
But a friend of mine suggested to upgrade to cool mono CCD/CMOS instead and be blown away by the result.
I'm pretty sure NB with achromat would do the job, nice round tight stars. But not so sure about LRGB, hence this question posted everywhere.

Before this, I had a lot of doubts because I definitely gonna get a lot of halo in L, but suggestions to use RGB or even Ha to replace L seems to work well. But still, like some here pointed out, individual R G B bands are too broad, especially towards the violet/blue, and post processing part to deal with different star sizes proves to be tough.

So before I committed to a cool mono cam, I was hoping to see some examples of how this is done.