Go Back   IceInSpace > Equipment > Equipment Discussions

Reply
 
Thread Tools Rate Thread
  #1  
Old 07-12-2017, 05:29 PM
cresskh's Avatar
cresskh (Cress)
Registered User

cresskh is offline
 
Join Date: Aug 2017
Location: Malaysia
Posts: 5
APO performance with achromat + mono + LRGB

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?
Reply With Quote
  #2  
Old 07-12-2017, 05:42 PM
pluto's Avatar
pluto (Hugh)
Astro Noob

pluto is offline
 
Join Date: Dec 2011
Location: Sydney
Posts: 1,982
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?

Last edited by pluto; 07-12-2017 at 05:56 PM.
Reply With Quote
  #3  
Old 07-12-2017, 06:02 PM
glend (Glen)
Registered User

glend is offline
 
Join Date: Jun 2013
Location: Lake Macquarie
Posts: 7,033
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.

https://www.astrobin.com/275791/
Reply With Quote
  #4  
Old 07-12-2017, 06:13 PM
RickS's Avatar
RickS (Rick)
PI cult recruiter

RickS is offline
 
Join Date: Apr 2010
Location: Brisbane
Posts: 10,584
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.

Cheers,
Rick.
Reply With Quote
  #5  
Old 07-12-2017, 06:17 PM
Peter Ward's Avatar
Peter Ward
Galaxy hitchhiking guide

Peter Ward is offline
 
Join Date: Dec 2007
Location: The Shire
Posts: 8,090
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.

Last edited by Peter Ward; 07-12-2017 at 07:07 PM. Reason: ipad keyboard
Reply With Quote
  #6  
Old 07-12-2017, 07:46 PM
Merlin66's Avatar
Merlin66 (Ken)
Registered User

Merlin66 is offline
 
Join Date: Oct 2005
Location: Junortoun Vic
Posts: 8,904
Peter,
Have you examples showing your concerns?
Reply With Quote
  #7  
Old 07-12-2017, 08:15 PM
Atmos's Avatar
Atmos (Colin)
Ultimate Noob

Atmos is offline
 
Join Date: Aug 2011
Location: Melbourne
Posts: 6,980
Quote:
Originally Posted by Merlin66 View Post
Peter,
Have you examples showing your concerns?
Spot diagrams
Reply With Quote
  #8  
Old 07-12-2017, 10:51 PM
Merlin66's Avatar
Merlin66 (Ken)
Registered User

Merlin66 is offline
 
Join Date: Oct 2005
Location: Junortoun Vic
Posts: 8,904
Colin,
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......)
Reply With Quote
  #9  
Old 08-12-2017, 06:28 AM
Atmos's Avatar
Atmos (Colin)
Ultimate Noob

Atmos is offline
 
Join Date: Aug 2011
Location: Melbourne
Posts: 6,980
Quote:
Originally Posted by Merlin66 View Post
Colin,
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......)
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.
Reply With Quote
  #10  
Old 08-12-2017, 08:03 AM
Slawomir's Avatar
Slawomir (Suavi)
Registered User

Slawomir is offline
 
Join Date: Sep 2014
Location: North Queensland
Posts: 3,240
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.
Reply With Quote
  #11  
Old 08-12-2017, 08:55 AM
Merlin66's Avatar
Merlin66 (Ken)
Registered User

Merlin66 is offline
 
Join Date: Oct 2005
Location: Junortoun Vic
Posts: 8,904
Cress,
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.
Reply With Quote
  #12  
Old 08-12-2017, 08:59 AM
RickS's Avatar
RickS (Rick)
PI cult recruiter

RickS is offline
 
Join Date: Apr 2010
Location: Brisbane
Posts: 10,584
Quote:
Originally Posted by Slawomir View Post
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 suspect that the wings of the PSF are a much greater factor in this than out-of-band light.

Cheers,
Rick.
Reply With Quote
  #13  
Old 08-12-2017, 10:12 AM
Merlin66's Avatar
Merlin66 (Ken)
Registered User

Merlin66 is offline
 
Join Date: Oct 2005
Location: Junortoun Vic
Posts: 8,904
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.

Last edited by Merlin66; 08-12-2017 at 10:22 AM.
Reply With Quote
  #14  
Old 08-12-2017, 11:55 AM
Slawomir's Avatar
Slawomir (Suavi)
Registered User

Slawomir is offline
 
Join Date: Sep 2014
Location: North Queensland
Posts: 3,240
Quote:
Originally Posted by RickS View Post
I suspect that the wings of the PSF are a much greater factor in this than out-of-band light.

Cheers,
Rick.
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
Reply With Quote
  #15  
Old 08-12-2017, 02:29 PM
RickS's Avatar
RickS (Rick)
PI cult recruiter

RickS is offline
 
Join Date: Apr 2010
Location: Brisbane
Posts: 10,584
Quote:
Originally Posted by Slawomir View Post
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
Interesting comparison, Suavi.
Reply With Quote
  #16  
Old 08-12-2017, 04:27 PM
Slawomir's Avatar
Slawomir (Suavi)
Registered User

Slawomir is offline
 
Join Date: Sep 2014
Location: North Queensland
Posts: 3,240
Quote:
Originally Posted by RickS View Post
Interesting comparison, Suavi.
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.
Reply With Quote
  #17  
Old 08-12-2017, 11:57 PM
cresskh's Avatar
cresskh (Cress)
Registered User

cresskh is offline
 
Join Date: Aug 2017
Location: Malaysia
Posts: 5
Quote:
Originally Posted by Merlin66 View Post
Cress,
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.
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.
Reply With Quote
Reply

Bookmarks

Thread Tools
Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT +10. The time is now 10:12 PM.

Powered by vBulletin Version 3.8.7 | Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Advertisement
Testar
Advertisement
Bintel
Advertisement