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oeagleo
sage
Reged: 01/02/11
Loc: Utah, USA
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Imaging DSO's, Achromat, or Apochromat?
#5237604 - 05/24/12 03:51 PM
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Hello, all.
I know that the APO scopes are "better" at giving true color around bright stars, and planets, but I'm curious. Would an Achromat (doublet?) give true color on DSO's, like say, M101, or Andromeda? Would any color variation be correctable with filters, or through Photoshop?
I have a Celestron 9.25, but was thinking of "branching out" into a shorter FL scope for objects that are too large to capture with the Celestron. I have an SBIG 8300C camera, as well as an Imaging Source video with the 618 chip.
Thanks for any input.
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jgraham
Postmaster
Reged: 12/02/04
Loc: Dayton, Ohio
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Re: Imaging DSO's, Achromat, or Apochromat?
[Re: oeagleo]
#5237633 - 05/24/12 04:17 PM
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There is no doubt the for imaging a apo is much better than an achro and if you can afford an apo that's the way to go. However, there's no reason for those who already own or can only afford an achro (my hand is up) not to use it for deep sky imaging. I've imaged with an ETX-60 (lots of fun), and ETX-80 (not quite as much fun), and an LXD75 AR-6 (powerfully neat). The achro doesn't have any noticeable effect on the deep sky object itself, but it does cast a violet halo around the brighter stars in the image. There are ways of reducing the halo, both with filters and processing techniques, but with my AR-6 I've pretty much decided to leave it alone and I'm happy with the images without any special filters or processing. I posted one recent image taken with my AR-6 here in the beginners forum and a couple more in the DSLR forum.
Also, before I forget, this only applies to one-shot color imaging. There is absoultely no problems with RGB imaging. All you need to do is to reset the focus for each color and voila! No halos. I also sum the R+G+B data to make a synthetic luminance. This has an added benefit that it usually means I end up averaging more images than I would with a traditional luminance and I end up with a tad lower noise than I would with a conventional LRGB image.
P.S.
See…
http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/5233292/page/0/view/collapsed/sb/5/o/all/fpart/1
http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/5236367/page/0/view/collapsed/sb/5/o/all/fpart/1
http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/5231339/page/1/view/collapsed/sb/5/o/all/fpart/1
Edited by jgraham (05/24/12 04:19 PM)
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CounterWeight
Postmaster
Reged: 10/05/08
Loc: Cloudyopolis, OR.
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Re: Imaging DSO's, Achromat, or Apochromat?
[Re: oeagleo]
#5237668 - 05/24/12 04:41 PM
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This question gets asked a lot. However you couch it, light lost though filtering is image / detail information that is lost. My main reason for using APO triplet is to get the best possible image I can for the time I spend imaging. There are already enough curve balls in imaging that I don't want to from inception begin with something that I'll find distorted with halos, easily fixed with a different scope design. I think that goes to the heart of why the small short focal ratio 'apo' scopes are so popular with imagers?
-wider the fov or lower focal ratio ratio in an achromat is going to potentially give you even more color. As far as filtering tht out will depend on where the designer of the achro shifter the color so as not to be bothersome visually. Short answer is there are always tradeoffs available to the designer.
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oeagleo
sage
Reged: 01/02/11
Loc: Utah, USA
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Re: Imaging DSO's, Achromat, or Apochromat?
[Re: CounterWeight]
#5237697 - 05/24/12 05:02 PM
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Well, I'm thinking even a 5" Achromat on the EQ5Pro, would be a lot more stable than the ETX-125. I had it out the other night, and I wouldn't even think about imaging with it, even with the DFK video..
Thanks for the info, not having an unlimited budget, I think the Achromat might be the one for me; at least for now..
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BubbyD
professor emeritus
Reged: 04/22/10
Loc: Troy, Ohio
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Re: Imaging DSO's, Achromat, or Apochromat?
[Re: oeagleo]
#5237791 - 05/24/12 06:06 PM
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APOs have a good reputation in imaging not just because of their "color correction". APOs are primarily better because of the quality of their optics. Usually, especially in the past, much more care was given to creating Apochromatic lenses, which gave much better views than a cast lens that was formed by machine. Today, there are a lot of inexpensive APO's out there that probably won't give you the views an expensive Astrophysics, TEC, Takahashi, Stellarvue, etc. would.
The definition of an aprochromatic lense is one that brings all colors of the visible spectrum into focus at the same image plane. Only a 4 element system can do that in actuality, unless the f ratio is quite long. A triplet "APO" of f6 or less is likly not a true APO. I used to have a 4" f6 APO of very high quality made by Stellarvue. This was a $3000 telescope new. But I still had to refocus the blue filter because the blue bandwith's image plane was just a hair off the rest of the spectrum.
So, to answer your question, if you have a one shot color, an APO is going to be much better. But if you have a monochrome camera with decent filters, a achromat will probably serve you just fine. Just remember to refocus when you change filters.
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jgraham
Postmaster
Reged: 12/02/04
Loc: Dayton, Ohio
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Re: Imaging DSO's, Achromat, or Apochromat?
[Re: oeagleo]
#5237793 - 05/24/12 06:08 PM
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Yeah, the ETX mounts were never designed for imaging. I was able to use an ETX-60 in part due to its wide field of view and the high sensitivity of the DSI Pro III. I'm finding that imaging with an AR-6 and a modern camera to be very rewarding.
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avarakin
Carpal Tunnel
Reged: 07/13/09
Loc: Parsippany NJ, USA
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Re: Imaging DSO's, Achromat, or Apochromat?
[Re: jgraham]
#5239030 - 05/25/12 01:26 PM
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Achro should be equal to APO in case if you shoot narrowband. Narrowband is a very good technique if you need to fight LP or if you want to go really deep.
Alex
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daev
Technically Enlightened
Reged: 03/10/04
Loc: On the edge of the desert
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Re: Imaging DSO's, Achromat, or Apochromat?
[Re: BubbyD]
#5241996 - 05/27/12 11:01 AM
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The definition of an aprochromatic lense is one that brings all colors of the visible spectrum into focus at the same image plane.
Actually, and Apochromat brings 3 wavelengths into focus on the same plane, while an Achromat does so with only 2. Three-element designs can be true APO, and I think 3 elements is the minimum to achieve 3-wavelength focus.
daev
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PatHolland
Carpal Tunnel
Reged: 10/11/09
Loc: Clever, Missouri
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Re: Imaging DSO's, Achromat, or Apochromat?
[Re: avarakin]
#5242057 - 05/27/12 11:29 AM
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That's a good point Alex. I have the Explore Scientific AR152 (doublet) and intend to use it for narrowband imaging. I may try RGB through it at some point just to try it but the main purpose for having the scope is narrowband.
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jgraham
Postmaster
Reged: 12/02/04
Loc: Dayton, Ohio
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Re: Imaging DSO's, Achromat, or Apochromat?
[Re: PatHolland]
#5242067 - 05/27/12 11:39 AM
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That's a very good point. You can do some very good color imaging with just an H-alpha and O-III filter. If I recall right I had excellent results using O-III + 30% H-alpha for blue (synthetic H-beta), O-III for green, and H-alpha for red. I haven't tried it with my achro yet, but now that the haze of summer is here it may be time to give it a go.
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avarakin
Carpal Tunnel
Reged: 07/13/09
Loc: Parsippany NJ, USA
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Re: Imaging DSO's, Achromat, or Apochromat?
[Re: jgraham]
#5242087 - 05/27/12 11:53 AM
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Yep, I've done quite a bit of Ha+OIII and results were great - very sharp stars and nice colors. Also such images do not suffer from star overcrowding. Can't wait when spring galaxies, which are killed by LP in my red zone, will be gone and beautiful summer emission objects are up!
Alex
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Vondragonnoggin
Post Laureate
Reged: 02/21/10
Loc: Southern CA, USA
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Re: Imaging DSO's, Achromat, or Apochromat?
[Re: avarakin]
#5242102 - 05/27/12 12:02 PM
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This is good news for me. I have spent all I can to get started and my primary imaging scope will be my AR127. I have an OIII, and an H-Alpha.
Of course I have to learn to use my camera first (Baader modded T3i). I bought a field flattener for the achro.
I think I'm up for the challenge with it.
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BubbyD
professor emeritus
Reged: 04/22/10
Loc: Troy, Ohio
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Re: Imaging DSO's, Achromat, or Apochromat?
[Re: daev]
#5243095 - 05/27/12 11:41 PM
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Actually, and Apochromat brings 3 wavelengths into focus on the same plane, while an Achromat does so with only 2. Three-element designs can be true APO, and I think 3 elements is the minimum to achieve 3-wavelength focus.
daev
No, that is the way 3-element telescope makers define Apochromaic. In optical physics, Apochromatic means all wavelengths of visible light have focal planes of equal distance. If you look at a 3-element design's graph of focal plane distance, it will resemble a sine wave. That sine wave gets steeper the faster the lense is. A four or five, even six element desing can get it much flatter (think very high end low light cine type lenses), but a triplet APO isn't Apochromatic. If you don't believe me, email Vic at Stellarvue. He used to put "superapochromatic" on some of his scopes, which is just a marketing word for "more apochromatic than anyone else's triplet at this focal lenght". Lol. I don't think he usus it anymore since a few manufaturers came out with 4 element designs of the same or similiar diameter.
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