revans
Carpal Tunnel
Reged: 09/26/05
Posts: 1517
Loc: Fitchburg, MA
|
|
First of all... I'm not a great or an experienced deep sky imager and I live in very light polluted skies. But I hear it again and again... don't touch a short tube achro for deep sky imaging... you'll be sorry.... too much chromatic aberration.... you have to get an ED scope and if you can afford it you really have to get an apo scope. Stay away from those short tube achros... you'll be sorry...
This philosophy begins to get burned into your mind after a while... but is it really true today with all of the image processing software available out there? I really thought that you can get decent color images of deep sky objects with an F5 achromat with proper, but still quick and easy post-processing. And I've done some preliminary images that seem, at least to me, to validate this. Attached are two half hour images of M27. No filters used. Camera was a Canon Rebel DSLR. The first is an unprocessed image from my Orion 120 mm F5 achro. The second is a processed version of the first.
So... I'm not claiming that I am a great imager or that this is a great image... but can anyone say that the processed image is ruined by chromatic aberration? Couldn't you imagine the processed image as having been taken with an ED scope and not an achromat.
Anyway... I hope this generates some honest discussion because I think the achromat is under rated by almost everyone... and maybe even maligned unfairly... 
--------------------
Rick Evans
http://www.freewebs.com/revans_01420/
"The universe is there for us to see, but it cannot be understood without learning its language -- mathematics." Galileo Galilei
Edited by revans (10/28/09 01:45 PM)
|
revans
Carpal Tunnel
Reged: 09/26/05
Posts: 1517
Loc: Fitchburg, MA
|
|
Now here is the processed image...
PS - The cost of an Orion 120 mm F5 achromat is $320 new. The cost of a Takahashi 120 mm F7.5 apochromat is about $3750. I'm not aware of a 120 mm F5 apochromat... but they may exist...
--------------------
Rick Evans
http://www.freewebs.com/revans_01420/
"The universe is there for us to see, but it cannot be understood without learning its language -- mathematics." Galileo Galilei
Edited by revans (10/28/09 02:03 PM)
|
Djarum
sage
Reged: 03/12/09
Posts: 219
|
|
Wow. Thats awesome.
Dj
--------------------
Celestron Astromaster 130eq with a 60mm finderscope
Bushnell Legend 10x50
TMB planetary 5mm EP
Enhanced Wide Angle 6mm EP
Plossl 10mm EP
Plossl 32mm EP
Ultima 2x barlow
Baader M&SG and UHC filters.
|
snart
professor emeritus
Reged: 04/07/05
Posts: 596
Loc: Cincinnati, OH
|
|
All the software did was to turn purple bloated images into white bloated images. The images, however, are still bloated! When you can get pinpoint star images with that short f-ratio achro then give me a call...
--------------------
Celestron C100ED w/ Moonlite focuser
6" f/9.25 Newtonian - GEM
Meade 10" f/10 Schmidt-Cassegrain
18" f/4.5 Newtonian - Dob
4mm & 5mm TMB SMCs, 6mm, 7mm, 9mm, 12mm UO HD Orthos, 4.8mm, 7mm, 11mm, 16mm, 20mm TV Naglers, 24mm, 32mm, 40mm TV WFs, 55mm TV Plossl.
|
revans
Carpal Tunnel
Reged: 09/26/05
Posts: 1517
Loc: Fitchburg, MA
|
|
I take your snarkily expressed but very valid point to heart... but sharp images of stars should be equally sharp in an achro or an apo of similar lens quality. Assuming I focused well, then I should get equally sharp images with an achro or an apo of similar quality. The only issue I am addressing in this post is chromatic aberration and nothing more... and that is because no one yet makes an extremely high quality achromat lens at F5 that I am aware of... but at the same time I'm also not aware of a 120 mm F5 apo lens either...
--------------------
Rick Evans
http://www.freewebs.com/revans_01420/
"The universe is there for us to see, but it cannot be understood without learning its language -- mathematics." Galileo Galilei
Edited by revans (10/28/09 02:31 PM)
|
Retentive
member
Reged: 02/16/08
Posts: 15
Loc: Florida
|
|
Mr. Evans, I too own a 120ST and agree with your assessments. I dont expect my telescope to perform as an APO. That being said many times I read here that it is a total waste to buy or try to use an achro even if that is all you can afford. I applaud anyone who chooses to spend thousands of dollars for their telescope, I spent my scope money on a 350+ HP hemi jeep cherokee which I enjoy driving. I know there are people who enjoy their car even though they cant go off road or go very fast but i dont belittle them for that.
|
revans
Carpal Tunnel
Reged: 09/26/05
Posts: 1517
Loc: Fitchburg, MA
|
|
I really believe that processing advances over the years have made it possible for us to tame chromatic aberration at least in deep sky images taken with achros. But at the same time, it is also true that the optics have other aberrations that are controlled for in apos. What would be fascinating, and I think likely possible, is to have someone manufacture an F5 achromat with a high quality lens free of these other aberrations. I think the Orion 120 mm F5 lens is OK and it is fine for my visual and even my imaging needs... but imagine the possibilities if its achromat lens were made just a bit more precisely... I don't think it is the type of glass that is holding achros back... it is a need for better figuring of the lenses...
--------------------
Rick Evans
http://www.freewebs.com/revans_01420/
"The universe is there for us to see, but it cannot be understood without learning its language -- mathematics." Galileo Galilei
|
imhotep
Pooh-Bah
Reged: 02/14/07
Posts: 1429
Loc: Tampa, FL
|
|
Quote:
... but sharp images of stars should be equally sharp in an achro or an apo of similar lens quality. Assuming I focused well, then I should get equally sharp images with an achro or an apo of similar quality.
First of all, cool comparison. It's an interesting and important topic. You've got sharpness and CA seperated in your mind though, and they are very much linked under the heading of image quality. It is hard to say 'I've got pinpoint stars' when the points are surrounded by a halo of any color, don't care if its blue or white.
Realize this -- if your stars have halos around them, you're target is also not appearing as clearly and with as much detail as you could expect to get with a better corrected objective. Processing to make the halos blend in by changing their color to roughly match the stars they surround isn't correcting the real problem, it's only taking the spotlight off of it somewhat.
BUT...you may be addressing a more general and subjectively phrased criticism that has been launched against achromats, that being "they can't take good astrophotos" or something to that affect. As far as that goes, I think you've demonstrated otherwise. That's a neat image and if it brings pleasure to yourself and others who view it, then that's awesome. BUT, you do have a fair amount of CA in there regardless of what appearance it takes on after processing. People talk about processing OUT the CA which is sort of possible, but you can't process IN the detail you would get with a well corrected apo.
--------------------
Curt
8" F5 Newtonian Astrograph
Orion 80ED CUSTOM
Edmund Optics 4.25" 'Palomar' Newtonian, 1968 vintage
|
imhotep
Pooh-Bah
Reged: 02/14/07
Posts: 1429
Loc: Tampa, FL
|
|
Quote:
That being said many times I read here that it is a total waste to buy or try to use an achro even if that is all you can afford.
Yeah I'm with you and the OP on this, and I don't agree with that sentiment. My 80ED has made me very happy for the low amount of CA this scope seems to produce. The images I've captured with it are certainly worth the $350 I paid for the OTA. But with that said, I know it's not an apo and I know the images would be better if it was.
--------------------
Curt
8" F5 Newtonian Astrograph
Orion 80ED CUSTOM
Edmund Optics 4.25" 'Palomar' Newtonian, 1968 vintage
|
starrancher
professor emeritus
Reged: 06/09/09
Posts: 620
Loc: Northern Arizona
|
|
Quote:
Mr. Evans, I too own a 120ST and agree with your assessments. I dont expect my telescope to perform as an APO. That being said many times I read here that it is a total waste to buy or try to use an achro even if that is all you can afford. I applaud anyone who chooses to spend thousands of dollars for their telescope, I spent my scope money on a 350+ HP hemi jeep cherokee which I enjoy driving. I know there are people who enjoy their car even though they cant go off road or go very fast but i dont belittle them for that.
Cool Beans ! ....I'm a Jeep man too !
It's a Jeep thing , ....Some folks just don't understand .
--------------------
LXD75 AR5
LXD75 SN8
Series 4000 Plossls
Misc. other stuff
Fort Rock , Az .
|
Al Miller
sage
Reged: 08/25/08
Posts: 415
Loc: San Antonio, Texas
|
|
I also own and use a ST-120 achro. I love it! I get pretty decent images with my small-chip DSI cameras that more than satisfy the one person it needs to... me. I don't care about outdoing the other guy's perfect image, that's already been done. I find that only the brighter stars are bloated but, I don't get absolute pinpoint stars from my reflectors either (and I focus with a Bahtinov mask). My ED refractor is better but not by a huge amount. I too don't expect too much and use my cheap equipment as an observing aid as well as to take some pretty good images. I'm happy. I use the cash I save to get other toys, Astronomy or otherwise. My little ST-120 makes a nice H-alpha imaging scope too. In H-alpha, all the stars are points!
Rick Evans... I think your second image may look even better with a bit of color balancing to reduce the blue. Nice job none-the-less.
BTW... I also have a Jeep and just got myself a nice Camaro. 
--------------------
Meade 10" LX200 Classic
Celestron C8 XLT OTA
Orion XT8 Classic Dob.
Meade AR-5, f/9.3 refractor OTA (LXD55 version)
Antares 4.1", f/12.4 refractor w/EQ5
Orion 110ED, f/7 refractor OTA
Orion ST120, f/5 refractor OTA
Orion ST80, f/5 refractor OTA (guidescope)
Meade DSIc, DSI IIc and DSI II Pro
Celestron NexImage (x2)
Orion StarShoot autoguider
Atlas EQ-G mount
Edited by Al Miller (10/28/09 04:13 PM)
|
Jeff B
professor emeritus
Reged: 12/30/06
Posts: 679
|
|
Next time, try focusing on the blue stars. I'm not an imager...yet... but I know achromats. The red and blue focus of achromats are shifted on the same side relative to green/yellow and by similar amounts. If you focus on the blue & red the green/yellow will be out of focus. But how many green stars and nebulas have you seen lately? Yeah, there are yellow stars but the reds and blues tend to be more numerous and brighter. You would might end up with bloated yellow stars but the red & blue may appear sharper. Ditto with nebulas as they emit a lot of light in the red & Blue ends. The Pleiades might be a worthwhile target to test this. Worth a try?
Jeff
|
galaxyman
Pooh-Bah
Reged: 04/04/05
Posts: 1472
Loc: Limerick, Pa
|
|
Quote:
I really believe that processing advances over the years have made it possible for us to tame chromatic aberration at least in deep sky images taken with achros. But at the same time, it is also true that the optics have other aberrations that are controlled for in apos. What would be fascinating, and I think likely possible, is to have someone manufacture an F5 achromat with a high quality lens free of these other aberrations. I think the Orion 120 mm F5 lens is OK and it is fine for my visual and even my imaging needs... but imagine the possibilities if its achromat lens were made just a bit more precisely... I don't think it is the type of glass that is holding achros back... it is a need for better figuring of the lenses...
You nailed it about the over all quality of the optics, particularly at the eyepiece.
For instance my observing colleague also owns a APM achro. His is the 7" f/6 that gives outstanding views from low power to high power (300x plus) on all objects.
As with ALL scopes, good quality optics is essential.
Karl
E.O.H.
Chesmont Astronomical Society
Telekit (Swayze optics) 22" F/4.5 Dob
Homemade (Parks Optics) 12.5" F/4.8 Dob
TMB 8" F/9 Refractor”The Beast”. One great achro
ES 6" F/6.5 achro- NEW
Orion 4" f/6 Refractor. Also not bad for an achro
Celestron 10x60 binos
--------------------
So many galaxies, so little time!
|
old_frankland
professor emeritus
Reged: 03/28/05
Posts: 524
Loc: San Francisco Bay Area
|
|
The 120ST is indeed excellent with narrow band filters, with smaller CCD cameras. Plus, working in H-alpha gets you completely around the local light pollution. Frankly, I think folks are a little too hung up on color. Working in monochrome with H-alpha narrow band is like discovering a whole new univers.
http://www.lafterhall.com/atk16_b33.html
http://www.lafterhall.com/atk16_ic410.html
My 2 cents...
--------------------
Cheers,
Jim
Livermore CA
http://www.lafterhall.com/astro.html
3" APO, 4" ED refractor, 9" SCT, GM8
|
BradC
sage
Reged: 01/31/07
Posts: 448
Loc: St. Louis, MO
|
|
Quote:
... but sharp images of stars should be equally sharp in an achro or an apo of similar lens quality. Assuming I focused well, then I should get equally sharp images with an achro or an apo of similar quality....
I'm not sure that's true.
My understanding is that Chromatic Aberration is literally when different colors reach their proper focus at different positions of the focuser (ie, each wavelength has different focal lengths). All the more sophisticated lens designs and exotic materials in APOs are made that way in an attempt to get all colors to focus at the same plane.
The reason you see the purple fringing is because the purple light is de-focused at the exact point at which the other wavelengths are in focus.
Assuming my optic science is correct here, then that leaves you with a few choices to deal with the problem:
1. Post-process the image to reduce the appearance of fringing, as you've done. I really like your result, so in my opinion there is absolutely nothing wrong with this approach.
2. Use a physical filter to reduce or eliminate the purple light when taking the image. Probably allows for more pinpoint stars, but may loose some detail in some DSOs.
3. Use a color wheel to take individual color exposures, adjusting the focus for each one. Then combine and process.
I'd have to dig back through the forum archives, but if I remember right, this last approach (the most labor intensive) could give you the best quality raw frames, that with the right processing might rival what you could do with much more expensive equipment.
--------------------
My God! It's full of stars!
Zhumell 10" dob - First Light Report
Celestron OptiView 10x50 LPR Binoculars
|
revans
Carpal Tunnel
Reged: 09/26/05
Posts: 1517
Loc: Fitchburg, MA
|
|
Well... I'm pretty new to processing and so far I only tried to deal with the CA issue. But I noticed that the astronomy package I use does have a bright star select and a star size reduction algorithm. So... I gave it a try.
Maybe its not for purists... and also I must say that I really could do a better job with more practice and this is just a first attempt.... but anyway here is the result of trying to reduce the bright star bloat with post-processing. I'm not really happy with the overall processing yet, but it is a start and I think it could get a whole lot better with practice...
It certainly seems possible to me that both CA and star bloat problems in achro deep sky images likely can both be managed in a post-processing that only takes a few minutes and isn't very complicated...
--------------------
Rick Evans
http://www.freewebs.com/revans_01420/
"The universe is there for us to see, but it cannot be understood without learning its language -- mathematics." Galileo Galilei
Edited by revans (10/28/09 04:55 PM)
|
Kal
sage
Reged: 07/31/06
Posts: 201
Loc: Sydney, Australia
|
|
Quote:
The 120ST is indeed excellent with narrow band filters, with smaller CCD cameras. Plus, working in H-alpha gets you completely around the local light pollution. Frankly, I think folks are a little too hung up on color. Working in monochrome with H-alpha narrow band is like discovering a whole new univers.
http://www.lafterhall.com/atk16_b33.html
http://www.lafterhall.com/atk16_ic410.html
My 2 cents...
I agree with you 100%. When doing narrowband imaging, the achro can be a powerful and affordable OTA. While an achro can't focus all wavelengths at once for a single shot color image, it can focus individually all of the wavelengths needed for narrowband imaging. Plus, as you mentioned, you get past light pollution as well!
To the OP, no offense but I really don't like the results you obtained after processing. Of the two I'd prefer the one with all of the CA :/
--------------------
CG-11 • 130mm f6.3 StarFire EDF Gran Turismo • SV90TBV • ETX90EC • 25x100 BINOCULARS • Toucam 840K hacked to a 900NC • DSI Pro
|
tcat
sage
Reged: 06/02/08
Posts: 233
Loc: Va
|
|
That is a very nice image, Rick! If you feel pleased with the result as well (and the equipment cost savings!) then you need not worry about the apo purists that reside here from raining on your parade. I agree totally about achromats being underrated. I have been satisfied with my little ST80 for many years and only recently moved up to an ED scope so I could pump the power up a bit more for lunar and planetary viewing.
I hope you will share more images with us in the future. You are evidently on a very fast AP learning curve!
--------------------
Tom
-------------
Orion 102ED / AT Voyager
ST80 / EQ-1
NexStar6 SE
|
revans
Carpal Tunnel
Reged: 09/26/05
Posts: 1517
Loc: Fitchburg, MA
|
|
Kal,
Narrow band imaging would be nice to try but I suspect the exposure times would be too long for me. Sorry you didn't like the CA removal etc... I suppose taste in astropics is like wine tasting... everyone has a different palate. Personally I don't like the violet halos and was very glad to be rid of them 
--------------------
Rick Evans
http://www.freewebs.com/revans_01420/
"The universe is there for us to see, but it cannot be understood without learning its language -- mathematics." Galileo Galilei
|
Joe Ogiba
Post Laureate
Reged: 02/14/02
Posts: 4069
Loc: NJ USA
|
|
I tested my 120ST with my Pentax K-7 in HD video mode last summer and it looked great for a low cost scope.
photo of setup
test video
--------------------
Pentax PF-80ED,Meade 102ED APO,Orion EON 72,120ST
Apex 127,C6 XLT,CR150,C9.25,XT10
Zeiss 7x42 FL,Canon 10x42L IS WP,15x50 IS
12x36 IS II
Garrett Optical 28x110 HD-WP Signature Series
Oberwerk BT-80 45, Apogee RA-88-SA
Denk II Power x Switch binoviewer w/13mm Ethos, 20mm Pentax XW's, 20mm Widescan III's.
17mm Ethos, 22mm Nagler, 40mm Pentax XW, 14mm Pentax XL, 5.2mm Pentax XL, 8-24mm Pentax XL Zoom
|
Previous
Index
Next
Threaded
Edit
Reply
Quote
[Re: 