|
M.Taylor
Vendor
Reged: 10/11/06
Posts: 326
Loc: Claremore, Oklahoma
|
|
Theorectical limits aside, the slightly softer look to the AR6 M16 image probably has a lot more to do with the doublet objective, 1200mm focal length, collimation, sky conditions while imaging, and how the image was post processed. In the case of these two images, where the AR6 and FLT 132mm APO are concerned, we're really comparing apples to oranges, a $300 AR6 1200mm doublet vrs. a $3700 925mm triplet but what's really amazing to me is how close in quality the two images really are considering the differences between the two scopes.
I think that in astrophotography, given decent subs to begin with, we often overlook the most important and determining factor to outstanding final results, the person doing the post processing. I've been at this hobbby for about 4~5 years now, given my experience level I believe that if I were to give my AR6 subs to say Dean, Budd, Richard Crisp, Rob Gendler or some other well advanced person in the hobby they would be able to produce a better, sharper, image than I produced with the FLT132 subs.
That said, to answer the orginal question, yes of course there is a need for the APO refractor. But I think it's importance is much more pronounced when LRGB imaging is the goal.
Quote:
Those are nice images, and I hate to criticize anyones images! But the AR images are soft. This is what one would expect from a scope that is operation at well under the diffraction limit of 1/4 wave.
I'm not saying that it isn't fun to do, or that the images stink! They obviously don't! But one cannot produce anywhere near as nice and sharp a narrowband image with and achro as with an APO. So is there a need for APO - yes, the produce sharper, better, more detailed narrowband images.
Can one use an achro, save some money and still produce nice (but lesser) images? Yes. Is it worth the money? That value judgment has to be made by each individual.
Philip
Quote:
... As you can see from the M16 images above the difference is not that much.
--------------------
Matt Taylor
Antique Photons Observatory
Meade Beta Tester & 4M Advisor
MI-250, WO 132mm APO, Orion 80ED
Meade AR6 & 80ED APO, WO 66ED Triplet
ST2000XM, DSI Pro II, DSI Pro III
DSI Tutorials On DVD: www.mallentaylor.com
|
gordianknot
professor emeritus
Reged: 09/04/05
Posts: 542
|
|
Your images look darned close to me. I downloaded them both, resized and aligned them and they look virtually identical (especially if you take artifacts from resizing & rotating into account). You can tell they are different images, but I believe that's because the exposure of the images differs slightly. I honestly have to look at the originals to remember which is which.
I'm attaching it, I hope you don't mind.
Edit: Just to clarify my comment about exposure. Stars are clearly more visible and brighter on the left hand image and the background is brighter.
Edited by gordianknot (12/25/07 05:00 AM)
|
Zuts
member
Reged: 09/22/07
Posts: 11
Loc: Sydney, Oz
|
|
Do we need APO's? Wrong forum but, some people still like to look and so yes we do still need APO's.
Paul
|
Miguel Lopes
scholastic sledgehammer
Reged: 01/04/07
Posts: 912
Loc: Portugal
|
|
Quote:
Do we need APO's? Wrong forum but, some people still like to look and so yes we do still need APO's.
Paul
Not the wrong forum. The context was narrow band imaging, so its the correct forum.
gordianknot, your post was most helpful. And you showed me a way to quantifie it.
I just received my Orion ST 80 for guiding (with barlow). Then switched functions and placed my DSI Pro on the ST80.
Although these was one of my first auto-guiding imaging session, i could spot some problems that i could not blame seeing or the mount.
The first was astigmatism. The second was the bloat caused by the use of color filters, although it performed well with h-alfa. So yeah, for the outstanding images being captured by amateurs these days, it sucks.
But... its a 99€ scope. I could setup one as guiding and other as imaging scope, each weighting 1.1Kg, for 200€!
this was a quick processing i made. And be gentle, its my first guided image 
M42
M. Taylor, great AR6 image! gordianknot made a good comparison. So my question can be reformulated to:
"On a limited budget for imaging is APO a priority?"
--------------------
Astrology is the science for ignorants. Astronomy is the science for those who feel ignorant. - Miguel Lopes
|
Bert
professor emeritus
Reged: 02/17/06
Posts: 671
Loc: Australia
|
|
I recently bought a Skywatcher 150mm F5 Achromat with a view to collecting faint narrowband data. It has very good correction for an achromat see here
http://aberrator.astronomy.net/scopetest/html/refractor_150_3.html
I have fitted a Borg helical focuser and use a Hutech FR/FF #7887 which gives a good flat field and reduces the scope to 670mm FL at f/4.5. The helical focuser scale gives me a means of quickly adjusting focus after each filter change.
Here is a large image taken with a Canon 5DH 1.1 MB
http://members.optusnet.com.au/~fmlee/hel_nb_acr_01_6.jpg
Details
Here is a quick combined image. No flats.
HA 8x8 min at 1600ISO
O3 6x8 min at 1600ISO
HA red to Red
O3 green to Green
O3 blue to Blue
I dont know what a 150mm F5 APO would cost but it would not be cheap. The Skywatcher scope cost $850 Australian.
Here is an image with narrowband from the Skywatcher and colour data from a 100ED combined in images plus.
http://avandonkbl.bigblog.com.au/data/2/13839/image/COLNBM425089120071018073640.jpg
Bert
Edited by Bert (12/25/07 08:20 PM)
|
Ben Ritchie
Lost in Space
Reged: 01/31/05
Posts: 4389
Loc: Bosham, UK
|
|
Bert - nice image (although it needs flats!)
One thought about the larger achromats - while the 'scope may be cheap, the mount requirements for imaging with a 6" f/8 (like the AR6) are substantial. You're not just going to stick it on a HEQ5 and take 15-minute narrowband subs.
Quote:
The reason is spherochromatism, the change in the spherical correction with wavelength. This is a different issue from the longitudinal error which it what one primarily sees in an achromat. Spherochromatism cannot be focused out, it is fixed by the design, focal length and aperture of the lens.
Out of curiosity, if sufficient care was taken in design and manufacture, could a quality 4" f/5 (say) achromat be made that was good for narrowband imaging - or is spherochromatism always going to be a problem? In principle it seems to be a good option for narrowband imaging, 'fast' and the narrowband filters mean that you don't need the colour correction of a FSQ anyway. But it might not be possible, or prohibitively expensive.
--------------------
Observatory:- Astro-Physics 130EDT StarFire, Baader SkySurfer V, EQ6/pro
Outside:- TeleVue 76is/Vixen Porta II
Eyepieces:- 2-4mm Nagler zoom, 5mm & 6mm Radians, 9mm Nagler, 13mm Ethos, 24mm & 34mm Meade SWAs
|
Bert
professor emeritus
Reged: 02/17/06
Posts: 671
Loc: Australia
|
|
The M42 image was taken solely with the 150mm achromat. Just wanted to see if you were paying attention. Where is the spherochromatism? Sad really. I was just trying to inform someone who asked a valid question.
Bert
|
blandp11
Carpal Tunnel
Reged: 04/19/05
Posts: 1963
Loc: Glen Ridge, NJ USA
|
|
I'm sorry, do you really think think I made up that achromats suffer from spherochromaticism? They do my friend, they do.
While your images are pretty, the do not have tight star images or the high resolution that APO's or reflectors can give, which is what one would expect from a scope operating with a poor wavefront at a particular frequency.
It is true that one can get much better images with an achromat and narrowband filters than with the usual tri colour filters, and as such is a great way to go. But you will not take images that can compete in resolution to what can be done with an APO or reflector.
The one way around this is to use *two* achromats, one designed and figured for green light (as most should be) and one designed and figured for red light. In this case you will have images at the designed wavelengths as tight as any single APO or reflector. Should I patent this? 
Philip
Quote:
Where is the spherochromatism? Sad really. I was just trying to inform someone who asked a valid question.
Bert
--------------------
Mostly refractors
|
|
|
Previous
Index
Next
Threaded
[Re: 
Edit
Reply
Quote
