I didn't want to pollute the any infor on the 10" RC thread any further, so I started one here to discuss the merits of each type of scope.
One rule, keep the comments civil and nothing personal. No sniping (OK that's two rules, I'd better come in again...).
Anyway, Bratislav and I imaged the same target within days of each other, me with my 8" RC as it came from Peter (it needed a tweak of the collimation and got a new focuser), so I'd say my pictures would be ever so slightly better now. Bratislav used his 10" newt, I don't know the focal length, but it seems to be a little less than the RC judging from the picture's FOV.
I have not done much to these pictures, and they are single subs, Bratislavs came up as a 60 second sub, mine is twice that. I calibrated mine, I think Bratislav's came to me calibrated, I used a touch of sharpening on mine as it showed some seeing induced errors (the base of the stars' peak has been spread, so Bratislav got the better seeing). I tried to do the same to Bratislav's picture, but it looked bad.
I have cropped the field showing part of the core and the best focussed bit of mine (I have since learned how to get most of the frame in focus). I think that the two images are virtually identical.
Let the discussion begin, remember that the Spanish Inquisition are watching and will lock the thread if you start to get nasty!
Almost indistinguishable... In the first pixel there is ONE "star" that is not in the second... about 2/3rds of the way across the image, down towards the bottom...
Appart from that, and the difference in exposure I agree - image sharpness and quality seem very similar..
The first image appears slightly brighter on my screen but both are very similar. You have two very short exposures here which is not telling us a lot. Perhaps if you both shot the rosette or similar we might have a better guide. What type of cameras are you using and are there any reducers/FF barlows etc being used by either of you. Also be intrested in the type of mounts you are both using.
The relatively short exposures takes the mount out of the equation.
What I was interested in here was the intrinsic sharpness of each type of scope. Is one design intrinsically sharper than the other.
So short exposures of a star cluster will work quite well for an optical comparison, I think.
I don't know about Bratislav, but I was not using any FR or FF or CC, just a pair of mirrors and a camera.
I also don't think that the camera will make much of a difference, as long as the image is well sampled.
Hmm... the Rosette with a 1600mm scope, you won't see much of it unless you have a really big CCD (I tried it once), I just took a pic of this with my ED80 and the ST10, still couldn't fit it all in!
I really can't see much diference between these two images...... The second one has a hot pixel...... That is how hard it is to pick a diference. Background brightness makes the two images appear diferent but that is about it. At best the second image has a little more contrast between stars with the bright stars standing out better against the star field.
Just one note - Stuart used the adaptive optics (I don't know at what guide rate). Mine was guided via separate scope at 0.5 Hz. The sub is pure raw, no dark/flat/bias applied and no sharpening of any kind - straight out of the camera.
I really can't see much diference between these two images...... The second one has a hot pixel...... That is how hard it is to pick a diference. Background brightness makes the two images appear diferent but that is about it. At best the second image has a little more contrast between stars with the bright stars standing out better against the star field.
Now tell us which one is which.
Hi Doug, the one with Bratislav in the name is his, the one with my name in the name is mine, I suppose I could have been very cunning and swapped them, but I didn't.
Quote:
Originally Posted by bratislav
Just one note - Stuart used the adaptive optics (I don't know at what guide rate). Mine was guided via separate scope at 0.5 Hz. The sub is pure raw, no dark/flat/bias applied and no sharpening of any kind - straight out of the camera.
From my very poor memory, I think that I was using something about 1-2Hz, there aren't many good guide stars in the region, so I may have also been in the 0.5Hz region as well, I should write this stuff down somewhere. The background difference may due to the fact that your sub is not calibrated.
Quote:
Originally Posted by multiweb
Altough those pictures are very close you can see the RC has rounder stars and better overall field off axis. In this particular case.
That could be the influence of the AOL. I also had to crop off the outer bits of Bratislav's picture to match my FOV, so we should be getting the central, best part of the illuminated circle.
The RC is a `coma free' design. Any Newtonian user would use a low cost coma -corrector ( ie Baader) if they were doing photography, so comparison with a Newt without a coma-corrector is not really saying anything much. BOth designs are equally sharp on axis , provided the optica are made to spec.
Altough those pictures are very close you can see the RC has rounder stars and better overall field off axis. In this particular case.
I guess people continue to read and see whatever they fancy.
Did you maybe miss these two crucial sentences from Stuart :
"I used a touch of sharpening on mine as it showed some seeing induced errors "
while mine is as raw as it can be, and :
"I have cropped the field showing part of the core and the best focussed bit of mine"
It didn't occur to anyone that this could well be the worst part of my picture (I had issues with collimation too, see CCDinspector analysis) ? And didn't occur that this area could well be on axis for Stuart (e.g collimation error) ? Let's say we compare the upper left quadrant now ?
And no comment on how Newtonian image looks brighter, despite half the exposure (60 vs 120 second) and el-cheapo OSC (lowly QHY-8) vs SBIG flagship, QE king of the hill ST10-XME ?
No commenting either on adaptive optics used on RC (by SBIG's own admission increasing sharpness and peak brightness by up to 30%) while my guiding is done via separate guide scope, prone to flex and all sorts of problems ?
In any case I'm done with this, feel free to think and believe whatever you want. Bash the Newtonians, buy RCs and be merry.
That is the beauty of forums, I suppose. It takes just too long to get that through this thick skull.
Last edited by bratislav; 25-01-2010 at 12:26 PM.
Reason: add a few things
I guess people continue to read and see whatever they fancy.
Did you maybe miss these two crucial sentences from Stuart :
"I used a touch of sharpening on mine as it showed some seeing induced errors "
while mine is as raw as it can be, and :
"I have cropped the field showing part of the core and the best focussed bit of mine"
It didn't occur to anyone that this could well be the worst part of my picture (I had issues with collimation too, see CCDinspector analysis) ? And didn't occur that this area could well be on axis for Stuart (e.g collimation error) ? Let's say we compare the upper left quadrant now ?
And no comment on how Newtonian image looks brighter, despite half the exposure (60 vs 120 second) and el-cheapo OSC (lowly QHY-8) vs SBIG flagship, QE king of the hill ST10-XME ?
No commenting either on adaptive optics used on RC (by SBIG's own admission increasing sharpness and peak brightness by up to 30%) while my guiding is done via separate guide scope, prone to flex and all sorts of problems ?
In any case I'm done with this, feel free to think and believe whatever you want. Bash the Newtonians, buy RCs and be merry.
That is the beauty of forums, I suppose. It takes just too long to get that through this thick skull.
There were two reasons for the crop. Firstly to get the images down to under 200k so I could post them, and secondly so I could show a region of the image that seemed to be pretty good for both telescopes. I tried to be fair with this. Bratislav's image has been rotated through 54.577 degree CW and expanded by 1.347 to match my image scale. I tried this both ways and it didn't seem to matter much, but I though that interpolating my image may have sharpened it up a bit, so I didn't.
If you look at your CCDInspector plot I think I have used an area that is still very good (blue to dark blue).
The sharpening I did was to fix some blur from either poor focus or seeing, it did not change the star shapes, just removed some low level blur from the image. I saw this as analogous to imaging at a shorter focal length, where guiding and seeing effects are lessened.
I'll leave to comments on the cameras, as I did say I didn't want this to be a pissing contest, those comment are clearly bait, so I'll ignore them (it's hard, but I should follow my own rules).
I have posted a crop from the upper left quadrant, which shows clearly that my collimation was off. Because I didn't save the psd files these have had different amounts of stretching done to them than the original ones, which shows that processing makes much more difference than the design of your optics to star brightness.
The relatively short exposures takes the mount out of the equation.
What I was interested in here was the intrinsic sharpness of each type of scope. Is one design intrinsically sharper than the other.
So short exposures of a star cluster will work quite well for an optical comparison, I think.
I don't know about Bratislav, but I was not using any FR or FF or CC, just a pair of mirrors and a camera.
I also don't think that the camera will make much of a difference, as long as the image is well sampled.
Hmm... the Rosette with a 1600mm scope, you won't see much of it unless you have a really big CCD (I tried it once), I just took a pic of this with my ED80 and the ST10, still couldn't fit it all in!
Cheers
Stuart
Why remove the mount? This is the reason why newts are difficult to use in astrophotography.
One would assume the newt had better mirrors as they are easier to make in respect to the two scopes on test here.
Agree
OK, just asking as both coma and field flattness must be assessed on both systems.
Agree but low well depth can make stars seem blotted and sensitivity of each camera should be taken into consideration.
If I remember correctly matching FOV and image scale is as easy as adding a high quality barlow to the newtonian so it should be possible . Perhaps a snap shot of the core would do it.
Stuart I don't see how this can be a even test to gauge the quality of both these scopes. Put them on the same mount at the same time with the same guiding at the same image scale with the same camera at the same place and then you might get a better idea of what is happening.
The reason to remove the mount from the equation is because I was interested in a previous statement that the RC was intrinsically sharper the Newt. This statement seemed to create a bit of a stir amongst the Newt community here, and an image was posted to show how sharp Newts can be.
Now, I'm a scientist, I could see that this could be measured, if we had two scopes, same target, same night, same camera, any differences would be in the optics as long as the exposures were short enough to exclude tracking errors (as the Newt mount and the RC mount might not be the same). This is our best approximation, same target, different night, but from close to the same location, different camera and different image scale.
It's not a perfect comparison, but I think it clearly shows that as long as the optics are good, then the pictures produced will be good. Many other influences (mount, collimation, focus etc.) will have far bigger effects on your photographs than the design of your optics.
The reason to remove the mount from the equation is because I was interested in a previous statement that the RC was intrinsically sharper the Newt. This statement seemed to create a bit of a stir amongst the Newt community here, and an image was posted to show how sharp Newts can be.
Now, I'm a scientist, I could see that this could be measured, if we had two scopes, same target, same night, same camera, any differences would be in the optics as long as the exposures were short enough to exclude tracking errors (as the Newt mount and the RC mount might not be the same). This is our best approximation, same target, different night, but from close to the same location, different camera and different image scale.
It's not a perfect comparison, but I think it clearly shows that as long as the optics are good, then the pictures produced will be good. Many other influences (mount, collimation, focus etc.) will have far bigger effects on your photographs than the design of your optics.
Cheers
Stuart
Hi Stuart
Yes I know what you mean about the ruckus. When I said the same mount I mean't the same mount i.e something big enough to carry both the newt and Rc whilst being guided by the same system. If you got two cameras the of the same type (i.e. SBIG ST10) then the only difference would be between the optics themselves and the minor differences between the cameras.
I have never doubted the quality of the newtonian mirrors but do recognise the difficulty and expense of mounting them properly. Seems to me everytime this argument comes up the newt boys avoid this issue preffering to concentrate on the cheap optics and comma correcter aspect.
I'm sorry that you felt that my comments were against you, that was certainly not my intention. If I didn't think it was worth bothering I would not have sent you the raw image. I am too interested to see the result, even more so as you use state of the art equipment (which as you well know I tried very hard to get at the time).
I am well aware that you tried to be as far and impartial as possible, it is very hard to match the images of different exposures and scale.
I am just getting tired from armchair expert's comments who demand this and demand that and then switch to something completely different. And constantly try to pick whatever suits them to argue their preferences. I to try to ignore them, but it is getting frustrating for me too.
Clearly some people can't use Newtonians - my advice is stay well away from those then. But that can't be a general rule - there are sharp astrophotos coming from Newtonians.
You had the advantage of adaptive optics, same instrument guiding (off axis), longer focal length, smaller pixels/higher QE CCD and supposedly "sharper" instrument (RC) (not your claim, I know). Yet, Newtonian image is just as sharp, if not sharper (see Terry's enhanced crops) than those from RCs. But all of a sudden this is now not enough, we have now to spend months with another object, then another.
Sorry, I have better things to do.
Provided both Newt (+ coma corrector) and RC are equally well made, you will see bugger all difference.
The mount and the seeing will make a world of difference. The attached images were taken with the same telescope/camera/mount. The only difference was the seeing improved markedly after the passage of a cold front.
. Perhaps a snap shot of the core would do it. 
