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.
That is why we are comparing examples of best FWHM's. Sooner or later everyone gets a decent seeing.
This is like comparing apples and oranges, all is not equal with gear, seeing etc....Both scopes have there merit and disadvantages.
Just check out the spot sizes in these pages at Dream Scopes. Don`t really need to know any more.
The F/8 newt has the best spot size but geezs it would be a bugger to mount and weigh a heap compared to the RC, not real practical in most situations
The F/8 newt has the best spot size but geezs it would be a bugger to mount and weigh a heap compared to the RC, not real practical in most situations
I suggested that an off the shelf F4 or F5 Newt with a quality barlow lens for less than half the price would be worth looking at if you wanted to do some longer focal length imaging . If I was on a budget I certainly would.
As we speak I know of 3 high end rigs being constructed in this country using 12" to 14" Newts and high end cameras ( Paramounts, Asrophysics mounts , plus ST11000 cameras etc ) so
the image quality of well constructed coma corrected Newts as astrographs is not unknown amongst advanced astro-photographers.
As we speak I know of 3 high end rigs being constructed in this country using 12" to 14" Newts and high end cameras ( Paramounts, Asrophysics mounts , plus ST11000 cameras etc ) so
the image quality of well constructed coma corrected Newts as astrographs is not unknown amongst advanced astro-photographers.
Indeed, I drooled over the ASA astrographs when Mike had one. If only they could have fixed that flexure problem. Anyway, time moves on so I found the RC at significantly less than the ASAs, easier to use (I have trouble balancing this scope on a GEM, I think I would have struggled seriously with a Newt) and a bit more focal length, which I have always liked.
Strange thing is I like to image galaxies, but they are difficult to get right compared to extended objects like nebulae, they also tend to be dimmer. What I really need is a huge F20 scope, but that's not going to happen. So I ended up with an 8" f/8, which allows me to do some of the smaller extended objects and some of the larger galaxies, the small ones will just be small. I also have the 10"SCT for planetary imaging and an ED80 for really extended objects. So now I know that if I want something at about 8-10" f4-6 I can go Newt shopping, get a coma corrector and I'm away.
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.
Hey Bratislav, no problems, just though I'd better stop the contest before it got started. Actually CCDStack did a pretty good job at this, I just read off the numbers. My equipment isn't state of the art anymore, which is almost certainly going to cost me more money sometime
Quote:
Originally Posted by bratislav
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.
Let the armchair experts sit in their armchairs, I like to sit in my garden chair in my observatory, taking pictures...
Quote:
Originally Posted by bratislav
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.
I don't think imaging at longer FL is an advantage, infact it would be the biggest disadvantage of them all, everything is much easier at shorter FLs. Smaller pixels is also a disadvantage, I'n imaging at a pixel scale of 0.87"/pixel, way oversampled, yours would be still oversampled, but not so much, Craig Stark has been writing on Cloudy Nights about this very point. So I reckon that's 2-all for advantages/disadvantages, which makes it a reasonably fair comparison. Terry's crops should not be taken too much to heart, I assume that he used the jpgs off the website, which are 8-bit, and stretched them, not really the best thing to do to images.
Still waiting for some 47Tuc subs from a CDK (c'mon Theo) and a refractor to do a real comparison.
Terry's crops should not be taken too much to heart, I assume that he used the jpgs off the website, which are 8-bit, and stretched them, not really the best thing to do to images.
Well there was enough there to show a clear difference in star sizes. I also realise there are other variables involved..but we weren't able to assess the effect of those given that they were made at 2 different times and locations, with 2 different camera/scope combinations. I suppose this is why spot diagrams are so useful.
I don't think imaging at longer FL is an advantage, infact it would be the biggest disadvantage of them all, everything is much easier at shorter FLs. ......
To get truly high resolution, you really do need focal length.
Sure to get an OK looking image, short FL's are very user friendly. Seeing. mountings, tracking errors etc. are all masked with modest conditions and equipment.
Small pixels are not without their problems...scattering and shallow well capacities don't help.
High-res deep sky however is a challenge (for me at least)...with good seeing more often than not playing a pivotal role.
I suggested that an off the shelf F4 or F5 Newt with a quality barlow lens for less than half the price would be worth looking at if you wanted to do some longer focal length imaging . If I was on a budget I certainly would.
As we speak I know of 3 high end rigs being constructed in this country using 12" to 14" Newts and high end cameras ( Paramounts, Asrophysics mounts , plus ST11000 cameras etc ) so
the image quality of well constructed coma corrected Newts as astrographs is not unknown amongst advanced astro-photographers.
So correct me if I am wrong Mark but are you saying I can use a cheap newt and coma corrector to image as long as I put it on a 15 - 20K mount .
Mark
Yes, Peter that is absurd. I should have put a disclaimer in there that you still have to be sampling at about 1/2 you're seeing, up to about 2"/pixel seems to work well, so I could easily shoot at 800mm rather than the 1600 I shoot at. The only thing that will suffer is image scale.
Quote:
Originally Posted by Peter Ward
To get truly high resolution, you really do need focal length.
Sure to get an OK looking image, short FL's are very user friendly. Seeing. mountings, tracking errors etc. are all masked with modest conditions and equipment.
Small pixels are not without their problems...scattering and shallow well capacities don't help.
High-res deep sky however is a challenge (for me at least)...with good seeing more often than not playing a pivotal role.
To get high resolution you need to satisfy Nyquist. If your seeing is exceptional, you'll get better results as long as you're sampling rate is high enough. If you've got a telescope in outer space then the resolution is defined by your optics, the mount is also rather expensive and the installation astronomical, possibly out of an airline pilot's range even...
To get high resolution you need to satisfy Nyquist....
Sorry, that's not quite right, 2 arc sec per pixel would result in tragic planetary images hence high res-planetary imagers use FL's that sample the sky well beyond Nyquist .
All I'm saying is you simply can't divorce the seeing that easily from the equation. Oversampling through longer FL's can and does give tighter stars and also helps with deconvolution.
I suggested that an off the shelf F4 or F5 Newt with a quality barlow lens
I have a 10" newt project lumbering along and initial FOV shots with the original tube and my little starlight camera look good to me but what barlow would you suggest. Ive never owned one and don;t know what a quality barlow is?
Sorry, that's not quite right, 2 arc sec per pixel would result in tragic planetary images hence high res-planetary imagers use FL's that sample the sky well beyond Nyquist .
All I'm saying is you simply can't divorce the seeing that easily from the equation. Oversampling through longer FL's can and does give tighter stars and also helps with deconvolution.
Well Peter, if you can take deep sky shots at 60fps then knock yourself out. It is only that and the stacking of hundreds of exposures that freezes out the seeing. As long as the image is adequately sampled, longer focal length on influences image scale. Some level of oversampling is required for deconvolution (particularly Maximum Entropy based algorithms) to work properly. As soon as the exposures ar more than a second or so you are at the mercy of seeing and Nyquist holds quite well. Like I said there is a good series of articles from Craig Stark at CloudyNights, read them for a proper explanation.
Robin - A Televue 2" 2x or 3x barlow is a good quality barlow...
Mark mentioned something in here that I mentioned earlier...
You just can not accurately compare two different optical systems with the method being used here...
Different mounts, different cameras, different seeing, different guiding (AO beats the pants of a separate guide scope, even at 0.5hz.. when you have a nice bright star and you operate it at or above 5hz you really get flawless images)
Mentioning the difference in brightness of the image despite the QHY8 being nowhere near as sensitive as the ST10 is pretty silly.. You have 10" of aperture at F/4.7 vs 8" at F/8... Your image is going to be brighter..
Everything has to be equal. mount, guiding, cameras, seeing, exposure duration... The works.. Otherwise this comparison will turn into "Yeah, but you have AO and a fancy pants CCD" "But you have shorter FL so less seeing and guiding induced errors" "Yeah, well my dad has a tank with infinity rounds and he'll blow your scope up"
Well Peter, if you can take deep sky shots at 60fps then knock yourself out. It is only that and the stacking of hundreds of exposures that freezes out the seeing. ....
Well....I wouldn't for different reasons ...that said Ed Grafton has taken many excellent Hi-Res planetary images using a few frames from a humble SBIG ST5... illustrating video rates are not required.
Also wanted to mention that there are many people using Newtonian telescopes on all kinds of mounts and achieving good results. A good quality fast 10" newt could be used on something like a G11 no worries.. and an 8" could be used on a GM8/HEQ5 without too much problem too provided you took the time to organize it all right...
A newt is no more difficult to mount than a long focus refractor. if anything, the refractors are harder, as their weight is spread apart further.. a 6" F/8 refractor holds most of its weight right at the front in the lens cell, the rest of the weight is right at the rear of the scope in whatever camera you decide to put on it... These would be 1.2M apart, the scope would easily weigh as much as a 8" F/3.6 astrograph, however the 8" F/3 would be shorter, and the distance between the weight would be even shorter again.. You don't need monolithic mounts to image with newts, you just need to know your mounts limits... Ie - 12" + EQ6 = No Go.
If you disagree, Speak to Clive (Alchemy). He imaged with a 12" F/5 newt on a G11 for quite a long time, and his results really spoke for themselves..
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...that said Ed Grafton has taken many excellent Hi-Res planetary images using a few frames from a humble SBIG ST5... illustrating video rates are not required.
