Newt and RC comparison

[rat156]

Hi All,

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!

Cheers
Stuart

[AlexN]

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..

Do we know what brand the 10" Newt is?

[marki]

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.

Mark

[rat156]

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

[Hagar]

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.

[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.

[tlgerdes]

Did you forget Fear and Suprise?:lol::lol:

[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. :)

[rat156]

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.



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.



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.

Cheers
Stuart

[Satchmo]

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.

[CometGuy]

If you stretch the images a little you can see the differences a bit more clearly. 10" Newt on the left and 8" RC on the right.

[bratislav]

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.

[rat156]

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.

Cheers
Stuart

[rat156]

I can't download the image, or expand it. At the present scale it's a bit blurry, so I can't really get your point.

Cheers
Stuart

[marki]

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 :P;). 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.

Mark

[rat156]

Hi Mark,

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

[marki]

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.

Mark

[Satchmo]

An RC is just a faster than normal ( usually F3) 'overcorrected' Newtonian primary teamed up with a hyberbolic curved secondary ;)

[bratislav]

Stuart,

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.

[Peter Ward]

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.

[bratislav]

That is why we are comparing examples of best FWHM's. Sooner or later everyone gets a decent seeing.

A bit sooner for people with AO ;)

[Garyh]

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


http://www.dreamscopes.com/pages/projects-04/ccvrc-08.htm
http://www.dreamscopes.com/pages/projects-04/ccvrc-07.htm
http://www.dreamscopes.com/pages/projects-04/ccvrc.htm

[Satchmo]

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.

[rat156]

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.

Cheers
Stuart

[rat156]

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:sadeyes:



Let the armchair experts sit in their armchairs, I like to sit in my garden chair in my observatory, taking pictures...



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.

Cheers
Stuart

[CometGuy]

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.

Terry

[Peter Ward]

I disagree for various reasons....

To carry the short FL argument to its absurd conclusion you only need look here http://www.atscope.com.au/BRO/gallery38ro.html
(a few hundred mm versus about 6 metres FL)

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.

[marki]

As do I but mine is a simple plastic chair :P;).

Mark

[marki]

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 :shrug:.
Mark

[rat156]

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.



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...

Cheers
Stuart

[Peter Ward]

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.

[Tandum]

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?

[rat156]

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.

Cheers
Stuart

[AlexN]

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"

Calling it as I see it..

[TrevorW]

and the final word

Nah !! nah !!! nan AAh :P:P:P :screwy::screwy::screwy:

[Satchmo]

No. The high end Newts I mentioned have conical mirrors, carbon fibre components etc.

[Satchmo]

Try a 2" Televue 2X 'Powermate' Its not a `Barlow' design but I think its 3 element and highly regarded.

[Peter Ward]

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.

[AlexN]

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..

[Aster]

10" F4.7 Skywatcher, Baader MPCC, EQ6, Unmodded 400D

[Peter Ward]

Agreed.. RC's are less taxing on a given mount (which is more than half of the equation IMHO).

An added benefit is the rear cells of most RC's have far less flexure with a CCD/filter wheel/AO etc combo being cantilevered off their cells compared to a Newt's ( often thin) sidewall.

[JohnH]

Is this comparison not a little apples vs oranges?

Many well respected "Astrographs" out there are "Newts" (well in layout anyway) - eg the TAK Epsilon 180 is regarded as peerless, there are others such as the Orion UK models like the AG8. To me it seems that (in practice at least) a well corrected Newt can perform very well - at lower F numbers - so perhaps the conclusion is if you need F8-10 go for RC if you want F3-5 then a well corrected newt is the better option. Or have I got it all wrong?

[gregbradley]

There are many incredible Newtonian images around. The conical mirror is easy to make so its easy for manufacturers to get really high accuracy with modest manufacturing.

RC optics are hard to make hence the high cost. Their advantage though seems to be long focal length and no coma. They are also relatively compact and the camera is at the end not on the side of the tube.

The disadvantage of a Newt as I see it is they are very long for their focal length and the camera has to be mounted on the side opening the door to flexure issues, and length of backfocus for all the accessories used today.

The side mounted camera and short backfocus problems were highlighted with the ASA scopes where flexure and focus slop were a problem (apart from the mirror mount etc). Tubes are only so strong even carbon fibre.

But when the ASAs got it right they were awesome in the 1200mm focal length band.

RCs and similar designs though seem to totally dominate the long focal length/galaxy type imaging scope area. Not so great for widerfield though.

I see A&M has a compromise RC which is F5 and wide field yet an RC 360mm aperture I think it is. It probably is a simple matter to extend the focal length if you wanted to do galaxies.

Ceravolo astrograph is an attempt to service both widefield and long focal length and seems to do very well. But its very expensive.

Planewave CDK seems to the be the current market leader in long focal length compound scopes.
Someone mentioned wanting to see CDK20 images. There is one good one of the Jellyfish neb on their site but here are a few from Bob Fera who is a great imager using a CDK17:
http://www.feraphotography.com/CDK17/HH.html

Click through his site to see more examples. Impressive. Looks more like an RCOS 20 inch result.




Orion Optics UK, A&M, RCOS, Ceravolo, Starizona Hyperion, Deep Sky Instruments are other hot instrument makers. It is quite competitive these days in this band.

Greg.