RC's and Newts and Theory

[Outbackmanyep]

It seems to have a HUGE secondary! Is this normal for a RC?

[netwolf]

The larger secondary provides a faltter field. Good for DSO not so good for Planet imaging.

[Gama]

Provides a Wider illuminated field.



Theo.

[toryglen-boy]

i dont know much about the RC design, or the pros and cons to it. Would this better than an F5 200mm reflector for DSO imaging?

thanks

[Octane]

Duncan,

Depends on what focal length you want, really.

I'm sure there's more to it, but, essentially, for a lot of people, that's what it'd come down to. Approximately 2 metres focal length compared to 1 metre.

Regards,
Humayun

[pmrid]

Unequivocally yes.
Peter

[gbeal]

Unequivocally yes, but only if your mount, your guiding etc all works OK at the longer focal length.

[toryglen-boy]

thats quite a resounding answer Peter, with no explanation !!

care to elaborate?


;)

[AlexN]

True... Despite it being lighter than a 8" F/6 newt, weight isn't the only thing that dictates whether or not a mount will accurately track for photography... I know with my current rig I can achieve 30 minute guided exposures no worries with the 480mm focal length of my scope. however I dare say the 8" RC on the HEQ5 would be difficult to achieve 10 minute subs, and being an F/8 system, I'd say 10 minute subs would be about the minimum you'd want to run in the 8" RC to achieve very nice images...

I would love to get another one one day.. We'll just have to wait and see how things go..

[toryglen-boy]

i hear you, i will stick to my 200mm F5 reflector i think, when i use the MPCC i get a pretty flat field anyway, and its more forgiving on guiding.

:)

[AlexN]

Yep... Providing your optics are sharp, and the field is flat, there is not much reason to go to the RC unless you want much longer focal length... The RC will better illuminate a DSLR sized sensor, that said, an 8" F/5 newt will effectively illuminate the sensor enough that flats will take care of any vignetting...

[toryglen-boy]

Thanks for clearing that up Alex, its just as i thought.

:thumbsup:

[gregbradley]

It depends on what type of image you want to produce.

F8 200mm = 1600mm focal length = more closeup more highly magnified than F5 200mm = 1000mm focal length.

A common and very useful focal length for imaging is 1200mm.

F5 is a preferable focal ratio than F8. Not that there is anything wrong with F8 but it is more demanding on accuracy of tracking and will give more closeup views of objects. Maybe you prefer widerfield images. F5 will also get a bright image faster - more than twice as fast as F8 for the same aperture. So that may be good for your imaging requirements - perhaps you live in area where clouds often interrupt imaging. Well F5 is more productive than F8.

Collimation requirements for both will be important but a Newt is easier to collimate than an RC.

Then again F5 200mm will not capture too many galaxies whereas F8 at 200 will get quite a few.

So different strokes for different folks. No right or wrong about it but what type of image would you like to produce is the primary question coupled with what environment are you imaging from. Also what size camera chip as the bigger the chip the more you need a large corrected field which only some configurations of scopes can provide.

For a DSLR I imagine you are in safe territory with either scopes.

Also longer focal length often means a longer tube which is more wind affected.

If you travel to a dark site to image then F5 200mm would be far more productive and less likely to result in a nonproductive trip is clouds are hanging around. F8 200mm would require clearer less windy conditions to be productive but you could image a nice galaxy shot with it or a closeup of some DSO.

For less experienced imagers shorter focal length is always the way to go as the longer the focal length the more accurate everything has to be and the longer the exposure time to get a decent image.

Personally I think you would find most people would prefer an image taken from an F5 200 Newt well setup to a F8 200 RC. You'd get more positive feedback on most images (not all).

That is my experience anyway.

Greg

[toryglen-boy]

Thanks for that Greg

:thumbsup:


Yeah, i am gonna use the F5 Reflector, it serves me very well, and if i wanna view planets, then i can use my 12" dob.

infact, i have just put the 200mm SCT in the IIS classifieds off the back of these replies !!


:lol:

[Peter Ward]

Greg, your thoughts are no doubt appreciated by many and while it may seem I'm on your case, mate, I'm not, this was simply not right.



This old chestnut is false. With CCD imaging the limiting stellar magnitude and extended object S/N ratio is determined by aperture alone. I think it was Russ Croman... who did a pretty comprehensive analysis on SBIG users as to the how and why.




Again false. The GSO RC's have adjustable secondaries only. (the rest it locked in at the factory) . Newtonians have both primary and secondary adjustments.




Again false. In all Cassegrains (ie RC's), the light path is folded back through the primary..OTA length = FL divided by 3 or about 21 inches. Newtonians don't, so the OTA's are similar in length to the FL , 8" F5 so about 40 inches.

[Satchmo]

Hi Peter

I would have to support Greg on all three counts, but happy to stand corrected with convincing facts.

1. He didn't specify particularly limiting magnitude of stellar objects so we can assume that he had speed of imaging extended objects like nebulae and galaxies in mind . An F5 optics has an image intensity 2.5X greater than an F8 for the same aperture, so recording speed of extended objects will have to be similarly quicker. This applies to nebulous extended objects only.

2. Collimation of the GSO RC's depends on the optical center of the primary coinciding with the optical center of the squared on focusser. If there is any decenter error there from the factory , no amount of secondary angle tweaking will help. Are there any user adjustments for either of these mechanical adjustments ( SCT primaries are spherical so not as sensitive to decentering). There is nothing to stop manuafcturers making fixed primary mirrors in Newtonians so this potential 'feature' is not a privelage of RC scopes.

3. The short tube will help but an F8 scope is going to be more sensitive to vibration han an F5 whichever way you look at it as it is operating at high prime focus magnification.

[Peter Ward]

Mark,

1) I was incorrect in quoting Russ as the author of the F-ratio myth...it was in fact Stan Moore. Interested punters can read about it here:

http://www.stanmooreastro.com/f_ratio_myth.htm

Do you disagree with Stan's analysis?

2) You are assuming the primary is decentered by the factory. I've tested close to a dozen of these scopes and not one had a problem. Also last time I checked an F5 of anything was far more of a bugger to collimate than a F8

3) The point Greg made was the *tube was longer*.

It is patently not,

and will suffer less from wind loading as a result....though I do take the point the longer focal length could be more sensitive to seeing and tracking errors.

[multiweb]

What an interesting thread. :thumbsup: Got to agree with Peter on this one.

1)_ Recently attented an astro-photo meeting at the ASNSW and Mike BJ did a talk that reflects exactly what Stan Moore is talking about.

2)_ Don't know much about RCs, I trust Peter had enough in his hands to play with and knows what he's talking about. but I know my 5" f/5 newt is very fiddly to collimate from scratch.

3)_ Longer FL harder tracking. Longer tubes catch the breeze. That's a given :)

[Satchmo]

Peter

I had a quick skim of the article. I was distracted by the fact that you mentioned specifically stellar limiting magnitude. So it seems it matters not to the CCD that the physical image at the focal plane will be 2.5X brighter in the F5. I am a bit of planetary nebulae nut and its been my closet dream to make a photographic catalogue of far southern planetaries. I guess this means with a CCD I can shoot at F16 with no loss of speed over an F4, which makes my project all the more attractive.

Regards collimation: I would have been very nervous about purchasing a budget telescope with highly aspheric surfaces where you couldn't collimate the primary element but your findings allay my fears.

I don't really see the difference in difficulty of collimating a small Newtonian against a SCT or GSO RC. The secondary you don't routinely touch after it has been aligned, and at F5 a common or garden laser collimator gets you close enough. At F4 to F4.5 a squizz at high power at a defocused star tells you which primary collimation knob to move just as you would do to tweak the secondary on an SCT or RC.


[QUOTE=Peter Ward;490652]Mark,

1) I was incorrect in quoting Russ as the author of the F-ratio myth...it was in fact Stan Moore. Interested punters can read about it here:

http://www.stanmooreastro.com/f_ratio_myth.htm

Do you disagree with Stan's analysis?

2) You are assuming the primary is decentered by the factory. I've tested close to a dozen of these scopes are not one had a problem. Also last time I checked an F5 of anything was far more of a bugger to collimate than a F8

[Peter Ward]

Time for a group hug? :)

[gregbradley]

Yes that is a workable truth about the aperture and F ratio and exposure time. But like any theory I am sure there are deviations due to other factors and not all chips have exactly the same characteristics but yes I agree it is a workable rule that two 200mm scopes will expose the same brightness for the same time even if different f ratios. So if you want faster exposure times you up aperture. Although I know from an actual test that a Tak ED180 at F2.8 exposed to same brightness an image 2.5X quicker than an F5 FSQ106. Now that is 74mm more aperture but there is also a central obstruction. Not sure the total area of the aperture is over 2.5X. Probably more like 4% more area.

Collimation is not that big a point either for both scopes. F5 Newt can be tricky with a star test but if you use a cheap laser collimator it takes about 4 minutes (I used to have a Vixen R200SS F4 200mm). The RC optics though I had (RCOS 12.5inch) did require several steps including aligning the mirrors rotation-wise and using a Tak collimating scope as it was pretty impossible without but not hard with it ($400 collimating scope). The GSO may be simplified. I would not scare off based on collimation as it is a skill that should be learned anyway.

I am not criticising the GSO which is great bang for your buck. The question was whether to change the F5 200mm Newt for one.

What my main point was imaging scale. F5 200mm will give a wider field of view and that makes everything easier and more pleasing an image to more people. But then a lot prefer to image at a closer up scale so its what type of image you want to produce.

Greg.

[Satchmo]

Studying Stan Moore's 10 minute photograph of F3.8 vs F12 I see that the signal to noise is worse on the F12.4 ( as might be expected) but the resolution is only ever so slightly better.

I'm now convinced that it is better for me to go for an F3.5- F4 Coma corrected Newt than a long focus Cass as I will have a much larger field of view available for those larger objects and I will not be losing anything significant in resolution for those smaller objects ( I guess seeing is a factor here also). I can also use a 2" 2X Televue Barlow for F7-F8 should I need to take advantage of really fine seeing .

Thanks for posting that very interesting paper , Peter. CCD is certainly a whole other world from film.

[Paul Haese]

Just a point here. I think that my GSO RC has primary adjustment screws on it. Located at the rear are three hex headed screws 120 degrees apart on the inner mounting area. I am assuming this is either collimation adjustment or is it just the locking nuts to hold the primary in place? If it is the later how would they have maintained optical alignment. Surely this is conducted by way of a spring forcing against the primary. In which case primary collimation is possible???

[avandonk]

Hope this helps.

http://www.clarkvision.com/photoinfo/f-ratio_myth/

It is all about signal to noise ratio. Not brute strength.

Bert

[bratislav]

Yes, I do. For a given camera (i.e. given pixel size) and for those objects that are just above noise threshold, faster scope delivers more photons per pixel, which will result in a superior S/N. In fact it may be a matter of detecting the said nebulosity or missing it altogether.

Try those superfaint galactic Mandel-Wilson nebulae with your R-C and your STL11K one day :P

[Peter Ward]

:doh:

Sure, but the contribution from the sky background also *increases* as the f-ratio lowers. This lowers the S/N level. :P:P

Stan's analysis is quite sound.

[gregbradley]

Yes I agree with you Bratislav and I do think that Stan's paper is an
overall guide but it needs another datum to be fully practical although he acknowledges
the higher F ratio scope will give a lower signal to noise ratio.

My experience with a BRC250 at F5 and an RCOS 12.5 at F9 says that you can get a bright image from the BRC in 2.5 hours but the 12.5 at F9 will require more like 6 hours. Yet the BRC has less aperture. But the RCOS was a more zoomed in view of the object so its collecting a smaller slice of the available photons so the light is less compared to the nosie from the CCD itself.

And that is the point, at F9 you are looking at a small slice of something but with the same noise.

I must make an experiment of this some time to satisfy this point more clearly.

12 inch ASA at F3.6 clearly captures more nebula than similar close up images from 12.5 inch F9 RC's this is evidence that Bratislav is correct.

The BRC250 will also pick up faint nebula that other images I have taken of the same object with the 12.5 inch RCOS did even with longer exposure times. Different cameras nothwithstanding.


Greg.

[Peter Ward]

Well, this thread is certainly meandering!

The main limitation here is the ability of the camera to discriminate low contrast features deep in the sky background.

In practice, it is the seeing.... *more than any other factor*.... that will determine this.

Of course if I see some alternative (mathematical) rigor (none here so far :( )
that proves otherwise.... I'll happily revise my stance.

[Peter Ward]

Sorry...I had to think about why this was incorrect.....

This is the same as saying: increasing the pixel size will increase the S/N.

Bollocks :) (haven't said that for a while :) )

System gain indeed goes up, but the S/N does not.

[sjastro]

F-ratio = Focal Length/Aperture.

If you vary the aperture and keep focal length constant, then S/N ratio is a function of the f/ratio.

However the point to Stan Moore's article is what happens when you vary the focal length and keep the aperture constant. In this case S/N ratio is not a function of f/ratio.

S/N ratio is a function of the aperture or area of the primary mirror or lens. For example a 10" mirror at f/5 and a 10" mirror at f/10 will both collect the same number of photons, the only difference being the photons reach focus at different distances from the mirror.

Note that this does not apply when imaging using eyepiece projection. The S/N ratio is a function of the diameter of exit pupil.

The diameter of the exit pupil = focal length of eyepiece/ F-ratio.

Steven

[Peter Ward]

Clear an succinct. I get distracted by the details. Bless you Steven... :)

[Satchmo]

What practical effect is there then in having 4 times the # of photons hitting a CCD pixel per unit of time given two telescopes of given aperture , same camera and one at F8 and one at F4?

[sjastro]

The number of photons hitting a given pixel is determined by

(1) The brightness of the object being imaged.
(2) The aperture of the telescope.
(3) The exposure time.

All things being equal if f-ratio is altered by focal length, the pixel scale is the only factor that is changed. This leads to a change in the size of the field of view.

Regards

Steven

[Peter Ward]

Steve..would a rain gauge analogy be a good one? vis

x amount of rain falls from the sky (aperture), and we measure it with a rain gauge (small pixel), and say a swimming pool (large pixel)...in either case the level goes up by only x mm.

[sjastro]

I think the analogy is fraught with danger. The rain gauge can overfill (equivalent to saturation) and cause all sorts of complications.:)

The point is if you reduce the f/ratio by reducing the FL, and keep the exposure and aperture equivalent, you are not adding to the total signal hence the noise in the image (the statistical variation of the signal) is the same irrespective of the f/ratio.

The photons will fall over a smaller number of pixels and while it is true that any given pixel will experience an X-fold increase in the number of photon hits, the noise per pixel increases by the same factor.

Hence the signal to noise ratio doesn't change.

Regards

Steven

[iceman]

This new thread has been created for the continuing discussions about the optical/photographic theory of RC's, Newt's and CCD's.

[Satchmo]

Steve

So as the raw image projected on the CCD chip is 4X as bright on the F4 and an F8 scope, won't the brightest parts of the object fill the CCD pixel to full well capacity much faster . So if you can get more exposures in doesn't more stacked images that are at full saturation mean better signal /noise ?

I'm on a learning curve here , but this is one hurdle I'm not over yet...

[bratislav]

You can be as colorful as you want; that doesn't change facts. There is more that comes from a pixel than a simple background noise. You also have photon noise (or sometimes called shot noise), dark noise and readout noise. These are factors even in liquid nitrogen cooled front illuminated scientific CCDs, let alone consumer units we deal with. Those are constant for a given exposure (some will go up with exposure, some won't, like quantization noise or output amplifier or 1/f noise for example). So if you expose faint nebula for X minutes, faster scope will deliver more photons per pixel, you will get more electrons generated in the well and that will result in higher signal to noise ratio.

Stan's analysis is sound if, and only if, you use correspondingly larger pixel camera (with equivalent noise characteristics) on a longer f/ratio scope. But amateurs never do that - we simply use whatever CCD we've got. And in that case, faster scope will deliver higher S/N for extended objects. This advantage does diminish as signal goes up - so for bright objects S/N gets mainly dominated by background noise, and you can stretch the image, as Stan did, to make it look equal.
But for very faint extended objects that may be just above combined background/camera noise, this is not the case.

[AlexN]

if increasing pixel size does not increase S/N why would you ever bother binning 2x2? I suppose though, that is not physically increasing your pixel size, rather, adding 4 pixels worth of signal into 1?

[bratislav]

Binning also increases S/N ratio. See attached graph from scientific CCD manufacturer Hamamatsu

[Satchmo]

I thought he was making an unspoken assumption somewhere and you have pinpointed it.

[bratislav]

In fact, I've just read Stan's article properly now, and look what I have found ;) :

"But if the object and sky are both low-level (dim or short exp) then camera noise may become significant and degrade the image. Because camera noise is pixel based, this potential degradation is sensitive to the number of pixels used to capture an object or sky-area, and thus it is sensitive to f-ratio."

Clear now ?

[Peter Ward]

This is simply because you'd be operating in a regime not dis-similar to film.

CCD exposures that are so short as to be camera, rather than *sky* limited, are so full of noise you'd have no hope of getting a deep image hence your point is rather moot ;)

[Peter Ward]

You get more gain...ie signal. I'm not disputing that. You also get more noise. (rarely is there a free lunch with CCD's)

With a really high QE sensor...eg Hamamatsu, SITe, etc indeeed there is a slight S/N improvement....my thoughts are this is because the shot noise has less of an effect on high QE devices.

I'd expect this benefit is far less with front illuminated sensors eg Kodak.

BTW hands up from all those are actually running a Hamamatsu back illuminated sensor :)

[AlexN]

I dont even know what a hamamatsu sensor is.. :P I'm happily running a Sony sensor now, and will be running a Kodak in 6 ~ 8 months time.. Im happy to concede that I know little about CCD science.. All I know is I point my CCD towards the sky with a focused optical system attached to the front and take pictures! :)

[sjastro]

Hello Mark,

Before the pixel reaches saturation, the signal and noise increase by the same factor hence S/N ratio remains the same where the noise is purely confined to photon noise. That's because you're not actually increasing the total signal, instead the signal is distributed over a smaller number of pixels.

When a pixel reaches saturation it's impossible to measure the S/N ratio. The photon noise is the standard deviation of a series of measurements of a given pixel over a number of exposures. When a pixel reaches saturation it has reached its maximum value.

When the signal is increased by exposure time or by increasing aperture, the signal and noise increase at different rates.

By quadrupling the exposure (or doubling the aperture), the signal increases 4X but the noise increases by only 2X.

Hence the S/N ratio doubles.

Regards

Steven

[bratislav]

If anything, Hamamatsu sensors are far less sensor/quantization/amplifier noise limited than any of the commercial devices we're dealing with. That is, if camera noise is a factor in a Hamamatsu device, SBIG/QSI/etc will be just as bad (in reality much worse).

But more importantly, binned graphs (in this sense equivalent faster scope, i.e. more photons per combined pixel) continue to be quite clearly above the unbinned curve even when exposure is well into 1000's of seconds.
Check again y axis on that graph - it reads 'Signal to noise ratio'.
Yes, you can reach the similar S/N ratio with slower scope (== unbinned CCD), but it will cost you time. BIG time. For faint objects you will not even be able to expose long enough if a scope is too slow.

[Bassnut]

The reason for binning as I understand it, is that its done on chip, so the read out noise for example, will be 4 times less for 2*2 than for bin 1.

[gregbradley]

Thanks Bratislav I think you got that exactly right. I have actually observed this point with various telescopes and you have communicated what I have observed very well.

Faster scopes pick up the fainter nebula etc than slower scopes. Slower scopes will pick it up but at much greater exposure times.

Look at Thomas Davis's dust images with his 12 inch ASA F3.8 scope as a classic example.

You won't see all that dust background so clearly in a 12.5 inch F9 RCOS image without some serious hours of exposure time and even then probably not.

So the cost of long focal length imaging is patience and exposure time to get that lovely closeup image of things.

If you are in a hurry or you travel to image your F5 scope will give a brighter wider field with more of the dim/faint dust/nebula showing up.

This is exactly my experience on the subject.

On top of this of course is a low noise high QE camera as the next most important factor.

Hence in conclusion: ideally you want a high QE, really low noise camera (high cooling, excellent low noise electronics with minimal chip defects) with pixel size most ideally matched for sampling (.66 arc seconds/pixel) for your scope, with largest aperture OTA and the fastest F ratio you can for the image scale you want to image at. All on the best tracking and overmounted mount you can get. Then iamge from the darkest, best seeing location you can for the longest time you can for the resulting maximum best astrophotographic results.

Greg.

[AlexN]

Tom's images are actually a fantastic example now that you mention it.. Get any 12" F/8 scope with the same camera and try to extract the same amount of dusty data with equal sub exposure length, and equal total exposure.. I think you might be hard pressed...

[gregbradley]

Check again y axis on that graph - it reads 'Signal to noise ratio'.
Yes, you can reach the similar S/N ratio with slower scope (== unbinned CCD), but it will cost you time. BIG time. For faint objects you will not even be able to expose long enough if a scope is too slow.[/QUOTE]

Yes I experienced that last time I was imaging. It was 2/3rds moon and I imaged luminance on a galaxy and at 1x1 it was too faint with the light pollution but same exposure length 2x2 gave a quite nice luminance image wiht good contrast.

Greg.

[Peter Ward]

Good point Fred.....but this only applies if your signal-to-noise per pixel is dominated by read noise....this is very rare with deep exposures.

So I’ll modify my earlier position, you can indeed increase the signal-to-noise by binning but the disadvantages of using on-chip binning follow

-it will reduce the resolution of your image
-the relative number of pixels affected by cosmic rays increases
-the relative number of `hot' pixels increases proportionally to the binning factor.

As I said, no free lunch....plus the contribution of the skyglow goes up as well.

[Satchmo]

So in an F4 scope for a given number amount of time I have available I will be able to increase my signal to noise more efficiently than an F8 scope of same aperture as my raw image is already 4X brighter.

[bratislav]

The point may be good, but it is factually incorrect. If you bin 2x2, signal from the extended object adds 4 times, but noise adds sqrt(4) = 2, so signal to noise ratio goes up only 2 times.
On the other hand, if you had twice as large pixel camera, the readout noise is the same, signal still adds up the same, and S/N ratio does go up 4 times.

As pointed out, for well exposed frames it will be the photon noise that dominates the noise floor, and S/N ratio will be relatively independent from f/ratio. (you pay the price with much longer exposures though)

But in astronomy we deal with faint, sometimes extremely faint stuff. There may not always be enough photons to reach photon-noise limited region.

And that was my point from the very first post.

[Terry B]

This is an intresting topic. F ratio seems to important sometimes and as stated by some it can have a bearing on SN ration for very dim extended objects.
I have a question though.
I want to image a small dim object ie a small PN that may only be 20 pixels wide at f8. If I reduce the f ratio to f5.6 (1/2) then I will have a brighter images but it will only be 10 pixels wide. Obviously resolution will suffer in this situation and the only way to improve this would be to increase the aperture to increase the S/N ratio rather than making the image more concentrated on each pixel.
I cant see how this is different to fine structure in large extended objects.
At the lower f ration the dim clouds will have a higher SN ration but any small structure in them will be reduced.

[sjastro]

For an equivalent aperture and the same exposure time the answer is no.
It once again boils down to the fact your are still dealing with the same number of photons in both cases.

The best way of explaining this is with an example.
Suppose with your f/8 scope you take an image of 4 equally bright stars. Lets assume the photons from each star fall on four separate pixels. You therefore have 4 separate pixel S/N ratios.

Now you change from f/8 to f/4 by reducing the FL.
All the photons from the 4 stars are now superimposed on a single pixel and saturation hasn't occurred.

While the photon count on that pixel has increased by a factor of 4 as has the signal, it is due solely to the sum total of photons from 4 separate stars. It is not the same as increasing the signal from a single star by 4X which will increase the S/N ratio (N=Photon noise) by 2X.

You would probably find the S/N value will not be significantly different.

Stan Moore's comparison image supports the case.

Regards

Steven

[bratislav]

But in one case you have all the photons in one pixel (== one readout+thermal noise figure) while in other case you have the same number of photons split over 4 pixels, with combined electronics' noise that is sqrt(4) or 2 times higher.
This may well be insignificant in cases where signal is so high that shot noise grossly dominates readout & thermal noise.

It is not insignificant for really faint stuff.

Let's be realistic - how many images do we see of Mandel-Wilson stuff with a f/16 Cassegrain equipped with a small pixel camera ? All I've seen are done with instruments f/5 or faster, with CCDs covering several arc seconds per pixel.

Any volunteers ?

;)

[Satchmo]

Steve

The case where 4 stars at F8 get merged into one at F4 seems like an a quite esoteric case and just further confuses me..I think we are just getting bogged down about signal to noise.

Can you turn your explanation towards imaging diffuse objects..galaxies and nebulae.

The general consensus seems to be that the brighter the image or the longer the exposure the better the signal to noise ratio. F4 will record faster with better signal to noise on very faint diffuse objects. Stan Moores two pictures at F4 and F12 show this clearly..for a 10 minute exposure there is less detail and definition in the diffuse areas and the the F12 shot looks dimmer and grainy. There is a slight very impovement in the sharpness of the stars. The F12 instrum,ent would only show 1/9 the sky area of the F4, so it doesn't appear to be a very good trade off.

I'm not sure how this stellar s/n theory translates to diffuse objects that aren't point sources. Will I have no gain in image depth/contrast imaging faint nebulae and galaxies at F4 rather than F8?

[Terry B]

At F4 the image will be smaller than at F8 (1/4size) for the same diameter scope.
I think that this is OK for big nebulae but very few galaxies are big and making the image 1/4 the size limits what you can image.

[coldspace]

I do understand some things of whats been discussed here on this interesting thread, but although I do not fully understand I know the faster I make my 12 inch F10 Meade the better my Mallincam hypercolor video shows objects like faint nebulae and galaxies, but this also has its limmits to sky fog out of my images.

At f7 the Mallincam shows a fair amount of detail in say Trifid, lagoon etc,etc at 14 seconds exposure/intergrations.
When I drop the F ratio down to F5 using my optec reducers alot more detail/colour appears on my CRT/LCD screens.
Going down to F3.3 really shows alot of detail but on some bright objects such as Omega centaurus/laggon/trifid at F3.3 my Mallincam goes into saftey shutdown mode to protect itself from overload.
On the Orion nebular I can't use more than 7 seconds at 3.3 as te object just way over exposes and the camera goes into saftey shutdown as its a bright object so I either slow the scope to F5 or F7 or turn the gain right down and keep the exposure/intergration times down to 7 seconds or less.

At 28 and 56 second intergrations I have to lower the gain down a few notches on alot of the brighter objects at F3.3 but at F7 or F10 I can crank the gain right up and run the intergrations at 28 or 56 seconds with no problems but I only use these settings and F ratios if am chasing a really faint/ small planetary or a newly discovered Mag 18 supernova.
Alot of guys overseas attach the hyper-star F2 system to their Cats, and are getting fantastic results with the Mallincam in pristine dark sky but they soon found out that at F2 the images would washout or the camera would go into safty shutdown from sky glow in light polluted skies.
This is why F.3 will do me as I live in a light polluted environment

So for my camera at least in practise F3.3 will be the fastest I will run at the lower intergration times for wide field extended objects and F7 and longer intergration/exposure times with a little more gain for chasing small dim objects.
Not sure if this means anything to anyone but thats what I find using my camera/scope setup in practise.
I think you will get more detail at F4 than F8 as thats what happens to my setup but you will also get to the sky fog limmit alot faster so shorter exposures will be required.

Regards Matt.

[sjastro]

I'm highlighting an extreme case for pixel S/N ratio as opposed to object S/N ratio to show that S/N ratio is independent of f/ratio for a constant aperture.

Ultimately it doesn't matter whether the source of the photons on a single pixel is from an extended object or a point source.

The point is the photon noise which is the statistical variation of the signal is independent of f/ratio for a fixed aperture.

You are not recording any faster at f/4. The amount of information in the image is a function of the diameter of the mirror (or lens). You are collecting the same number of photons at f/8 at the same exposure.

The important difference is in the pixel scale. The effects of camera noise such as read out noise and thermal noise can be more easily discerned in faint objects as one increases the f/ratio. That's because the pixel scale allows the noise to be resolved in the image.

That's probably where the confusion lies. The apparent lesser noise in a smaller f/ratio image is falsely attributed to a stronger signal.

The only criticism I have of Stan's article is that he has rescaled the images. That makes it very difficult to make direct comparisons. In fact by down sampling the f/12.4 image he has probably made the image look smoother than what it actually is.

Stan however has made the very valid point the f/12.4 doesn't need 10 times the imaging time of the f/3.9 scope unless a photograhic emulsion is used.

Hopefully I've anwsered that.:)

Regards

Steven

[coldspace]

Thanks for the explanation Steven,

Now I understand why with my video system because its so sensitive and maximum exposures are only 56 seconds it shows alot of noise at the longer focal length that I try so this in turn cause degration to my images.
And at the shorter focal lengths the image seems alot better as noise is less apparant.
Is this right?

Matt.

[sjastro]

That's correct Matt.

Also since you are using very short exposure times, your images are probably dominated by camera read noise instead of photon noise.

Regards

Steven

[Satchmo]

OK, So is field of view the only reason that we should choose one F ratio over another or would choosing apparent less noise be valid also.

[sjastro]

The advantages of using a lower f/ratio are

(1) Larger FOV.
(2) Image quality is less dependent on seeing conditions.
(3) Easier to guide for long exposures.
(4) Camera related noise is less apparent due to higher pixel scale.

The disadvantages of a lower f/ratio are.

(1) Potential loss of resolution using too high a pixel scale.
(2) Images could be undersampled which reduces processing latitude.
(3) Possible reduction in optical quality (ie increased star spot sizes, off axis aberrations etc).

Regards

Steven

[gregbradley]

Nice and clear explanation Steven. Thanks mate.

That is what I have observed to be true.

Greg.

[Bassnut]

So how does cam QE affect s/n ratio ?. For instance if the apature was reduced to capture half the number of photons and the cam QE doubled ,that doubles the photon conversion to electrons efficiency and produces same number of electrons, to I suppose give an equivalent exposure time.

Optical quality differences aside, is the S/N ratio (thermal, readout etc) the same?.

[CometGuy]

Fred,

In the worst case s/n ratio increases as a square root of increase in QE. The worst case being where the greatest contribution to image noise is from the sky background.

Here is a question for the forum: I want to photograph the horsehead nebula over an area of 1 degree wide field. Therefore I've determined I need 500mm focal length with the CCD I am using. Which of the following - well corrected optically - instruments would you choose?

a. 500mm focal length f10
b. 500mm focal length f2

Answer that and tell me whether you think focal ratio is not important..

T.

[Peter Ward]

This is a no brainer...50mm *aperture* vs 250mm...I pick the latter...but it wasn't because it was an F2 (though that would be cool....who makes such a neat optic?)

How about we do the numbers the another way? ie.

250mm aperture 500mm FL
2.5metre apetrure say make it 7500mm FL ;)

[CometGuy]

Exactly...no brainer..f ratio is important

[Satchmo]

As I understand it now there is no point in paying for large fast highly corrected instruments . A 20" F5 with coma corrector will produce similar results to an exotic design 20" F2.5 in the same exposure time for 1/10 of the cost, with a narrower field of view.

[Terry B]

This makes no sense.
Optical physics states that resolution is proportional to diameter of the scope.
You need to compare your 500mm fl F10 scope ie 50mm diameter with the same scope with a focal reducer on it as this is what people do with their scopes ie a 50mm diameter f2 scope with a fl of 100mm.
I think I would rather use the 500mm fl scope rather than a 100mm mild telephoto lens.
If this is the case why do we bother with telescopes at all?
Just look at the sky with a camera lens.

[sjastro]

That's correct. The f/2.5 won't go any deeper.

As you know the rule doesn't apply to photographic emulsions. The f/5 will require 4X the exposure.

A good example of Quantum Mechanics at work.

Regards

Steven

[TrevorW]

I brought my RC because it looks good

:lol:

[CometGuy]

Why doesn't this make sense?

You are assuming I am interested in ultimate resolution. What happens if I want to make a wide angle image of the milky way? Or what happens if I want to do a survey for Asteroids, comets or nova? I'll want the fastest focal ratio practical.

Aperture, focal length and the ratio between them are all important factors.

Terry

[Peter Ward]

I didn't pick the F2 because it was a F2....it was because you put up a 50mm vs 250mm *aperture*.....which wins every time

[dpastern]

mmm whilst admitting I know jack about astro imaging, I find this all odd. When it comes to terrestial imaging, the exposure triangle (aperture, iso, shutter speed) all come into play. As an example, a 500mm f4 telephoto lens, used at 1/1000 second, ISO 400 and f4 will provide twice as much light as the same shutter speed and iso but f5.6. Why is it different for astro imaging? Logic indicates to me that f stop (at the same focal length ratio and aperture) *does* make a difference. Not so much in the amount of light hitting, but in the amount of time required to hit a particular level of exposure. I fail to see why astro imaging is any different to terrestial, digital or film be damnéd.

Dave

edit: so a f3 10" newt will gather light twice as fast as one that is one stop slower in terms of f ratio (but same aperture, i.e 10"). Am I just misunderstanding what you guys are all saying, or just completely off track here with my thinking?

[Peter Ward]

No it won't. Sure we've been haggling over details in the limit cases, but
a 10" is a 10" you will only get a marginal performance increase (using a CCD) with a faster system.

[dpastern]

But (excuse my non astro logic here) doesn't make sense. It is completely the opposite to terrestial photography. Physics are the same (at least that's how I was taught) - photography and photons are the same at night or day. Sorry, I'm just having a very hard time wrapping my head around it all Peter.

Dave

[CometGuy]

Dave,

I don't think anyone is wrong, just looking at the it from different angles.

We all agree FOCAL RATIO = FOCAL LENGTH / APERTURE

For instance Stan Moores article states that f-ratio has little impact on image depth when APERTURE is kept constant. On that basis his article is sound. I would even add that as focal ratio is increased stellar limiting magnitude is improved since the sky background appears darker.

However, if you keep FOCAL LENGTH constant then varying focal ratio has a big effect as you are varying aperture. This is how photographers tend to think - i.e what focal length do I need to frame my subject. Of course in astrophotography we often find ourselves not having enough focal length.

Sorry, if I am getting over pedantic...

Terry

[Peter Ward]

With camera lenses the aperture can be varied with a constant FL.
So a 'wide open' a lens is operating at its maximim aperture. Not surprisingly it gives its brightest image at this point....hence the amount of flux coming down the pipe, so to speak, is soley (give or take) determined by the aperture. Does that help?

[coldspace]

Hi Steven,

Just so I can get my head around all this, the f2.5 for a given aperature won't go any deeper than say an f7 or f10 for the same aperature and same exposure time with the same camera. Only change is the field of view with less noise?

Just when you see companies like Starzona marketing their hyperstar lenses to make your f10 to F2 and over 30 times faster I wonder if they are not telling you the whole story on this. Their speed comparisons and their images and other advantages really make you want to buy one, not that I will be.
I might email a couple of guys I know in the states who got these hyperstar lenses in the last year and see what they really think.
http://starizona.com/acb/hyperstar/whatis.aspx

If you look at the comparison shots of M8 are not really good comparisons as one shot is using a hyperstar C14 with a top CCD camera and the other is using a 6 inch scope and film.
Its a pity they don't post comparisons of objects on their web site showing same scope,camera,exposure times with only the f ratio changes.
Interesting topic and thanks everyone for the input.
Maybe someone if they can get some time put up some images of a well known object using the same scope, camera and exposure time with only the f ratio decrease to see what happens.

Matt.

[gregbradley]

Hi Matt,

Firstly they compare a CCD image to a film image in the M8 example. Film is different because it loses sensitivity over longer exposure times.

Also 2ndly you would have a very widefield image at F1.8 compared to a small slice of sky at F10.

Greg.

[sjastro]

Hello Matt.

Greg has summarized it well.

Their f/ratio vs exposure data is simply misleading. It is only applicable to film.

On the comparison images, the Lagoon Nebula being a red emission nebula falls right in the sweet spot of the ST-X10ME's QE of 85%. Even if the film used was a hypered Tech Pan, the QE over a 70 minute exposure would probably be no more than 1% (and that is also considering the non linear behavior of film over long exposures).

Ultimately all it proves is how superior CCD's are to film.:)

An uncropped full resolution version of the Lagoon will not only reveal how noisy the image is, but also the off axis performance of the Hyperstar.

Regards

Steven

[dpastern]

yes, that sort of makes sense to this lad. So, if I understand it all correctly, since the f stop on an astro setup is fixed and non changeable, then the amount of light coming through the aperture is fixed, only the exposure length and aperture width will affect the final exposure. Correct?

Dave

[Peter Ward]

Pretty much. Provided you are not pushing the noise floor of your CCD, way oversampling the data or shooting from bright urban skies etc. etc. ..as covered earlier..

[dpastern]

Well, I've learnt something today. Thanks Peter.

Dave

[coldspace]

Thanks Steven and Greg,

It all makes sense now.

Regards Matt.

[Satchmo]

I can see now why short focal length /large aperture instruments with a well corrected field are so highly prized. Having field of view to play with is as good as money in the bank.

[gregbradley]

That's how I see it too Mark.

If F ratio makes no difference, well why not image DSO at F40 then with 8 inches of aperture. The extreme example helps show the point.

But the troubles with some ASA F3.6/3.8 scopes shows how critical everything becomes in quality required to achieve a practical scope at the faster f ratio. But see Wolfgang Prompers Namibia images using one and a FLI 6303 camera and look at how deep the images are and then look at how short his exposure times were. But they also aren't 12 inch RCOS closeup galaxy shots either and that scope would be no good for that sort of work. So it is hard to have scope that is all things to all people. Often 2 or 3 scopes are required to fit the sweet spot for different types of images. The Cervalo Astrograph is an attempt to capture both grounds fast F ratio for the wide field and slow f ratio
for the galaxies or closeups. You can see a difference in resolution between the 2 f ratios and that is from the same scope and same camera. A good example of this theory that it is a workable guideline but not a truth.

F5 -7 is not a bad range for imaging. The Tak 180ED is F2.8 and they routinely showed lots of faint nebula in the usual imaging targets not normally seen.

Greg.