APO telescopes for imaging, does aperture make a difference?

[JamesF]

I think there's a need to be quite specific with terminology when discussion how changing f-ratio works because it can be open to interpretation otherwise.

Changing the f-ratio of the OTA with a reducer won't mean that any more photons enter the OTA than without the reducer.  The "photon collection rate" of the OTA is entirely dependent on aperture.

What reducing the f-ratio does is increase the "photon density" for each photosite.  That is, the number of photons that arrive at a photosite in a fixed period of time.  This happens because by reducing the focal length (to reduce the f-ratio for the same aperture), each photosite corresponds to a larger area of the sky.

This can make it look like more photons are arriving overall because the sensor will cover a larger area of sky, but those additional photons were still delivered to the camera before the focal length was shortened.  They just didn't hit the sensor.

Perhaps it's useful to consider a setup where the entire image circle of the OTA ends up on the sensor with room to spare.  If you reduce the focal ratio by a factor of around 1.4 then the image area will halve and twice the number of photons will arrive at any given photosite (in a fixed period of time) as before, so those photosites will saturate twice as fast.  The total photon count over the entire sensor will not increase however.  They're just concentrated in a smaller area.

Of course if your sensor is smaller than the image circle (which is probably desirable then reducing the focal ratio can make it appear that the camera sensor collects more photos overall in the same amount of time.  They're not "new" photons though.  They were always there.  The sensor just isn't big enough to have collected them.

I think

James

[Earl]

the SW ED80 and matching reducer is probably the best introductory setup, it has its weak points, but price is not one of them.

[kalasinman]

Just to be clear, I have never had the slightest doubt that a short tube and wider FOV will make a brighter image. My problem with that side of the discussion is one of emphasis. For myself, I am more concerned with framing a target, so the FOV has to fit. IMO making the target smaller in the frame, then having to crop to the framing I want seems a waste. I'm just a beginner, but it seems I get a dimmer, noisier image this way all else being equal.

[Earl]

Just to be clear, I have never had the slightest doubt that a short tube and wider FOV will make a brighter image. My problem with that side of the discussion is one of emphasis. For myself, I am more concerned with framing a target, so the FOV has to fit. IMO making the target smaller in the frame, then having to crop to the framing I want seems a waste. I'm just a beginner, but it seems I get a dimmer, noisier image this way all else being equal.

Its about target selection for the gears strong points is all.

If your prone to a touch of over sampling which is common for wide filed imaging the PI Drizzle Intergration can help smooth the blocky stars out quiet well ( I Think thats the right way round, or is in undersampling?... dunno LOL)

[KyleStoke]

Its about target selection for the gears strong points is all.

Surley Earl has hit the nail on the head here.

Pick the correct target no point imagine a small object in a massive field of view then cropping and resizing, all the extra photons imaged will just be cut and you will be left with a low res result.

[graemlourens]

One thing i havn't seen mentioned yet here is the weight issue.

80ED will certainly be lighter than the 120ED.

Depending on your Mount, tracking could be much easier with ED80 as less weight means less stress on the mount.

It also depends on how dark your sky is and if you're in lightpollution. If bad sky, more aperture will not help at all.

I have an ED80 myself pretty much from the beginning.

I have invested tons of money in stuff, but the ED80 is still with me as my only imaging scope. Before i upgrade that, its going to take a while. First an observatory

Regards, Graem

[Macavity]

If you fancy a bit of an intellectual workout:

http://www.astrometrica.at/papers/PointSources.pdf

Then, of course, you have to consider extended sources...

I sense that their conclusion: 

... faint point sources with various telescope setups, highlighted

that environmental conditions, telescope equipment, and CCD

detector must harmonise to operate at peak performance. 

is probably about right though!

[ollypenrice]

The aperture of a telescope determines its resolving power (R or Dawes Limit). The equation for this is R=11.6/D in cm.  

The resolving power of a telescope in DSO imaging is also affected by atmospheric seeing and the mount's guiding performance.

It makes sense to match the pixel size of a camera to the resolving power of a telescope so that any image is neither under sampled (blocky stars and loss of fine detail) or over sampled (detail being limited by a small pixel size). The "sweet spot" for us amateurs imaging with commercial mounts which aren't located on a mountain top in Chile seems to be to image at about 2 arc seconds per pixel.

If we take this figure and apply the maths of Harry Nyquist and his chums (who basically said that we should sample at least at twice the frequency of what we're looking at), we get a optimal sampling rate of  about 1 arc second per pixel.

Plug this figure into the Dawes Limit equation and you get the optimal diameter of an imaging scope D = 11.6/1 cm = 11.6cm. 

This is, I believe, why you see so many 4" / 5" class refractors in use by amateur imagers out there.....they strike the right balance between cost and real world imaging conditions.

Moving onto focal ratio - I'd like to use some real world experience here:

I bought my first scope - a SW100ED f9 on the basis of of SaN review before I had any idea that I wanted to get into imaging. When I plugged my DSLR into it, I found that I really struggled to get anything decent out of it, so on the advice of well known and respected dealer I "upgraded" to a WO 70mm f6 scope - and voila! Much brighter images! This is because even though the new scope had 25% less aperture, it had a shorter focal length and was focussing light from a bigger portion of the sky onto my camera chip.

I now have a 106mm f5 scope, which gives me even brighter images and (this is the proof of the pudding coming up) - I've just added a 0.73 reducer to it - making it a 106mm f3.7 scope.

Yesterday, I took a load of flat frames using the same flat panel as I've always used and found that (with an Ha filter as an example) while I had to use exposure times of about 3.6 seconds to get a good flat frame with the scope at f5, I found I only needed 1.8 seconds get the same ADU value flat frame using the same aperture scope with the same camera from the same light source.

I was staggered by this decrease in exposure time until it dawned on me that the area of aperture had increased as a square of decrease in focal ratio (i.e. (5/3.7) squared) = 2.04.

The f3.7 scope was therefore collecting light from twice the area of the flat panel, focussing this double amount of light onto the camera chip - and therefore halving the exposure time from that of the f5 scope. This is reality - not a myth.

Translated out under the sky, the f3.7 scope will provide a FOV of twice the area that the f5 scope would provide, therefore individual objects of any given brightness in the FOV will illuminate half their previous area on the chip - and will therefore appear twice as bright as on the same chip in the f5 scope.

I've just posted a first light from the new set up of the Pelican Nebula in Ha in the DSO imaging thread, and there is no difference in the average ADU of a 15min sub at f3.7 and a 30min sub at f5...just a wider field of view and a sampling rate of 2.4 instead of 1.76 arc secs per pixel.

Using this in the OP's situation - personally, given the choice between the ED80 and the ED100 - I'd save my money and get the ED80 - and if you get the reducer as well, I'm sure you'll be delighted with the f6.4 over the reduced 100ED's f7.7.

Hope that hasn't confused things!

Steve

Sure. But this tells us only about extended objects which fill the chip both reduced and unreduced. Obviously a luminescent flat panel fills the chip entirely (and if it didn't it would be a useless source of light for flats.) The F ratio myth discusses objects which do not fill the chip at native FL so that reducing the FL without increasing the aperture has no effect whatever on the number of object photons captured.

So imagine a very small (and entrirely useless ) flat panel which gave a small central glow on your chip at native FL. Take its picture. Now put in a focal reducer so that the same useless panel gave a rather larger central glow on your chip. The number of photons counted in each case will be precisely the same. The 'F ratio myth' is the name given to the claim that the reducer will pull in more photons. Very obviously it won't.

So, back in the real world, we have a certain aperture with which to catch M42 at F5 or M42 and the Running Man at F3.6. (Invented but about reasonable!   ) If you are going to throw away the Running Man by cropping the F3.6 image you would do better to shoot it at F5. You'll get the same amount of M42 light and better resolution. If you want them both then use F3.6. You won't have the resolution on M42 or the RM but, in the same time, you'll have better S/N ratio for the global image. (The reduced image, if you like, has light from M42 and the Running Man so it has more light than the unreduced one from M42 alone.)

Olly

[Steve 1962]

Thanks Olly - I agree with everything you say ...and I particularly enjoyed your red ink. (anyone would think you were once a teacher!)

I think JamesF has summed it up nicely:

Perhaps it's useful to consider a setup where the entire image circle of the OTA ends up on the sensor with room to spare.  If you reduce the focal ratio by a factor of around 1.4 then the image area will halve and twice the number of photons will arrive at any given photosite (in a fixed period of time) as before, so those photosites will saturate twice as fast.  The total photon count over the entire sensor will not increase however.  They're just concentrated in a smaller area.

 

Of course if your sensor is smaller than the image circle (which is probably desirable   then reducing the focal ratio can make it appear that the camera sensor collects more photos overall in the same amount of time.  They're not "new" photons though.  They were always there.  The sensor just isn't big enough to have collected them.

For me, the reducer has increased my FOV enabling me to image bigger objects - and made them "brighter" - albeit "smaller and less resolved".

Steve

[dph1nm]

This is something I hadn't heard before. So are you saying the size of a camera's photosites influences the f/ratio "speed" for imaging

Yes, and given how many different cameras there are with different pixels sizes, it make something of a nonsense of using f-ratio as the measure of "speed".

NigelM

[ollypenrice]

Thanks Olly - I agree with everything you say ...and I particularly enjoyed your red ink. (anyone would think you were once a teacher!)

I think JamesF has summed it up nicely:

For me, the reducer has increased my FOV enabling me to image bigger objects - and made them "brighter" - albeit "smaller and less resolved".

Steve

Oh God, I only chose red because it shows up OK! The shame of it!!! Will I ever be free of that damned profession?     

Yes, and given how many different cameras there are with different pixels sizes, it make something of a nonsense of using f-ratio as the measure of "speed".

NigelM

...though it does give a measure of speed for 'your' camera so it's not entirely daft in these conversations.

Olly

[feras281]

right folks lets say I start off with the humble skywatcher 80mm ED which has an F ratio of 7.5 or .......................go all out .......and buy the APM 152 ED Apochromatic telescope 6 inches for £3000 which has a similar F ratio of 8  in one big purchase would I get more or less the same Astrophotography images but the bigger telescope will give me just an advanatge in viusal usage only?

would that be fair assesement thanks?

[ollypenrice]

Focal length determines what will fit on your chip. You have to start with this question: what do you want to photograph? It makes sense to pick a focal length which frames the object as you wish to frame it.

Do you want this

or this?

The second image was done on a larger chip but the key difference is focal length. In the first image, 328mm and in the second 980. Don't foget the bigger chip in the second, though. I'm sorry but I can't give you exactly like for like because I've changed the kit around between images.

If you want the 'closer view' (longer focal length) can you guide it? Will it give you a reasonable pixel scale? http://www.12dstring.me.uk/fov.htm That calculator is helpful.

The F ratio of 'real world' apos gets slower as the aperture goes up. This is because it gets harder and harder to colour correct refractors as they become bigger. Sad but true. To the best of my knowledge you can buy a 106mm F5 but not a 150mm F5 apo.

Olly

[Zakalwe]

right folks lets say I start off with the humble skywatcher 80mm ED which has an F ratio of 7.5 or .......................go all out .......and buy the APM 152 ED Apochromatic telescope 6 inches for £3000 which has a similar F ratio of 8  in one big purchase would I get more or less the same Astrophotography images but the bigger telescope will give me just an advanatge in viusal usage only?

would that be fair assesement thanks?

The focal length of the Skywacher 80 is 600mm. The focal length of the AM is over double at 1216mm. That will make a heck of a difference.

[Alien 13]

As Olly said the "what do I want to image" is probably the most important question you need to ask it determins whether you need a DSLR/Lens/lightweight tracking mount or Heavyweight guided setup with CCD and filters, every thing flows out of the answer to that single question

Alan

[Zakalwe]

I think what you are knocking your head against is focal speed. The ED 80 has an aperture fo 80mm, focal length of 600mm and a focal speed of f7.5. If you then got an (imaginary) 150mm objective with a  focal length of 600mm, then the focal speed would be f4. The image scale would be exactly the same (assuming the same camera was used), but the f4 would gather light at a much, much faster speed.

The image circle would also be greater, allowing you to use a much larger sensor.