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For imaging I thought aperture was king?

swag72

By swag72
in Imaging - Discussion

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ollypenrice

ollypenrice

Posted
Posted
Hey, I use my CPC800 (which is f/10) to image (the recent Ring Nebula I posted was using this scope and I think its a pretty darn good scope for imaging things that are small like planetary nebulas, smaller galaxies, etc.

On the other hand, if it were faster than f/10 I probably wouldn't have to do 30min subs to get the faint halos on M57 that I did ....

Also, with that said, I also have an 80ED piggybacked that I generally use to image just for the wider FOV with the small-chip ccd I am using (and it is a bit faster at f/6.25)

Yes indeed and it's a great image but the target is a bit of a special case. In the main you wouldn't want to be stuck with just an F10 system, surely? I find the TEC slow enough at F7, I have to say, and that is twice as fast as F10. Your ED80 will get in 39 minutes what F10 would need 1Hr and 40 mins to collect. That's not what I'd call a bit faster! Also we both use cooled CCD but think of a DSLR imager limited by thermal noise on sub length. Where you see really great DSLR images they are done in fast systems.

I know I bang on about this but beginners get drawn in by advertising without realising just what a big decision it would be to buy an F10 only system.

Olly

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johnb

johnb

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Posted

Follwed this with interest, from what I read I dont think my unused

Vixen 80MF refractor which is f11.4 is of much value for imaging or did i not fully understand were as my Skywatcher 250 is f4.7 is ideal ?

John B

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ollypenrice

ollypenrice

Posted
Posted
Follwed this with interest, from what I read I dont think my unused

Vixen 80MF refractor which is f11.4 is of much value for imaging or did i not fully understand were as my Skywatcher 250 is f4.7 is ideal ?

John B

The big Newt will need coma correction and careful mounting but yes, why not?

Olly

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ollypenrice

ollypenrice

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What I think is that we are not talking about the same thing. If you focally reduce a telescope then it is perfectly true that you don't bring in any new information about an object that fits on the chip of both reduced and native systems. (In reducing you do bring in new objects because of the wider field.)

In the reduced system you will produce an impressioon of higher quality (better S/N) only because the image of the object-in-common is smaller. It is perfectly OK to talk of the f-ratio myth in this context. There is a myth there to be exploded.

However, as an imager I thnk first in terms of focal length and what that focal length is going to give me in terms of 'framing' or square degrees on the sky.

I will choose my equipment accordingly with a particular group of objects in mind.

Let's say I want a metre of FL. I can have this at F5 in Mak Newt, f7 in a refractor or F10 in an SCT. In considering which option to go for aperture is king because f ratio is really king. The f5 scope will be four times faster on the faint stuff than the F10. There is no myth to be exploded here.

The article in the link is fine, but in reality would I want to use an f10 scope on galaxy x (filling the frame) and present it at 25 percent of full size, or would I rather take galaxy X and its neighbours at F5 and present this wider image fullsize? Galaxy x will be the same in both. If I want just galaxy x at 100 percent in the f10 scope I will have to expose for four times as long.

The reason that this becomes so divisive a conversation is that the whole context of the picture(s) in question is not considered.

Olly

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Space Cadet USA

Space Cadet USA

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Celestron have been saying since the launch of the Edge series that a focal reducer will follow. Where is it? It seems others now make suitable ones...

Olly

Has anybody had actual experience of one using of these third party FRs on an Edge scope?

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dph1nm

dph1nm

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The article in the link is fine, but in reality would I want to use an f10 scope on galaxy x (filling the frame) and present it at 25 percent of full size, or would I rather take galaxy X and its neighbours at F5 and present this wider image fullsize? Galaxy x will be the same in both. If I want just galaxy x at 100 percent in the f10 scope I will have to expose for four times as long.
That's your choice, but there you are simply saying that the shorter f-ratio gives a larger field of view withe same detector. Which is true - but I could take the f10 scope and use a different detector and get an identical image to your f5 scope. Who is to say which is best? Your option might be cheaper (large pixel chips tend to be expensive) but there is no physics in it.

And whatever depth you reach with a short f-ratio scope I can do with a longer one of the same aperture in the same time, if I am allowed to chose an appropriate CCD (or bin up yours). So in that sense aperture is king when it comes to depth.

The advantage of short f-ratio is really one of increased field of view for the same cost of detector, not one of depth (as ever I add the proviso that one has not to be read-noise limited on either scope).

NigelM

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Tim

Tim

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FWIW, although I usually image at f5.3 with the Skywatcher MN190, I have switched to my trusty C9.25 tonight to get in closer to M57.

With bright objects like M57 you dont need such long exposures to get detail in the bright regions, so at f10 I am happily working on 240 second subs. Going to switch to Ha soon, and that will be a bit longer, but not the 2 hour subs I was taking last week :)

Its horses for courses, with the big slow long f/l scopes you can get in nice and close, until the seeing limits what you can do really. Ideally, have a scope for every occasion :)

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MartinB

MartinB

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Hey, I use my CPC800 (which is f/10) to image (the recent Ring Nebula I posted was using this scope and I think its a pretty darn good scope for imaging things that are small like planetary nebulas, smaller galaxies, etc.

This illustrates an important point. Assuming the same camera is being used, for a given sub length M57 will contain more noise using an 8" F5 scope and a 10" at F10. It is simply that the noise with the faster scope will be less visible because M57 will be smaller and make up less of the FOV. For a small target like M57 an F10 SCT makes a lot of sense, but only if you have pristine seeing!

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MartinB

MartinB

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This illustrates an important point. Assuming the same camera is being used, for a given sub length M57 will contain more noise using an 8" F5 scope and a 10" at F10. It is simply that the noise with the faster scope will be less visible because M57 will be smaller and make up less of the FOV. For a small target like M57 an F10 SCT makes a lot of sense, but only if you have pristine seeing!

Sorry, wrote this without realising the discussion had moved onto a second page. My points had already been covered!!

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MartinB

MartinB

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OK so what we now need is not only a range of scopes but a range of cameras as well! Eek...

Olly

Not necessarily, the range of 583 cameras with there small pixels can readily be binned for long focal length imaging as well as working very well unbinned with small refractors.

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dph1nm

dph1nm

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The other thing about f/no is that it shouldnt matter if your taking a stellar photograph, eg a star cluster, as they should be point sources (and so the light doesnt get spread)
Not so unless you are under-sampling the seeing (which wide-field imagers often do mind you). Stars will have a size equal to the seeing disk, which is fixed in arcseconds - so if you use a longer focal length on the same camera they will cover more pixels.

NigelM

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ollypenrice

ollypenrice

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Not necessarily, the range of 583 cameras with there small pixels can readily be binned for long focal length imaging as well as working very well unbinned with small refractors.

True. I keep thinking that an OSC 583 would be great for camera lens imaging but then I couldn't bin in the ten inch. Curses and double curses! Why is it so complicated? Of course if I went for the QSI it might work with built in filter wheel but would it work at f3.2?

Olly

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ollypenrice

ollypenrice

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Sara, I hope to be reviewing one soon so it will be interesting to see how it performs. I also have a guest coming with experience of the scope, though I don't think the scope itself will be coming this time.

My own feeling is that F8 is slow but not out of the question. I'd rather have a 16 inch f4, sure! I used a Meade 127 productively at F7.5 and our TEC is F7. Fortunately many of the likely targets, like planetaries and galaxies, are tolerably bright. The faintest things tend to be dusty nebulae and these are widefield by their nature.

But in the real world lots of people use the EQ6 and would maybe like to get the longest workable imaging focal length they can on this mount, particularly if they already have a short FL scope. You have to bear in mind both the weight and the accuracy likely to be acheived by the mount and here something like the 8 inch RC sits well. It is well within weight and the FL of 1.6 metres, while not likely to be a cynch, should be possible with care.

In a nutshell, for those who want 1.6 metres on an EQ6 or similar, a slow f ratio is something they will have to live with. I think the numbers stack up quite well on this instrument, as they do on f reduced 8 inch SCTs and even on the C9.25.

If you have a millionaire's mount the world is your oyster, of course.

I was in the market for something around this size but interesting events have started to occur here so things are on hold...

Olly

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ollypenrice

ollypenrice

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I really don't know. The HEQ5 is reputedly as accurate as the NEQ6. I did once read that it might tend to be better. But at long FL you really do have to avoid the jitters. One good thing about the RC is that it is short and this has a huge bearing on stability. There is also a 6 inch version of the scope. However, you then start to ask yourself whether a 4-5 inch refractor might not be better. Maybe.

Thanks for your kind comment.

Olly

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lukebl

lukebl

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Posted

Just one thing to remember about using HEQ5s, is that people sometimes forget to factor in the additional weight of guidescope and the camera itself which will add a lot of weight over and above the scope itself. I used an HEQ5 with an 200p and guidescope and it was creaking a bit, although it did work.

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