You don't need aperture for imaging?

[Kaptain Klevtsov]

Gordon said this in another thread

"Oh, and yes.... sell the newt! Unless you are into visual stuff, the extra aperture doesn't count for much, and the central obstruction is an.... obstruction"

It has got me intrigued, as it implies that there are some imaging issues that I don't understand (especially the role of the CO in degrading the image) so it's open for discussion. :scratch:

For my sins, I've got aperture fever WRT DSO imaging, 8" being the minimum that I currently consider workable. Gordon has suggested (on several occasions) that a refractor would give a better image. Having had some improvement with my Saturn imaging by changing from a 200mm Newt. to a 120mm Achro. it seems that there is definitely something in what he says, but I need to at least attempt to get my head round the difference, and what causes it, in some kind of theoretical way.

Over to you guys, I'll add my biased ramblings later after I've seen some repies so as not to appear too ignorant.

Captain Chaos

[Jamie]

Its just MHO but the best DSO images that i have seen taken by amatuer astronomers

have all been taken with refractors (smallish refractors).

In saying that the best "Planetary" images i have seen to date have been taken

with a reflector (bigish reflectors)

Its not a case of what is best here but like you say CC "You don't need aperture for imaging"

[geppetto]

This should be interesting 8)

Not qualified to comment much but sort of lean towards the more aperture

the better be it reflec or refrac.

What teases you is the huge amount of top class images taken through

ED80's which sort of goes against the gut feeling for aperture.. :?

Also don't fully understand the effect of the CO when your talking about large aperture

scopes. Yes, you loose a percentage of the light path but if the primary is a big one

isn't there enough light gathering capabilty left for the CO to be insignificant?

The one thing I have definitely proved to myself over the last couple of years is that

aperture rules when it comes to capturing detail.

All good stuff

[GordonCopestake]

I'll qualify what I said: You dont need aperture for LIGHT GRASP, but you DO need it for resolution, but for DSO's you dont need much resolution IMO. For planetary thats different as you can't see the smaller features with a small aperture.

I believe that you can simply replace aperture with exposure though. So an ED80 will give a better image having no central obstrutuction than a C8 will due to the 33% CO. HOWEVER the exposures will be different (ignoring the f/10 of the C8 over the f/7.5 of the ED80) as the C8 has massivly more aperture than the ED80.

At the end of the day what you are looking for is the HIGHEST quality image, and forget the aperture. So a Tak Sky90 is better than an 8" newt but you'll need a longer exposure. Of course the best setup is high quality AND large aperture, but we havn't all just won the lottery

Of course I am NOT an imager so everything I say could be wrong!

[FLO]

This is a good question and a difficult one to nail down as several factors are involved.

More aperture does = more resolution, which in turn allows greater magnification. Its a law of physics and can be taken as fact. Whenever a reviewer claims a small aperture shows more detail than a large aperture, you need to consider optical quality, type and seeing conditions to determine why. The size and resolution of the imaging sensor also plays a part.

I suspect that the reason the ED80 (and its like) perform so well is that they are generally used at the low-med magnifications that its aperture can easily support. When pushed to higher mags (for planets and lunar) the limits of its 80mm aperture become apparent. Also, its unobstructed light-path offers high acutence (edge definition) for enhanced 'bite' and delivers more compact, less bloated stars (I am not sure why, something to do with its energy being better contained?).

After saying that, a Newtonian designed for high magnification (f8+) will have minimum central obstruction, a narrow light-cone and be free of colour fringing so - as Ian (Lunator) has demonstrated - is capable of high definition, contrast and acutence.

Of course, I am only just beginning to image thru a telescope so reserve the right to change my opinion - often

[Kaptain Klevtsov]

Taking Gordon's example of an ED80 versus a C8, I've done some maths. The C8 we'll take at it's native f/10 so a focal length of 2000mm. The ED80 will need Barlow lens(es) to get to that focal length, which I would expect would reduce the image quality at least by a little, because of extinction due to extra glass, some air:glass surface reflection due to imperfect transmission, re-reflection of said reflected light etc. We need to multiply the focal length, somehow, to get to 2000mm. The ED80 would end up at f/25 for the same focal length (2000/80).

Given those f/ ratios, I would need to increase the time for each sub. by the square of the f/ number ratios, so a 30 second sub. with the C8 would be equalled by a 187.5 second sub. with the ED80. That's 6.25 times as long (25/10 = 2.5, 2.5 squared = 6.25). (This is for DSOs remember, my main target, but the subs for planets would be very much shorter making the difference less of an issue.)

So I presume that we have to consider weather ( ) I can manage to do enough subs at the much longer exposure time before the window of opportunity passes, on a practical level, AND if I can get by with fewer higher quality subs with the ED80. Perhaps the higher quality ED80 subs would give the same, or better, result for the same total time?

Fascinating stuff, eh?

Captain Chaos

[Astroman]

I can't think of a single professional refractor taking DSO images. All are reflectors. I'll go with CC's maths-and add, I don't think you could tell a reflector image from a refractor image using his exposure formulae. For us mortals, the less time we have to track an object, the better it turns out, so it seems obvious to me that the more aperture you have the better. Period.

[GordonCopestake]

Straight reflectors or CAT's Astroman?

[Astroman]

Most are classical Cassegraine's or Ritchey-Cretien's. Some add a tertiary mirror for added focal length. Some image at prime focus, most at Cassegraine focus.

[russ]

Well I'm not qualified to comment but all I do know is:

The best planetary imagers use an SCT, in particular - C9.25, C11 and the C14 - which gives a good clue to the answer of another debate (Blue vs Orange )

And the best deepsky imagers use refractors, normally 66mm - 150mm.

Russ

[John]

For visual and imaging purposes does a central obstruction have an equal impact on light grasp, resolution and contrast or does it impact one or more of these more than another ? - up to now my understanding has been that the CO has little if any effect on light grasp, a little more but still not massive on resolution but does reduce contrast (eg: detail on planetary surfaces) somewhat - or have I got the wrong end of a few sticks here

Sorry if this question has already been answered in this or other threads.

John

[Kaptain Klevtsov]

The more questions and answers that I can gather the better John.

Just to muck things up I've done a better Saturn tonight with the Newt. so maybe it was to do with seeing?

There seem to be some good points raised here, let's keep it going as I might will need either a big Frac. or some kind of compound 'scope for when Mars turns up later in the year.

Captain Chaos

[John]

If I a) had the dosh and had a big mount and c) had a roll off roof shed to keep it in I think I'd look seriously at a 6"-7" Mak-Newtonian. The design seems to combine aperture / mimimal obstruction / a reasonably fast focal length / no false colour in a way that really could make it a true all round scope for both visual and imaging.

Orion (USA) did market a small (3.6 inch) off-axis reflector which had no central obstruction at all - as far as I know that did not take off though - maybe there's more of a niche for some innovative designs now ?.

John

[Kaptain Klevtsov]

If I a) had the dosh and had a big mount and c) had a roll off roof shed to keep it in I think I'd look seriously at a 6"-7" Mak-Newtonian.

John

Any particular models to take a look at John?

Captain Chaos

[FLO]

...up to now my understanding has been that the CO has little if any effect on light grasp, a little more but still not massive on resolution but does reduce contrast (eg: detail on planetary surfaces) somewhat - or have I got the wrong end of a few sticks here

Good question. My understanding is that it effects contrast (the larger the obstruction, the more the effect). If that is true then applying regular photographic theory, it will effect the observer more than the imager because when imaging, so long as the resolution/detail is there, contrast can be added during image processing. There is even an argument against excessive contrast because contrast is, after all, an exaggerated difference between hilight and shadow - which can cause loss of information due to blown hilights and 'blacked out' shadows.

Can astro-imaging be that different.

Thinking further, resolution can be effected if the detail depends on contrast or acutence. Also, if the only thing defining the boundary between detail is colour, then chromatic abberation from an achromat refractor will be more detrimental than loss of contrast from a Newtonians central obstruction.

Am looking forward to comparing different scope types 8)

<edit> Can't help but think that ultimately, the telescope gathers the information and the image processing defines and presents it in a way that is technically useful and/or pleasing to the eye. So, assuming the optics are capable, aperture rules!

I just know some Smart-Alec imager is going to find this thread and blow me out of the water

[John]

If I a) had the dosh and had a big mount and c) had a roll off roof shed to keep it in I think I'd look seriously at a 6"-7" Mak-Newtonian.

John

Any particular models to take a look at John?

Captain Chaos

I guess the Russian's (Intes) are the most readily available. I looked very seriously at a 6inch F6 Mak-Newt a little while ago but while the image quality was no doubt superb it was very far from portable - would have needed an HEQ5 mount to do it any justice.

Also the collimation looked a little daunting :shock:

John

[jakey]

My take is as follows, buy large aperture for visual use (as large as you can afford.)

For astrophotography, buy the best magazine you can afford.

Life is so much simpler with that formula.

[russ]

My take is as follows, buy large aperture for visual use (as large as you can afford.)

For astrophotography, buy the best magazine you can afford.

Life is so much simpler with that formula. Very Happy

excellent.....and think of the money you save too

[russ]

Just take a look here: http://www.dhinds.co.uk/pages/photos.php

Adam, that's the trend I had in mind

[barkis]

Aperture lives, and it always will. The fever that is it's bedfellow, will never be eradicated. The technological advances in deep space detection, will only be valid when coupled to the largest of photon collectors. ie massive light buckets.

Ron.

[Astroman]

But you know what? You don't need lots of aperture to decent imaging. You can do wonderful things with a decent mount and an ED80, no doubt.

It's just a heck of a lot easier with a 4 meter scope and a 4Kx4K chip. :sunny:

[Kaptain Klevtsov]

To pick up on what Steve posted earlier, the lack of contrast isn't an issue for us imagers. All we do is slide a slider, or click on a button and the contrast is back. In fact the lack of contrast might even compress the dynamic range of the target so that it fits better into the response curve of the camera. Never even considered that, thanks Steve. We even get to choose which bit gets decompressed most by using the gamma adjustment!

Adam's link to some wonderful images now means that I'm in the market for a Professor and a Guru, oh and a Takhashi Sky 90. In real world land though, I'd struggle to justify the Tak. (to me, never mind anybody else!) so I'm going to wait and see what Pans ( even I don't get that pun, as I don't use eyepieces ) out regarding the "purity" of a Frac. versus the photon gulping reflectors of whichever variety.

I'm still very wobbly, as Russ has pointed out that the best DSO images are done using itsy bitsy refractors. The links prove the concept, absolutely, but can I do it on a budget, and does the budget redefine the kit?

This is getting to be a fun thread, I like this!

Captain Chaos

[GordonCopestake]

You already have an ED80 and an EQ6 CC, you have the kit that many people use to produce fantastic images. You've made a load of fantastic images yourself so know what you are doing. My advice would be to choose a target, M81 or something, do a session with the ED80. Then change to the Newt and do a a similar session on the same night. Compare the results. Personally I think that the image quality of the ED80 will win over the aperture of the newt as long as you adjust the exposures to suit.

I've come to the conclusion that for DSO imaging the highest quality refractor you can afford is the best instrument, for planetary imaging the highest aperture (for resolution) and the associated focal length is the best bet. Right tool for the job and all that

[MikeD]

I have an 80Ed and a 10" newt and if it will fit in the fov (most galaxies) I will always go for the newt.

[Kaptain Klevtsov]

Your idea is obviously sound Gordon, but do I add a Barlow lens to the ED80 to up the FL to 1000mm just to make things fair?, or even a focal reducer to bring the ED80 down to f/5, would that be fairer?

I currently use Starry night to see how big the target is in relation to the FOV of the chosen camera with the newt. and ED80, then mount the one that fits best. NGC7000, M42 or M31 are more than a little awkward with the Newt. and SC3 for example, so I'd go for something shorter in focal length, or use a camera with a bigger chip (such as the DSLR). For planets, its something long with lots of Barlow things added for good measure and a webcam.

For very dim stuff, its a no brainer as well because the little ED takes far too long to scoop up enough light to get an image. For these things I think you do need aperture for imaging (or, I imagine, observing) otherwise there's nothing there.

I know that its apples and oranges that we are comparing here, but the apples are a lot bigger and a lot cheaper than the oranges. ( Not that kind of orange Adam, I'm not knocking colour schemes here )

Now, for the next toy on the shopping list, do I go for an ED100, a C8, 200K, or a Mak. Newt? I miss the focal length of the OMC140 that I had, but it was slightly too slow (f/14) for imaging, and I never got round to guiding it. As the Astro-budget is currently somewhat depleted, and there's nothing to point at, it's a rhetorical question for now, but an interesting one nonetheless.

Captain Chaos