#41  
Old 17-09-2017, 07:50 AM
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Originally Posted by gregbradley View Post
I am not totally convinced of these arguments. I have both types of scopes - the AP Honders 305 F3.8 1159mm focal length scope and a Planewave CDK
17inch 432mm aperture scope at 2936mm focal length.

The Honders is better at widefield. It can do galaxies well with small pixels (Sony 694 sensor) but I get a better result from the CDK. I have seen Peter Wards images with similar setups show the same results.

It may be more the aperture rather than the focal length but there is a point where large aperture is hard to physically handle without being in a more compact form like an RC or CDK etc. A 17 inch Newt would be very hard to handle and wind prone as well.

Perhaps its the old adage "aperture rules" at work more than the focal length but the 2 concepts are really intertwined.

Small pixel cameras come with their own baggage as well. Small wells, Sony
CCDs are more prone to fixed pattern noise too.

Greg.
To cover most deep sky objects and coupled with the right two cameras, I recon you have the perfect combination of scopes there really Greg but aaaand hey, I could be wrong ...I'm gathering Andy is also not in the position to acquire a 17"CDK and a 12" AP Honders with the two mounts and cameras required either...?

At the end of the day, people like Andy need to do the image comparisons for themselves and do them properly ie compare full res versions, allow for image display size, use of AO, type of camera/pixels used and site quality etc. and critically judge the level of difference and be able to differentiate whether these differences are perceived or indeed real and then do what they wish with the budget they have. After already doing this though, I know what I recon

Mike
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  #42  
Old 17-09-2017, 12:06 PM
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The way I understand goes something like this....

A telescope is a photon funnel. The bigger the aperture of the funnel, the more photons end up hitting the sensor.

Traditionally, large photon buckets (pixels) were necessary to achieve acceptable SNR from the stream of photons hitting the sensor, largely because of high read noise and significant dark noise.

The only problem with tradition is that over time it gets outgunned by the technological revolution.

CMOS sensors are now starting to get competitive with their forebears with a combination of high QE and low noise, resulting in (commonly) smaller pixels that are able to achieve similarly acceptable SNR from fewer photons. That doesn't mean that chasing the faintest of fuzzies doesn't take a long time, but what it does it mean is that it isn't impossible with more humble kit.

Nothing is ever going to beat aperture...but what's happening with the newer technology is that there are more ways to achieve acceptable SNR that don't require the outlay of a family car. It doesn't diminish the amazing work done by the dedicated few with deeper pockets, but greater accessibility in this hobby can only be a good thing.

Some day I might buy a similar imaging newt that the OP is considering, hence my interest but in the meantime, I'm having as much fun as I can handle with a small scope.
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  #43  
Old 17-09-2017, 07:12 PM
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Originally Posted by Camelopardalis View Post
............

The only problem with tradition is that over time it gets outgunned by the technological revolution........
Beg to differ.

For sure, sensors have improved immensely.

But while I cherish my optics, I upgrade my camera bodies often, simply to take closer scrutiny of what the glass has to offer.

I've put together a small web page here to illustrate what I'm on about.

Sure, going from a 52mm diameter objective, to a 100mm is not the same as comparing a 300mm vs 400mm aperture telescope.

But I'd suggest we are simply talking shades of grey here.

To argue a 3" objective with say 1 micron pixels, can capture the same detail as a 30" and 10 micron pixels is a nonsense. You just need to decide where along the continuum you want to operate.
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  #44  
Old 18-09-2017, 05:02 AM
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An interesting short article talking about aperture (and also benefits of adaptive optics) that I find relevant for our discussion: http://www.cfht.hawaii.edu/en/outreach/NHN/size.html
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  #45  
Old 18-09-2017, 06:01 AM
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Originally Posted by Camelopardalis View Post
Traditionally, large photon buckets (pixels) were necessary to achieve acceptable SNR from the stream of photons hitting the sensor, largely because of high read noise and significant dark noise.

The only problem with tradition is that over time it gets outgunned by the technological revolution.

CMOS sensors are now starting to get competitive with their forebears with a combination of high QE and low noise, resulting in (commonly) smaller pixels that are able to achieve similarly acceptable SNR from fewer photons. That doesn't mean that chasing the faintest of fuzzies doesn't take a long time, but what it does it mean is that it isn't impossible with more humble kit.
The main improvement has been a reduction in read noise (there are CCDs with high QE as well.) This allows the use of much shorter subs. It doesn't make a lot of difference to the total integration time required as you still have to overcome shot noise. Going large on aperture and pixels is still a winning combination for those who can afford it.

Cheers,
Rick.
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  #46  
Old 18-09-2017, 06:25 AM
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Originally Posted by strongmanmike View Post
To cover most deep sky objects and coupled with the right two cameras, I recon you have the perfect combination of scopes there really Greg but aaaand hey, I could be wrong ...I'm gathering Andy is also not in the position to acquire a 17"CDK and a 12" AP Honders with the two mounts and cameras required either...?

At the end of the day, people like Andy need to do the image comparisons for themselves and do them properly ie compare full res versions, allow for image display size, use of AO, type of camera/pixels used and site quality etc. and critically judge the level of difference and be able to differentiate whether these differences are perceived or indeed real and then do what they wish with the budget they have. After already doing this though, I know what I recon

Mike
Thanks Mike. I wasn't suggesting though that he should spend mega dollars it was more about the theory that you can get exactly the same results.
Its certainly close but that extra bit does seem to come from aperture. GSO has made large aperture a lot more affordable in just a few years.
Just putting in a different point of view.

Greg.
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Old 18-09-2017, 12:54 PM
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Originally Posted by gregbradley View Post
Thanks Mike. I wasn't suggesting though that he should spend mega dollars it was more about the theory that you can get exactly the same results.
Its certainly close but that extra bit does seem to come from aperture. GSO has made large aperture a lot more affordable in just a few years.
Just putting in a different point of view.

Greg.
Hey no problems Greg, all good fun musing I was just trying to illustrate that a relatively inexpensive fast Newt around 1100-1200mm FL, coupled to a small pixel sensitive camera does pretty ok on small objects and (unlike the smartR's link to the CHART32 class outfit ) could perhaps fit in Andy's budget . Hey and then if one attaches a bigger chip you would also have a bigger field = win-win

Of course aperture is desirable but it comes at a $ cost and in typical Aussie conditions often fails to give the commensurate returns on the significant investment required in the OTA, suitably massive mount and necessary observatory.

I'd happily have your gear though

Mike
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  #48  
Old 18-09-2017, 01:55 PM
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This is such an interesting thread

I have been thinking..(oh no!)... and came to a conclusion, that perhaps it wouldn't be entirely incorrect to assume that small pixels are more demanding on optics than large pixels, except for the size of corrected circle?

I feel it is not so easy to achieve the same level of off-axis optical correction on a smaller telescope with small pixels, as it would be with a longer focal length combined with large pixels, given both are giving exactly the same image scale. A small telescope will have more significant field curvature that needs to be corrected/flattened, and then of course diffraction becomes more significant with smaller apertures.

And since most amateurs are on a fairly tight budget, rare are setups that can take a full advantage of small pixels with quality small optics, good mount and excellent dark skies. I feel we more often see images taken with sensors with small pixels that are put at the end of a less than perfect (“mass” produced) optics, less than perfect mount and in less than perfect conditions - city backyards/balconies. It would be a rather rare sight to see an FLI 16803 attached to a budget but large telescope riding on EQ6/8 and set up on a balcony in a city.

So perhaps in many cases effects of diffraction on a small telescope and limitations of small pixels are actually not the main “issues” affecting quality of data?
What say you?
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  #49  
Old 18-09-2017, 03:16 PM
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The title of the thread......Minimum fl.....
As the focal length increases so to does the size of the seeing disk - larger seeing disks can accommodate larger pixel sizes and still achieve good sampling.
It think it comes back to larger apertures = fainter targets, longer focal lengths = smaller FOV (unless we increase the camera frame size)
For 20 years the f10 SCT almost ruled supreme....add the x0.63 reduce and you have a long focal length solution.
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  #50  
Old 18-09-2017, 04:37 PM
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This is a reasonably complicated discussion as there are so many factors that have to be taken into account.
Mike uses a 12" F/3.8 with a ICX-694 which gives a sampling around 0.82"/pixel. This just so happens to be almost identical to a 12" F/8 with a 16803/11002/6306 which all have 9 micron pixels (I think the 6303 does anyway). Technically its closer to a F/7.5 than F/8 but as a standard RC is F/8 lets roll with that.

Both scopes need correctors of various kinds, both can be built both optically and structurally good, both have the same aperture and image scale (or near enough). Both of these should perform very similar to one another but one has about twice the focal length of the other. Both of these should perform similar to a 12" f/3 with an QSI690 as the image scale remains the same as does the aperture.

In terms of cost effectiveness I would say.
12" Newt + ICX-694
12" RC + 9 micron pixel (choose your poison)
RH300 + ICX-814

The difference between all of these systems them comes down to the quality of the optics and mechanics. GSOs are generally optically pretty good but mechanically leave a bit to be desired. A reasonable newtonian isn't overly expensive and even some of the premium ones are still less than what a comparable RC would cost. Throw in a smaller (cheaper CCD) and CONSIDERABLY cheaper filters, a good newtonian is difficult to beat from a price/performance aspect.

Suavi does bring up an important point, smaller pixels are most definitely more stressing on any optical system.... but likely not in this case. One of the benefits of higher resolution imaging is that your seeing conditions do help mask a lot of problem. Peter Ward a while back made the comment that he thought he had his RC nicely collimated until he had a night of very good seeing which then showed that it was a smidgen off.

A good example of this is with some of the Celestron Edge 14" scopes, some of them have an on-axis performance of something like 20 microns but because they're a 14" f/10, that equates to 1.16" resolution which when using a 11002 sensor still equates of a FWHM of 2 pixels under average seeing. If a small refractor had 20 micron on-axis spot sizes you'd send it back.

The cheapest option would be to get a reasonable 10" F/5 with a RCC Coma Corrector (think I got that right). Teleskop Service make a 10" F/5 ONTC with carbon fibre tube and various mods that they can do pre-shipment.
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Old 18-09-2017, 05:01 PM
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I thought Andy said his budget was $2k? Also worth remembering he has an EQ6. Can't believe people are discussing RH and CDK optics in this context. Perhaps his price was a typo and he meant $20k?
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Old 18-09-2017, 05:20 PM
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I think for under $2k it will have to be a Newtonian. And EQ6 will only carry so much, so I will second Colin - a 10" f/5 should be a one of the best options. I would personally go for 10" f/4 as it would be a bit lighter and easier to guide, but more challenging to collimate. Also, drizzling good quality data can always give that extra detail.

Thank you Colin for clarifying a few things, for me anyway. As for telescope options, would a 140 mm triplet at f/6.5 with ICX814 (0.84"pp) yield similar data to say RH300 or 12" fast Newton, or would diffraction get in the way with a 140mm aperture? I understand that a refractor would be slower with the same camera.
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Old 18-09-2017, 05:29 PM
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What about a s/h 10", 11" SCT with a x0.63 reducer???
Costs may be in the right ball park....
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  #54  
Old 18-09-2017, 05:35 PM
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I think for under $2k it will have to be a Newtonian. And EQ6 will only carry so much, so I will second Colin - a 10" f/5 should be a one of the best options. I would personally go for 10" f/4 as it would be a bit lighter and easier to guide, but more challenging to collimate.
That's the path I went down but it isn't as simple as just buying a GSO newt. He will need:
1. a better focuser with a QSI683wsg8 as this is a heavy camera (I've got the 683ws8)
2. a decent coma corrector - at least $300 worth I suspect
3. if it's f4 he'll need good collimation tools (Catseye)

He might need to reinforce the tube where the focuser attaches.
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Old 18-09-2017, 06:32 PM
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.......... would a 140 mm triplet at f/6.5 with ICX814 (0.84"pp) yield similar data to say RH300
Is this a rhetorical question?

To quote from the book of The Castle: "Tell 'em their dreaming!"

With a 140mm vs 300mm objective you are virtually in the same territory as my previously uploaded link.

The images were not fudged. If you can't see a difference in the roll-over
images......... might be time to see an ophthalmologist
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Old 18-09-2017, 06:50 PM
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Is this a rhetorical question?

To quote from the book of The Castle: "Tell 'em their dreaming!"

With a 140mm vs 300mm objective you are virtually in the same territory as my previously uploaded link.

The images were not fudged. If you can't see a difference in the roll-over
images......... might be time to see an ophthalmologist
Im not sure how daylight snapshots of an object a few hundred meters away really compares to long exposure astro photography through the entire atmosphere and on a moving mount

I didn't see your comment on the information from the article I provided a few posts down - I would be really interested in reading your opinion on that
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Old 18-09-2017, 07:05 PM
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Im not sure how daylight snapshots of an object a few hundred meters away really compares to long exposure astro photography through the entire atmosphere and on a moving mount

I didn't see your comment on the information from the article I provided a few posts down - I would be really interested in reading your opinion on that
The houses were not a few hundred metres away....several kilometres in fact.

I thought they pretty clearly showed how aperture influences image resolution.
(sampling between the two systems by dumb luck was almost identical)

Deep sky imaging simply has a few more layers of image abberration by looking through a tad more atmosphere, keeping the shutter open for longer and having to pan the camera....none of which reduce the effect of your starting point.

The image simply degrades even further from there.

Last edited by Peter Ward; 18-09-2017 at 08:52 PM.
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Old 18-09-2017, 07:12 PM
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Just read the article Suavi. Interesting.
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Old 18-09-2017, 07:45 PM
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Reviewing the Ray-Fan plots and spot diagrams in "Telescopes, Eyepieces Astrographs - Design, Analysis and Performance of Modern Astronomical Optics" I think the argument may not be so much about aperture as scope design. Fast refractors inherently have relatively lower resolution compared to slower, large aperture catadioptric systems (e.g R-C, corrected Dall-Kirkham, corrected Newtonian).

Tiny pixels only produce better resolution if the optics support it and even the amazing FSQ-106ED won't benefit from tiny pixels. Take a look at these spot diagrams which use a 100 micron scale: http://www.takahashi-europe.com/en/F...tics.spots.htm Measuring generously, at the centre of field you're looking at 10 micron spot size. And that's a high-end refractor/astrograph...

Cheers,
Rick.
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Old 18-09-2017, 09:04 PM
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..........Tiny pixels only produce better resolution if the optics support it ............
Cheers,
Rick.
I agree

Leonard Cohen said it way better than me: Hallelujah
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