What do you think is the best scope in the world under 20 inches at the moment?

[Slawomir (Suavi)]

This interesting discussion has induced in me a question about the impact aperture has on imaging.

Given the same sampling (in arcsec per pixel), would a large scope with large pixels capture more detail in comparison to a small scope with small pixels?

I know that it would make a difference for point sources of light (stars), but not sure about extended objects such as nebulae.

I also understand that small pixels mean shallower well depth so you need to have shorter exposures, but in case of Sony sensors read noise is also significantly lower than that of large chips and their sensitivity is relatively high, so apart from lesser FOV, perhaps not as much difference after all?

Any enlightening comments will be extremely valued and appreciated.

[Shiraz (Ray)]

Originally Posted by Slawomir

This interesting discussion has induced in me a question about the impact aperture has on imaging.

Given the same sampling (in arcsec per pixel), would a large scope with large pixels capture more detail in comparison to a small scope with small pixels?

I know that it would make a difference for point sources of light (stars), but not sure about extended objects such as nebulae.

I also understand that small pixels mean shallower well depth so you need to have shorter exposures, but in case of Sony sensors read noise is also significantly lower than that of large chips and their sensitivity is relatively high, so apart from lesser FOV, perhaps not as much difference after all?

Any enlightening comments will be extremely valued and appreciated.

if the atmosphere is the limiting factor (eg for scopes above about 6 inches in Australia), the scope size does not determine resolution. To obtain best possible image detail, the angular size of the pixels should be around 1/2-1/3x the seeing ( ie 0.67 - 1 arc sec in Australian 2 arc sec seeing). Oversampling does not get you more detail (it can't because the sky gets in the way and it doesn't take any account of what you do). Some find that oversampling provides for more freedom with deconvolution, but if you do oversample, you take a big hit in sensitivity - 2x oversampling requires 4x the time spent imaging. In terms of sampling under seeing-limited conditions, all that matters is the angular subtense of the pixels - for example, a 1m fl scope with 4.5 micron pixels will produce an identical image to a 2m scope with 9 micron pixels - both have the same pixel scale. The sensitivities of the two systems will depend on the aperture, but if both have the same aperture, the performance will be identical. ie a 1m f4 scope with 4.5 micron pixels is functionally identical to a 2m f8 scope with 9 micron pixels.

the debate about well depth is a furphy - part of the marketing by camera makers with high read noise chips. what matters is the dynamic range (the ratio of well depth to read noise) and most chips get to around 70-80 dB, regardless of well depth. If you have low read noise, you can use short subs and the average of many short subs with a low read noise chip will be the same as the average of fewer, longer subs with a noisier chip. this explains dynamic range fairly well http://www.ccd.com/ccd111.html . Some typical dynamic ranges are 8300=69dB, 694=71dB, 3200=77dB, 16803=80dB. Low well depth can cause saturation problems if you insist on soaking the chip with photons in super long subs - something you had to do with older chips, but don't need to do if the read noise is low.

There is only a difference in system performance for extended and point source objects if the system is heavily undersampled - wide-field vistas showing a carpet of myriads of almost identical point stars come to mind.

[Slawomir (Suavi)]

Thank you Ray. I was thinking along the same lines, but you have explained it very clearly. Dynamic range is a much better measure of camera's capacity to record cosmic realities than well depth just by itself.

It seems then that 16803 at the back of a 2m fl quality scope that is also fairly fast would be hard to beat, given that this rig would be sitting at the top of a quality mount and somewhere dry and dark. Unfortunately we only have two kidneys and that might not be enough...

[clive milne]

There is a basic premise in this discussion that a few people have skirted around and even less have addressed directly, and that is the arbitrary nature of the parameter that defines the fundamental boundary.


The way professional institutions approach the subject is along these lines: Maximise the science return per dollar invested... ALL other aims are inconsequential. .....period.....

[Atmos (Colin)]

It truly is a very difficult question to answer as the best telescope is basically the one that you're going to use the most. If you have a small refractor you can be limited to larger objects in the sky. If you have a huge telescope, it can be wonderful at smaller objects but taking an image of Orion Nebula would end up becoming a 1000 image mosaic! For planetary you could be better with a 4-6" refractor with a 5x barlow.

As has been mentioned, the telescope and camera combination makes all the difference! As an example, most would argue that a 11" F/3 would flog my Meade 10" F/10 with a 0.63 focal reducer. If they use a KAF8300, a VERY popular and good camera and I use a Alta F77, due to the pixel sizes (5.4 vs 12) and QE peak (56% vs 96%), the second setup is nearly twice as fast! Of course, it is the difference between 8.3MP vs 0.26MP.

Furthermore, if you're just doing AP then the first setup would be better, albeit needing nearly twice the amount of exposure time to reach the same magnitude, the extra FOV is more than worth it. Of course, for scientific analysis, the second setup is probably going to be okay even with the 512x512 sensor.

To sum it all up, the best optical system is essentially whatever you're planning on doing with it.

[sn1987a (Barry)]

The best scope under 20" is a 28" Dob and just wait for the AP tech to catch up with it. (Think outside the square man!).

[Atmos (Colin)]

How about...

http://www.officinastellare.com/prod....php?idProd=12

When throwing a 16803 sensor (9 micron pixels) on it, you will get pinpoint stars across the entire image as the RMS remains below the pixel size.

Plus it's a 16" F/3.8!

[gregbradley]

Yes Colin those astrographs look very nice. iDK sell something similar as well. Yuri from TEC makes a couple of astrographs too.
I'd have to see some proof of performance though as these are all newish to the market and not many examples of it in action.

Greg.

[Don Pensack]

Originally Posted by gregbradley

A contentious question. I mean for deep sky imaging as visual would have a whole lot of different values.

Perhaps it should be by aperture. 4, 5,6,8, 10, 12, 14, 16, 17, 20 inch aperture of any type.

Greg.

You need to specify fixed (observatory) or portable.
And is cost an issue?
And a preference for type?
Photographic use or visual?
At each size, you could pick a "best of breed" for each type, though finding commercial refractors over 10" might be tough.
Some of the scopes will exceed a quarter million dollars.
Don Pensack
Los Angeles

[Atmos (Colin)]

Removing money aside for a moment, I personally believe that the best telescope that one could buy would be a custom built 500mm (19.68") F/5 refractor.
At this point, to get the quality that we're all going to want (near perfect colour correction, a very high correction and flatness), it's going to cost some $5-10,000,000 to fabricate. Throw in some high quality barlows, 0.6 & 0.4 field reducers for F/3 and F/2 wide fields.

By playing with barlows and field reducers, you have an all round telescope for both wide and narrow field astrophotography wile having now central obstruction so even tighter stars.

[Slawomir (Suavi)]

Although certainly not an F/5 telescope, this one fits into 20inch refractor category. Just need a scissor lift to look though it...

http://upload.wikimedia.org/wikipedi...l-2-modern.jpg

[clive milne]

Originally Posted by Slawomir

Although certainly not an F/5 telescope, this one fits into 20inch refractor category. Just need a scissor lift to look though it...

http://upload.wikimedia.org/wikipedi...l-2-modern.jpg

Must use depleted uranium for the counterweights judging by the size of them.

[raymo]

Love the ship's steering wheel. Am I missing something, but why is it so tall? even at the zenith the eyepiece would be way unreachable.
raymo

[cometcatcher (Kevin)]

Originally Posted by clive milne

Must use depleted uranium for the counterweights judging by the size of them.

They also did em wrong. They should be using more further up the shaft!