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

[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.

[glend]

There can be many different answers based on circumstances. What mount, fixed or transport to dark site, weight to lift, type of imaging, budget of buyer, room for storage or observatory, etc etc....

Personally I believe the 'sweet spot' is around 10" , still light enough to handle and transport, doesn't require a monster mount and cost effective in most optical designs except refractors. Big enough for DSOs.
If there were no barriers to entry, i'd opt for a Planewave CDK - the smaller one.

[alocky]

Mirror, mirror, on the wall, who has the highest Strehl of all?

I would have to argue that it would be a 19.999" something with good optics. Assuming it is appropriately mounted, the sensor samples the image adequately for the focal length, and the system is well designed to eliminate 'problems' ie flex, stray light. I've guessed the CDK style, which is why I'm buying one. Others are opting for RC, and the best will always come from one of the custom optics houses. The fact that there are multiple options still commercially viable means there is no correct answer!
I heard the r-h was ok too, but I'd hate to think what a mangin mirror near 20" would cost.
Cheers,
Andrew.

[Eratosthenes]

Andrew,

I think a 19.999745" is the way to go - overall it performs better under various conditions.

I think it's worth the extra cost and the additional weight compared to your 19.999" should be okay if you have a friend to help you with loading onto the truck. (I would also consider the 19.99999614" which is heavier and out of my price range. Of course there are some 19.9" scopes coming out of China on ebay for about $75 - stay clear of them, the plastic mirrors are light but are near impossible to allign)

[LewisM]

I dunno.

I had once a Takahashi FCT-100 - considered the BEST 4" telescope of all time, heck, even considered the best optically of ANY refractor before or since. Yes, it had a reputed Strehl of 0.998 (call it parity really) - absolutely diffraction limited in every sense of the word. Also, VERY rare telescope (I paid a ransom for it). A telescope Roland happily tips his hat to. Only downside to it was it was pre-MC days, so the contrast wasn't as good as recent offerings (but still better than many!). Tak still makes a limited custom offering in the guise of the FCT-250 and FET-300 - both telescopes that I recall costs in excess of $100,000 (and come with their own custom mount, installed by Takahashi personnel).

It is both a visual (mind blowing) and imaging scope, as it laid the foundations for the FSQ legend.

So, as 4" goes, FCT-100 hands down. It is legendary, though many have not even ever heard of it.

For modern renditions for imaging, of course the FSQ-106N cannot be beaten. The FSQ-106ED may have slightly better correction, but I, and many others, feel the contrast and colour rendition of the N better to the ED, plus many of the N's can be found with better optics (higher Strehls) than the ED's.

I think for the larger refractor apertures, the Tak TOA 130 and 150 are right up there, but Roland and Yuri sure give them a VERY good run for the money and surpass them in many ways. The Tak FSQ130 is an unproven beast right now, so the jury is out on that one.

Mirrors? They hang on a wall... I know NOTHING about them, as my ugly mug tends to rack them :)

[alocky]

:lol:
True - but that aperture can exceed the 20" criteria on a warm day, although i believe you can get the optics made from a single flawless piece of diamond from Zeiss.
After Greg's recent purchase, this question and the coincidence of my children watching Disney's Snow White I had visions of Greg in his observatory preparing a poisoned apple...
Cheers,
Andrew.

[RugbyRene]

Seriously you guys? No wonder these forums seem so impsoing to new posters. Someone asks what is a very valid question and you guys turn it into a joke. Instead of making fun of the question maybe, just maybe you could contribute something worthwhile.:mad2:

[alocky]

OK, fair enough. Since I've never contributed anything worthwhile before I shall attempt to now. The 'best' optical system I assume to be defined by spot size across the imaging sensor area, since Greg asked about imaging (which, incidentally, he is not exactly a novice at). This will be the smallest for the largest aperture of any given optical design. End of story - very basic physics. Assuming the designer and optician have done their job there are a few options commercially available and I would argue none are 'the best'.
Now as to why anybody actually considering undertaking the purchase of an imaging instrument of this size would be relying on the opinion of an open Internet forum -? Then I sincerely hope they do some more detailed research. You are not going to buy it from Australian Geographic, and it is probably not for the easily intimidated or humourless.
Better?

[marc4darkskies]

+1. Although, I think the most often suggested answer to questions like Greg's is ... "mine". (Think about it)



+2

[LewisM]

Make that +2 and +3 :)

[sn1987a]

http://www.motifake.com/image/demotivational-poster/0902/hey-demotivational-poster-1235455903.jpg

:P

[LewisM]

Ahhhh, thank the Gods they weren't watching Frozen.

[alocky]

Noooo 😱!
Last time they put that on I had to listen to Led Zeppelin I-IV three times each to clear out the ear-worms....

[Eratosthenes]

...good point. You should always buy OTAs in the winter time. A mate of mine managed to buy a classic 19.9999997621" research grade scope for the price of a 19.9999954" (he used the money he saved on a couple of Barlow lenses and one of those electric tooth brush gadget thingies)

[sn1987a]

How about one of these with each sub 20"?.

http://www.diyphotography.net/wordpress/wp-content/uploads/2015/05/Dragonfly_Telescope.jpg

[alocky]

That's definitely the way to optimise the weather window for narrowband! Either that or the owner is a trilobite...

[AstralTraveller]

Well he won't be needing it then ... being extinct and all.

[Paul Haese]

How long is a piece of string? You don't really expect a rational answer here Greg do you? :) Isn't that a budget related question?

My take on it.

If you had the money you could buy the technical best, but you would want to put it in the best conditions otherwise it is just a trinket really. It would never perform at its best in sub standard conditions and a lesser scope in better conditions might just perform better.

There are plenty of scopes I would love to own, but non would be considered the best for one reason or another. The same as there is not one screw driver that will drive all screws; there is not one scope that can do all the imaging that can be done.

So the best scope does not exist in my opinion. It is a technical myth. Something to dream about.

BTW a 4" scope would not qualify as the best scope under my criteria and many others I'll warrant. It might be a good work horse if it got worked. ;)

[gregbradley]

Some funny responses. Poisoned apple is in fact already made.:rofl:

4 inch scopes for imaging usually boil down to Tak scopes. TSA102, FSQ106N and 106ED. I am not sure how good the AP versions were, I've heard variable opinions. The 106N gave great colour and was very sharp but it vignetted (that strange black bar through bright stars along the perimeter), had very little backfocus but a rock solid focuser. FSQ106ED focuser is not rock solid, the focus lock is defective, the colour correction may be good but the scope has a slight colour bias and colour rendition is not as good as 106N. The FSQ130 may take all that a step forward.

125-130mm you've got AP 130 GT and TOA130 and TSA120. APM make some in this area.

140-180 you've mainly got TEC and APM and 2nd hand AP (quite rare).
I always thought one of the best imaging scopes of all time was the AP155. The AP140 is a scaled version of the AP155 (same lens, same body just a different lens cell). TOA150 is superb as well. Although I do see people selling them after a while. Not sure why, maybe they are portable imagers and don't like the weight. The TEC140 is the most popular scope in the 140 band. Yuri kicked a goal with that one. Yuri though does not make reducers/correctors (he's a visual guy).

RC's of course are fabulous. The latest 20 inch Dall Kirkhams etc don't seem to be making the same incredible images the RCOS 20 inch scopes made famous by Russell Croman, Jay Gabany, Ken Crawford and others.
Pauls 12 inch RC images show the GSO scopes can hold their own with some work and bang for buck may well be the best.

So possibly the older RCOS 20 inch is the best all time imaging instrument. Tiny stars, super sharp, they work well with the STL11 camera.

The only images I have seen rival the older RCOS 20 inch images (you don't see many these days) have been Martin's CDK17 images from Sierra Nevada. Otherwise they are largely unrivalled except for Adam Blocks RCOS 32 inch images!

Greg.

[Peter Ward]

Chasing an eclipse?.... then a 20" CDK is probably the wrong choice.

Like to visually observe planets?.....then a Honders Astrograph is also a bad choice.

I've found one instrument simply can't do it all....so it comes down to "horses for courses"...and sure, while we'd all like price not to be an object, that often doesn't apply.

A 99.9% Strehl, zero-CTE mirrored, carbon OTA, half-metre diameter optic is as probable as a lotto-win for many.

However, some excellent, purpose built optics can be acquired at very affordable levels.
Rolf's truss reflector is a superb example of an imaging machine that shames many commercially made telescopes.

That said, T-minus 80 days (or so) until my all-singing-all dancing 16" Alluna arrives...so I might have a different perspective then ;)

[Kunama]

My guess is that it is cloudy again in Sydney .............

@ Rene, being new to the forum you may not be aware that these threads sometimes come up, usually on cloudy days or at times when one's imaging gear is being serviced. In this case the OP is a seasoned astrophotographer who produces a string of nice images but sometimes gets bored and throws in some bait for conversation/entertainment.

It is a bit like asking how much water is in a lake, without specifying which lake, which season or like how long is a piece of string.

In this instance the only correct answer is actually " an 18" dob "

[Slawomir]

But a 4" fluorite scope could possibly make "the best" guidescope... ;)

EDIT: It might be impossible to identify the best telescope for astrophotography, but I can tell you that Orion ED80TCF is one of the worst ones...that have a glass lens of course.

[Bassnut]

You know, as you get older, certain truths emerge. Myths, hype, commercial BS subsides and crystal clarity finally smacks you in the face. Its a bit like with cars, in the end, the only things that really count, are Horsepower and a monster Subby in the back. life is for living. For DS Astrophotography anyway, aperture counts, astronomy is about zooming in, after that its a just a matter of time over money. Enough of tiny refractors and high quality wide fields. How many times can you image the same top 10 big objects?, boring, narrow field rules :D. The best scope is the largest aperture you can afford balanced with the amount of time you are willing to spend on processing to cover the lack of money you spent on the scope :P.

[Paul Haese]

OAG only :eyepop:. Though for some it might well be a good guide scope. :lol:; expensive but good.

Don't get me wrong I have two Taks and I like both of them, but they are not in the best scope category for me. Good work horses though. ;)

[Paul Haese]

That's why I have two systems Fred. Best of both worlds. Besides as you well know there are more than 10 fields to image Freddo. Quite a few in LMC just for instance. ;) If you do enough hours on things then you can stretch it out with the wide field stuff. :lol:

[Bassnut]

Thats pretty lame, best of both worlds?, whats best about the other one?.
With a woozy enough refractor, you can do the LMC in one go, what then?.

"If you do enough hours on things then you can stretch it out with the wide field stuff". What does that mean?. Stretch (crop) out the wide field shot to look like fake bad NF?. Or once you see something you like on the WF shot, do it again in NF?, why bother?.

[gregbradley]

I don't know Fred. Consider this. Do you achieve a narrow field of view only with long focal length? Or can you also achieve the same with a wider scope and a small chip effectively giving you a digital zoom.

With the excellent Sony sensors (I know you like QE and these have the highest short of the KAF3200ME) you can get quite a narrow field of view that can come up to the longer focal length scopes of the same aperture.

That's making 2 scopes out of one.

Your 10 inch RCOS with the way smaller ICX694 sensor would get twice the QE in O111 you are getting now and would be super narrow field of view.

Greg.

[Paul Haese]

LOL Fred, I think it is time we had another AAIC to run amok at.

[Bassnut]

The ICX694 sensor has a well depth of 13,000 over 100,000 for the 6303!. I cant even be bothered thinking of the implications of that, its a joke!.

[Paul Haese]

Not to mention the over sampling either. Shudder to think.

[gregbradley]

Its 20,000. C'mon Fred I thought you were hard core!

The KAF8300 has 25.5K and is producing some of the best images around. How do deep wells help with narrowband anyway as they are typically easy on the saturation of wells. 20,000 would be plenty for most images bar a couple. 40,000 would be heap. 100,000 is a luxury admittedly. I do like the 105,000 wells of the 16803. Its the horrid LRGB image that can cause an issue?

Greg.

[gregbradley]

It depends on what point you consider oversampling is occurring. Per Stan Moore that would be .5arc secs which his rig is not far from under good seeing.

The only problem with oversampling is a loss of sensitivity nothing else. Its already 77%QE so perhaps it goes down to 65% - still more sensitive than anything else and only 4-5 electrons read noise versus 15 of the 6303.

Greg.

[PRejto]

Here here! Is anything happening on that front? If not I think a group of us ought to get together and plan something. Any takers?

Peter

[glend]

What is AAIC for those of us who are not in the 'in crowd'?

[N1]

AAIC is what happens when the focus of the hobby has completed its shift from the sky to the ground. :P:poke::scared3:

[Shiraz]

:rofl:

[strongmanmike]

Well, yes, I love Fred to death :love:..... buuuut at SPSP I can reliably report to the astronomical community that his general knowledge of the sky was indeed extremely lacking :eyepop:...but luckily for him my impromptu and extensive sky tours went down well with him I think, he actually seemed very interested and even a bit excited, even when I showed him those dreaded things called star clusters :eyepop: :lol:

Mike

[rmuhlack]

Australian Astro Imaging Conference ;)

[Bassnut]

Yes, I must say your tour was an eye opener and most excellent Mike.:D:thumbsup::thanx:

[strongmanmike]

It was great mate, first time I had a scope at SPSP since 2005 so I loved it :thumbsup: I was really happy with how the scope performed too, so easy to setup and get going and use and 12" shows plenty under a truly dark sky too.

Mike

[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.

[Shiraz]

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]

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]

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]

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!).:P

[Atmos]

How about...

http://www.officinastellare.com/products_scheda.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]

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]

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]

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/wikipedia/commons/1/18/Rachel-2-modern.jpg

[clive milne]

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]

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