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Ultrafast ultracompact refractors?

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#1 JKoelman

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Posted 24 March 2013 - 01:18 AM

While the reflector guys are happily experimenting with ultrafast sub f/3 optics, in refractors we don't see such developments. I wonder why. Sure, fast focal ratios in refractors won't eliminate the climbing of any ladders, but I would love an ultracompact small refractor package easily airline hand-carried and backpack compatible. Just imagine what compact focal lengths like f/2.8 would enable: 100 mm aperture in a tiny 280 mm focal length, and 140 mm aperture within 400 mm focal length. And I guess (no experience, I am a purely visual observer) that such fast optics would also enable new avenues in imaging.

To control CA in such ultrafast refractors, I presume even relatively purple-tolerant visual observers like myself would require quality APO optics. And yes, field flatteners, high quality focusers, and ethos eye pieces would make all of this quite costly compared to the same aperture at larger focal ratios. But would there not be a market? Any thoughts?

#2 UniversalMaster

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Posted 24 March 2013 - 07:11 AM

You can get this in a camera lens already?

#3 bilgebay

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Posted 24 March 2013 - 07:49 AM

+1

Get yourself a Canon EF 200mm f/2.8. I love mine.

#4 Andy Taylor

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Posted 24 March 2013 - 08:33 AM

I built a 100mm F4.

CA - yup, plenty on bright objects but for cruisin' the Milky Way, Double Cluster etc it's a stunner. :bow:

#5 dan_h

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Posted 24 March 2013 - 09:49 AM

For visual use only, you really need to consider the restrictions put in place by the exit pupil requirements.
Photography is a different story and there are lots of lens available if you have the funds.

dan

#6 Tamiji Homma

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Posted 24 March 2013 - 10:53 AM

I've tried a few camera lenses with eyepiece. I have only one eyepiece (Siebert UW 28mm) that comes to focus to infinity with the adapter combination. I need to cut down light path of EF mount and T2 adapter to try better (well corrected for fast scope) eyepiece .

Canon 400mm f/5.6, works very well:
Posted Image

Canon 200mm f/2.8, outer 50% is mess:
Posted Image

Canon 100mm f/2, outer 80% is mess:
Posted Image

Canon 50mm f/1.4, outer 95% is mess, feel like everything out of focus, bohke, tiny center is sharp, though.
Posted Image

I decided to try tiny 50mm f/6.6 refractor.

http://www.stellarvue.com/sv50.html
It seems that objective lens is high quality.

I am hoping this one takes backpack transport abuse well.

Tammy

#7 JKoelman

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Posted 24 March 2013 - 12:38 PM

Thanks for the reactions so far.

Interesting experiments, Tammy. The simple conclusion seems to be that general purpose fast camera lenses can't compete with astro gear specialized in imaging high contrast objects at infinity. And at best they come to focus at infinity without leaving any room for a diagonal.

I thought I mentioned, but now realize I forgot, that I am a visual observer with zero interest in imaging.

I still wonder what would result if APO designs were optimized for faster focal ratios.

#8 Tamiji Homma

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Posted 24 March 2013 - 01:24 PM

I still wonder what would result if APO designs were optimized for faster focal ratios.



There are a few fast refractors with good color correction such as Televue NP 101(is) f/5.4, NP 127is f/5.2. They are really good.

Pentax used to make 100 SDUF II f/4 scope, basically same design as NP 101/127, Petzval based.

Televue web site states that they use 5" f/4 flat field custom scope to test their eyepiece design.
Why Choose Tele Vue Eyepieces

Pentax 100 SDUF II color correction isn't as good as NP 101. It isn't portable either, chunky scope :)

It renders nice Jupiter with 2x Powermate and Ethos SX 4.7 at 170x.

Here is photo, as you can see, it looks ridiculous, doesn't it? :)
Posted Image

Mostly used as wide field scope like this:
Posted Image

Tammy

#9 Aquarist

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Posted 24 March 2013 - 03:07 PM

I have some fast Nikon lenses but have not yet played with them for AP

#10 Tamiji Homma

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Posted 24 March 2013 - 03:59 PM

I forgot to mention in previous post about SuperWASP and Canon EF 200mm f/1.8.

This is interesting application of Canon EF 200mm f/1.8 camera lens for astronomical research. The lens is very popular among indoor sport and wedding shooters. It has been discontinued and replacement is EF 200mm f/2 IS, bit slower with Image Stabilizer.

http://en.wikipedia.org/wiki/SuperWASP

http://www.universet...ding-observa...

Tammy

#11 Jon Isaacs

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Posted 24 March 2013 - 04:04 PM

The NP-101 has a flat field and what seems to be perfect color correction. But it is far from compact, its 26 inches long without a diagonal and the dew shield retracted.

Refactotors suffer from two aberrations' field curvature and chromatic. Both are exacerbated by shortening the focal ratio. Scopes like the NP-101 begin as long focal length apos and are shortened to flatten the field. Add in the difficulties with astigmatism in eyepieces at f/3 and I just don't see it being a reasonable alternative.

Jon

#12 james7ca

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Posted 24 March 2013 - 07:20 PM

I recently purchased a used, off-axis guider that came with a t-mounted (both sides) 0.5X reducer and I've found that the reducer works for photography with my AT72ED refractor. That gives me a 215mm f/3 but as yet I haven't yet been able to do any tests at night to determine the usable field of view (I suspect that it will be very small, nothing near to full frame on my APS-C camera).

However, it's certainly a faster combination than any of the telephoto lenses I have for my digital cameras (the best that I can currently do at 200mm is f/5.3 with my 70-300mm zoom and the only current offering from Nikon that would be close to this AT72ED/0.5X combination is a 180mm f/2.8 for about $1000 U.S.).

As for the OP's wish, I'm not sure why you would want such a scope for just visual use. That fast aperture is going to cost you in weight and size and it won't help that much for visual use. It you just want a very small and light scope pick up something in the 50mm to 60mm aperture class. The only thing you will miss is magnifications over 100x, which probably wouldn't be that practical in the type of scope you are asking for anyway (i.e. there is really no perfect telescope that can do it all).

I've used a Tele Vue 127is (660mm f/5.2) and while it is great for wide-field views and similar astrophotography it really isn't the best choice if you need high magnification (although I've done planetary and lunar work with this scope that looks pretty good given its modest aperture). I suspect that any possible f/2.8 refractor telescope would offer the same type of compromises. However, as others have already mentioned, if you are interested in ultra "fast" astrophotography then you'd probably be better off getting a telephoto lens rather than hoping for a f/2.8 refractor telescope.

#13 james7ca

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Posted 24 March 2013 - 07:39 PM

...
I decided to try tiny 50mm f/6.6 refractor.

http://www.stellarvue.com/sv50.html
It seems that objective lens is high quality.

I am hoping this one takes backpack transport abuse well.

Tammy

Can you provide any feedback on the SV50?

#14 UniversalMaster

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Posted 25 March 2013 - 01:15 AM

I wonder if some dyepieces are better than others for camera lenses? The camera lenses must have a not too bad field to be used for ordinary photography, so if the eyepiece adds some form of correction it might hurt the image?

#15 Jon Isaacs

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Posted 25 March 2013 - 08:02 AM

I wonder if some dyepieces are better than others for camera lenses? The camera lenses must have a not too bad field to be used for ordinary photography, so if the eyepiece adds some form of correction it might hurt the image?


A camera lens forms an image at the focal plane from bundles of converging rays. All that is necessary is that those rays converge to a point on the focal plane and the image will be sharp and clear.

But the job of the eyepiece is more difficult because it must deal with the rays and their various angles. Some eyepieces are quite good at this, others not so good.

Jon

#16 JKoelman

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Posted 25 March 2013 - 08:36 AM

I forgot to mention in previous post about SuperWASP and Canon EF 200mm f/1.8.

This is interesting application of Canon EF 200mm f/1.8 camera lens for astronomical research.

Awesome. Yet, I guess exoplanet transit monitoring is an application quite tolerant to various forms of image blur. As long as the stars under study are individually resolved, one can measure their intensity variations.

As an aside, this begs the question: why are amateur astronomers not involved in exoplanet detection?

#17 JKoelman

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Posted 25 March 2013 - 09:20 AM

As for the OP's wish, I'm not sure why you would want such a scope for just visual use. That fast aperture is going to cost you in weight and size and it won't help that much for visual use. It you just want a very small and light scope pick up something in the 50mm to 60mm aperture class. The only thing you will miss is magnifications over 100x, which probably wouldn't be that practical in the type of scope you are asking for anyway (i.e. there is really no perfect telescope that can do it all).


I live in a large city, and have the pleasure of dark night skies only when traveling. When reaching remote locations I want to see the fainter stuff. I am ok with not having available large magnifications (planets I can observe from my light-polluted back garden), but I don't want to compromise too much on light-gathering capability.

Currently, depending on the location and the mode of transport, I tend to take binoculars and/or my ST80 with me. Giving up on high-power views, the ST80 is wonderful. But I wonder, if an f/5 achro can give such pleasing views, why would it not be possible to get equally good or better views at the same aperture with ED glass or APO designs in a faster package?

I guess what I want is best described as a rugged 80mm+ aperture ultra-compact high-quality spotting scope with 90° angled view. No problem if magnifications are limited to the double digit range.

#18 Alterf

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Posted 25 March 2013 - 09:28 AM

Borg. Hutech produces ultra-fast, ultra-compact refractors.

#19 dan_h

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Posted 25 March 2013 - 10:07 AM

But I wonder, if an f/5 achro can give such pleasing views, why would it not be possible to get equally good or better views at the same aperture with ED glass or APO designs in faster optics?


There is a real limit to what abberations can be corrected with a simple doublet objective no matter what glass is used. And it's not just color correction that is needed. Field curvature and off axis astigmatism will be real problems with large ultra fast objectives. This will require more elements to get a suitable view ( = more weight, more $$$).

And not to beat a dead horse, but it seems my original comment regarding eye pupils has gone unnoticed. The eye places a real limit on the range of useable exit pupils, hence on the aperature and f ratio. You can go as low power as you wish with as wide a field as you want, but the eye will always restrict the maximum useable aperture dependant on the f ratio.

For example, try a 30mm eyepiece with a 300mm FL objective to get 10X. With a 5mm exit pupil, this allows for a 50mm objective. If you go to a shorter focal length objective such as 240mm FL, that same 30mm eyepiece gives 8X and a 5mm exit pupil limits the aperture to 40mm. No need at all for 100mm objectives.

Rather than try to reinvent the refractor, why not simply machine a telescope tube into 3 or 4 screw together sections and use already available optics like Borg? Or for visual only, get a nice high end binocular.

dan

#20 JKoelman

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Posted 25 March 2013 - 10:59 AM

And not to beat a dead horse, but it seems my original comment regarding eye pupils has gone unnoticed. The eye places a real limit on the range of useable exit pupils, hence on the aperature and f ratio. You can go as low power as you wish with as wide a field as you want, but the eye will always restrict the maximum useable aperture dependant on the f ratio.

For example, try a 30mm eyepiece with a 300mm FL objective to get 10X. With a 5mm exit pupil, this allows for a 50mm objective. If you go to a shorter focal length objective such as 240mm FL, that same 30mm eyepiece gives 8X and a 5mm exit pupil limits the aperture to 40mm. No need at all for 100mm objectives.


Sure, if at fixed aperture you scale back the objective focal length, you also have to scale back the focal lengths of the eyepieces. On a (still hypothetical) 90mm f/2.8 my longest eyepiece would be close to 14 mm. With something like 70° AFOV this would yield close to 4° TFOV at 18x power.

#21 dan_h

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Posted 25 March 2013 - 11:20 AM

<< With something like 70° AFOV this would yield close to 4° TFOV at 18x power.>>

A 100mm f6 fitted with a 35mm pan gets us there now with a bigger aperature!

Make a custom tube with a removable focuser and the whole thing can fit in a lunch box with room for a sandwich.

dan

#22 John Rhodes

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Posted 25 March 2013 - 01:08 PM

I've used a Tele Vue 127is (660mm f/5.2) and while it is great for wide-field views and similar astrophotography it really isn't the best choice if you need high magnification (although I've done planetary and lunar work with this scope that looks pretty good given its modest aperture).


James,
What problem are you having with the NP 127 at high power ?
given it's diffraction limited and color free...

#23 james7ca

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Posted 25 March 2013 - 05:32 PM


I've used a Tele Vue 127is (660mm f/5.2) and while it is great for wide-field views and similar astrophotography it really isn't the best choice if you need high magnification (although I've done planetary and lunar work with this scope that looks pretty good given its modest aperture).


James,
What problem are you having with the NP 127 at high power ?
given it's diffraction limited and color free...

Visually, there probably isn't too much to complain about. However, once you add a Powermate or barlow to increase the effective focal length and then try to use that for high-magnification photography you'll see some slight color and the edges of the photographic field will be quite poor (given my experience, YMMV). I suspect this could have nothing to do with the quality of optics in the NP127is, it's just as likely to be caused by the Powermates. This is why I said you'd probably have to compromise somewhere to get a really fast, large aperture refractor.

In any case, given its modest aperture I've done some lunar and planetary work that I think is very good for an under six inch telescope (not the absolute best, but still quite good). So, I can't really complain, it's just that under high magnification photography I have seen defects in the optical system which might not be as much of an issue in a well corrected, longer focal length scope. IMO, if you want to do high magnification astrophotography you should get an SCT or similar catadioptric system (for planetary and lunar work, a lot more bang for the buck). However, an SCT isn't going to give you the wide, flat field that you can get with a short focus, astrograph-like refractor, so there you go.

I seem to recall a description of the NP scopes that said that their color correction was similar to a good f/15 or f/20 achromatic refractor, just in a shorter tube and with a much larger and flatter field. That may sound harsh, but once you add a Powermate (or similar) to convert your f/5.2 scope to something close to an f/20 focal length it may not be too far from the truth.

Lastly, I am no expert in optics or telescope design or astrophotography and the above is just my opinion. Furthermore, my sample of the NP127is may not not represent the best that is available (units vary).

Here is a recent image of Saturn that was taken with an NP127is and a 5X Powermate, it probably suggests what it possible with this scope:

Attached Files



#24 james7ca

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Posted 25 March 2013 - 07:56 PM

Here is another high magnification shot taken with an NP127is (prime focus + 4X Powermate which results in an f/21 system). The crater Clavius is at the bottom, Tycho toward the top. I've estimated that the smallest craters that are clearly shown in this photo have angular diameters of about 2 arcseconds.

Attached Files



#25 CounterWeight

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Posted 25 March 2013 - 08:08 PM

I recently bought the Tak 'Baby-Q', would need the reducer to get down into the high 3's. Not a lot of refractors that fast, would be nice if there were more offerings.






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