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Equipment Discussions >> Refractors

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JKoelman
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Reged: 05/16/11

Loc: Bangalore, India
Ultrafast ultracompact refractors?
      #5753174 - 03/24/13 02:18 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?


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UniversalMaster
professor emeritus


Reged: 11/20/08

Re: Ultrafast ultracompact refractors? new [Re: JKoelman]
      #5753363 - 03/24/13 08:11 AM

You can get this in a camera lens already?

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bilgebay
Post Laureate
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Reged: 11/06/08

Loc: Turkiye - Istanbul and Marmari...
Re: Ultrafast ultracompact refractors? new [Re: UniversalMaster]
      #5753401 - 03/24/13 08:49 AM

+1

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


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Andy Taylor
Twisted, but in a Good Way
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Reged: 09/24/08

Loc: Epsom - UK
Re: Ultrafast ultracompact refractors? new [Re: bilgebay]
      #5753455 - 03/24/13 09:33 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.


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dan_h
Carpal Tunnel


Reged: 12/10/07

Re: Ultrafast ultracompact refractors? new [Re: JKoelman]
      #5753638 - 03/24/13 10:49 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


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Tamiji Homma
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Reged: 02/24/07

Loc: California, USA
Re: Ultrafast ultracompact refractors? new [Re: JKoelman]
      #5753785 - 03/24/13 11:53 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:


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


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


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


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


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JKoelman
professor emeritus


Reged: 05/16/11

Loc: Bangalore, India
Re: Ultrafast ultracompact refractors? new [Re: Tamiji Homma]
      #5754018 - 03/24/13 01:38 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.


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Tamiji Homma
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Reged: 02/24/07

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Re: Ultrafast ultracompact refractors? new [Re: JKoelman]
      #5754147 - 03/24/13 02:24 PM

Quote:

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?


Mostly used as wide field scope like this:


Tammy


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Aquarist
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Reged: 08/27/12

Loc: Illinois
Re: Ultrafast ultracompact refractors? new [Re: Tamiji Homma]
      #5754358 - 03/24/13 04:07 PM

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

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Tamiji Homma
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Reged: 02/24/07

Loc: California, USA
Re: Ultrafast ultracompact refractors? new [Re: JKoelman]
      #5754456 - 03/24/13 04:59 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.universetoday.com/13406/superwasp-are-super-planet-finding-observa...

Tammy


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Jon Isaacs
Postmaster
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Ultrafast ultracompact refractors? new [Re: Tamiji Homma]
      #5754470 - 03/24/13 05:04 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


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james7ca
sage


Reged: 05/21/11

Re: Ultrafast ultracompact refractors? new [Re: Jon Isaacs]
      #5754898 - 03/24/13 08:20 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.


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james7ca
sage


Reged: 05/21/11

Re: Ultrafast ultracompact refractors? new [Re: Tamiji Homma]
      #5754920 - 03/24/13 08:39 PM

Quote:

...
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?


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UniversalMaster
professor emeritus


Reged: 11/20/08

Re: Ultrafast ultracompact refractors? new [Re: james7ca]
      #5755416 - 03/25/13 02:15 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?

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Jon Isaacs
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Ultrafast ultracompact refractors? new [Re: UniversalMaster]
      #5755694 - 03/25/13 09:02 AM

Quote:

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


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JKoelman
professor emeritus


Reged: 05/16/11

Loc: Bangalore, India
Re: Ultrafast ultracompact refractors? new [Re: Tamiji Homma]
      #5755723 - 03/25/13 09:36 AM

Quote:

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?


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JKoelman
professor emeritus


Reged: 05/16/11

Loc: Bangalore, India
Re: Ultrafast ultracompact refractors? new [Re: james7ca]
      #5755809 - 03/25/13 10:20 AM

Quote:

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.


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Alterf
professor emeritus
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Reged: 04/07/06

Re: Ultrafast ultracompact refractors? new [Re: JKoelman]
      #5755825 - 03/25/13 10:28 AM

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

Edited by Alterf (03/25/13 10:29 AM)


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dan_h
Carpal Tunnel


Reged: 12/10/07

Re: Ultrafast ultracompact refractors? new [Re: JKoelman]
      #5755915 - 03/25/13 11:07 AM

Quote:

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


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JKoelman
professor emeritus


Reged: 05/16/11

Loc: Bangalore, India
Re: Ultrafast ultracompact refractors? new [Re: dan_h]
      #5756032 - 03/25/13 11:59 AM

Quote:

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.


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dan_h
Carpal Tunnel


Reged: 12/10/07

Re: Ultrafast ultracompact refractors? new [Re: JKoelman]
      #5756077 - 03/25/13 12:20 PM

<< 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


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John Rhodes
Vendor (Televue Rep)
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Reged: 02/21/06

Loc: Torrance, CA.
Re: Ultrafast ultracompact refractors? new [Re: james7ca]
      #5756256 - 03/25/13 02:08 PM

Quote:


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...


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james7ca
sage


Reged: 05/21/11

Re: Ultrafast ultracompact refractors? new [Re: John Rhodes]
      #5756772 - 03/25/13 06:32 PM Attachment (42 downloads)

Quote:

Quote:


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:


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james7ca
sage


Reged: 05/21/11

Re: Ultrafast ultracompact refractors? new [Re: james7ca]
      #5756996 - 03/25/13 08:56 PM Attachment (50 downloads)

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.

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CounterWeight
Postmaster
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Reged: 10/05/08

Loc: Palo alto, CA.
Re: Ultrafast ultracompact refractors? new [Re: james7ca]
      #5757019 - 03/25/13 09:08 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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Jayo
super member


Reged: 02/03/07

Loc: Quebec City
Re: Ultrafast ultracompact refractors? new [Re: CounterWeight]
      #5757371 - 03/26/13 01:34 AM

I have the Canon 200 f1.8. Fast and fabulously sharp wide open.

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gezak22
scholastic sledgehammer
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Reged: 08/15/04

Loc: On far side of moon. Send help...
Re: Ultrafast ultracompact refractors? new [Re: Jayo]
      #5757401 - 03/26/13 02:02 AM

Quote:

I have the Canon 200 f1.8. Fast and fabulously sharp wide open.




... and thus very sensitive to temperature variations (Source)

I must say I am quite happy with my f/4.5 instrument. Any faster and I am losing too much light when narrowband imaging (Source, about two thirds down). f/1.8 will require temperature compensation, thus adding system complexity. I think in ~5-10 years when my next scope upgrade is due, the Borg 125 will suit me just fine - even at f/5.

Edited by gezak22 (03/26/13 02:03 AM)


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ManuelJ
professor emeritus


Reged: 12/19/05

Loc: Madrid, Spain
Re: Ultrafast ultracompact refractors? new [Re: james7ca]
      #5757432 - 03/26/13 02:49 AM

Quote:

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.




James, the Powermate does not work well with the NP. I had that combo in the past and it was horrible.

BTW, the TMB 1,8x and the Abbe 2x works perfectly. And you can use the Abbe in 4x format.


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james7ca
sage


Reged: 05/21/11

Re: Ultrafast ultracompact refractors? new [Re: ManuelJ]
      #5757458 - 03/26/13 03:09 AM

Manuel, were you trying to use the Powermate on DSOs or on the moon and planets? I've never tried to use the Powermate to photograph DSOs, but I'd think that anything even close to edges of the field would be "horrible" (as you put it). I've found that if you restrict the field to the center half it isn't too bad, but it certainly isn't flawless.

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ManuelJ
professor emeritus


Reged: 12/19/05

Loc: Madrid, Spain
Re: Ultrafast ultracompact refractors? new [Re: james7ca]
      #5757480 - 03/26/13 03:54 AM

Quote:

Manuel, were you trying to use the Powermate on DSOs or on the moon and planets? I've never tried to use the Powermate to photograph DSOs, but I'd think that anything even close to edges of the field would be "horrible" (as you put it). I've found that if you restrict the field to the center half it isn't too bad, but it certainly isn't flawless.




I'm speaking about visual configuration. The powermate introduces CA.


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Tamiji Homma
Post Laureate
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Reged: 02/24/07

Loc: California, USA
Re: Ultrafast ultracompact refractors? new [Re: james7ca]
      #5757928 - 03/26/13 10:48 AM

Quote:

Quote:

...
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?




Sure I will. It is on my way from Auburn CA.

Tammy


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John Rhodes
Vendor (Televue Rep)
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Reged: 02/21/06

Loc: Torrance, CA.
Re: Ultrafast ultracompact refractors? new [Re: james7ca]
      #5760049 - 03/27/13 11:36 AM

Quote:


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.




James,
We'd be interested to know where your recollection about NP127is color correction equivalency to an f/15 or f/20 achro came from,
since the actual equivalency is about f/90... according to Al.


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jrcrillyAdministrator
Refractor wienie no more
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Reged: 04/30/03

Loc: NE Ohio
Re: Ultrafast ultracompact refractors? new [Re: John Rhodes]
      #5760057 - 03/27/13 11:42 AM

Quote:

Quote:


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.




James,
We'd be interested to know where your recollection about NP127is color correction equivalency to an f/15 or f/20 achro came from,
since the actual equivalency is about f/90... according to Al.




F/15 to F/20 equivalence sounds more like the original Genesis model.


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james7ca
sage


Reged: 05/21/11

Re: Ultrafast ultracompact refractors? new [Re: John Rhodes]
      #5761004 - 03/27/13 06:27 PM Attachment (28 downloads)

Quote:

Quote:


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.




James,
We'd be interested to know where your recollection about NP127is color correction equivalency to an f/15 or f/20 achro came from,
since the actual equivalency is about f/90... according to Al.



I got that impression from something I read probably about one year ago, but I can't remember where so it has to be called an unsubstantiated report. It could have been in reference to the Genesis model, which certainly would be an incorrect characterization of the NP scopes (on my part).

In any case, I've worked for almost a year to remove all traces of what I will call false color from my NP127is, at least when working photographically at very high magnifications (as I said previously, visually I've not seen much to complain about). I suspect I've gone well beyond what most users attempt with their NP127is, using a 16 megapixel RGB sensor with the full range of Powermates (2X to 5X) to try to get the highest resolution pictures possible (with that scope and aperture). The results have been pretty good, but in the end I usually have to deal with a little bit of what I've concluded to be false color. One issue is that it is hard to find examples of anyone doing similar work, most lunar and planetary photographers are using scopes much larger than the NP127is so just for the reason of physics (resolution limited by aperture) I'm kind of working alone in this area.

Frankly, after reading ManuelJ's comments about the Powermates being "horrible" with the NP series of scopes I'm probably more likely now to attribute the color to the Powermates, which in my original post I mentioned as a possible culprit. In my experience I wouldn't call the Powermates bad, unless you are trying to use the entire field of an APS-C camera in which case you'd probably be disappointed with the results (shows up mostly on full-frame shots of the moon, since on planetary work I can keep the subject confined to the center of the field). When right on axis and at the center of the field I don't see much of a problem, the color is often vanishingly small to the point where most users would probably not even notice.

Truth be told, evaluating the performance of a scope while in the field can be a little difficult. Given all of the variables like seeing, temperature, object location (not at the zenith), mount stability, focus-plane tilt and/or collimation (which I haven't tried to adjust), focus accuracy (I've tried many methods), the transfer optics and sensor, and last but not least plain user error. I'm not fully ready to claim that the NP127is is lacking for high resolution photography as I'm still trying to optimize my technique. My latest changes are to use a Baader Clicklock (2" to Tele Vue imaging system, or 1.25 inch to T-Thread) to hold the Powermate which is then threaded directly to the NP127is' imaging system extensions and adapter (none of the standard extension tubes I've tried will really hold the Powermate and camera rock-solidly square to the focuser). I've also tried a Baader Fringe Killer filter with the Powermates but I haven't yet done a good A-to-B test to see if that results in a notable improvement.

In any case, can anyone point me to a good sample of lunar or planetary work that has been done with an NP scope? Something with a full set of technical details (scope, camera, exposure, transfer optics, etc.).

Below is another sample of the work I've done with my NP127is. It shows the shadow of Jupiter's moon Ganymede (quite obvious) as well as a transit of the moon Europa (look near the center of the lower white cloud band, that small spot on the clouds is the silhouette of Europa). The moons Ganymede and Io are visible off to the right. This image was captured under fairly good seeing (not great, just average or a bit better) with a 5X Powermate and an Imaging Source DBK 21AU04.AS camera (video stack processed in Registax v6). This has been slightly over sharpened (note the ringing on the edge of Jupiter and around the shadow of Ganymede), it's one of a series of images that I want to combine into an animation showing the movement of Jupiter's moons.


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orion61

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Re: Ultrafast ultracompact refractors? new [Re: james7ca]
      #5761371 - 03/27/13 09:39 PM

beautiful! job..
I used to have a Jagers 5" F5..
what a great scanning scope, I put it on a paralax mount and observe frome a lounge chair..
usually woke up wet about 2 or 3 hrs later with dew...


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Daud
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Re: Ultrafast ultracompact refractors? new [Re: JKoelman]
      #5761476 - 03/27/13 10:42 PM

The second attempt of short f.l. 100 mm scope, this time at F5.5, from Skywatcher

ESPRIT 100ED
The closest to OP's idea what the technology can achieve today?
The previous F5 version suffered from miscollimation despite commendable effort to protect the scope in shipping.

Edited by Daud (03/27/13 10:44 PM)


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james7ca
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Re: Ultrafast ultracompact refractors? new [Re: Daud]
      #5761707 - 03/28/13 02:51 AM

That shipping container is insane (in a good way).

Actually, something a little faster than the Esprit would be the Tele Vue NP127is or NP101is. The NP127is has a native f/5.2 speed and with a Tele Vue 0.8X reducer goes to f/4.2 at 5 inches of aperture. The Tele Vue optical tubes are also lighter than the Skywatcher Esprits. I believe that the NP127is has the fastest native aperture of any 5" APO that you can buy (or maybe I should say that is widely available for purchase, since there are probably some less well stocked, even more boutique units that go below f/5.2).

In any case, a few of the 4" Takahashi APOs can go down to f/3.6 with one of their matched reducers (but it will cost you).

Does anyone know of a faster combination at 4" or larger than the f/3.6 Taks?


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Andy Taylor
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Re: Ultrafast ultracompact refractors? new [Re: james7ca]
      #5761732 - 03/28/13 04:38 AM Attachment (22 downloads)

My 100mm F4 achro grab 'n go.

CA on bright objects but it's really for wide field cruisin'.


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Re: Ultrafast ultracompact refractors? new [Re: james7ca]
      #5761769 - 03/28/13 06:17 AM

Quote:

Does anyone know of a faster combination at 4" or larger than the f/3.6 Taks?





Will this come to focus with a 2 inch Stardiagonal?

The FSQs and NP's scopes are fast but not ultra-compact. They get their speed with corrective optics in the rear of the scope. The original poster was interested in a fast scope mostly to make it compact, a Traveler is compact, an NP-101 is not.

Jon


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JKoelman
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Re: Ultrafast ultracompact refractors? new [Re: Jon Isaacs]
      #5762144 - 03/28/13 10:42 AM

Quote:

The FSQs and NP's scopes are fast but not ultra-compact. They get their speed with corrective optics in the rear of the scope. The original poster was interested in a fast scope mostly to make it compact, a Traveler is compact, an NP-101 is not.




Right! Focal ratio is a proxy to "bulkiness", but a more direct measurement would be the aspect ratio: the bare tube length (dew shield retracted, diagonal removed) divided by the aperture.

My ST-80 and my EON 80 both reach an aspect ratio of 380mm/80mm = 4.75. Would the ST-80 be given a retractable dew shield, this figure could reduce to 310/80 = 3.9.

Googling for tube dimensions, it seem difficult to get significantly below this figure. The Takahashi SKY-90 has 350/90 = 3.9, and the Baby-Q 323/85 = 3.8.

The AP Stowaway achieves the same aspect ratio: 356/92.5 = 3.8.

I suspect scopes with lower aspect ratios are not feasible with current optical technology (the Pentax 100 SDUF II seems to be surprisingly long: 492 mm).

Having said this, I am sure Andy's f/4 will manage to dive below the 3.8 mark provided the dew shield is retractable.


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Andy Taylor
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Re: Ultrafast ultracompact refractors? new [Re: JKoelman]
      #5762244 - 03/28/13 11:44 AM

Quote:


Having said this, I am sure Andy's f/4 will manage to dive below the 3.8 mark provided the dew shield is retractable.




3.1 actually...


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Tamiji Homma
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Re: Ultrafast ultracompact refractors? new [Re: james7ca]
      #5762257 - 03/28/13 11:55 AM

Quote:

Quote:

...
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?




Hi James,

I received SV50A ED yesterday. Build quality is the first rate for every part of the package. I mean it is very solid build, ie: heavy for the size. R&P focuser is smooth. I haven't tested how much weight the focuser can take yet.

I did short terrestrial view during daytime and the Moon and Jupiter observation at night.

The little guy has indeed high quality objective lens. I see good color correction. Star test looks very good (128x with Pentax XO 2.58).

I used Ethos SX 3.7 mostly to observe the Moon and Jupiter. I am impressed by this little guy can pull off

With SCT->T2 adapter/Baader Prism/Mirror diagonal, I could use some of 2" eyepieces. 2" eyepiece is too big for this guy, though.

I'll take this guy to hiking

For size comparison AT65EDQ v.s. SV50A ED, hood/focuser retracted:



Here is when it is used.


Tammy


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Re: Ultrafast ultracompact refractors? new [Re: Andy Taylor]
      #5762379 - 03/28/13 01:11 PM

Quote:

Quote:


Having said this, I am sure Andy's f/4 will manage to dive below the 3.8 mark provided the dew shield is retractable.




3.1 actually...




Wow!

I should have predicted that value: my 80mm ShortTube has identical focal length, and as mentioned, with retractable dew shield it's length would measure 310 mm...

Found the original thread describing your f/4 build. What is the maximum magnification you use with this scope? Are ~40x views of deep sky objects still pleasing?


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Re: Ultrafast ultracompact refractors? new [Re: JKoelman]
      #5762507 - 03/28/13 02:28 PM

Quote:

Quote:

Quote:


Having said this, I am sure Andy's f/4 will manage to dive below the 3.8 mark provided the dew shield is retractable.




3.1 actually...




Wow!

I should have predicted that value: my 80mm ShortTube has identical focal length, and as mentioned, with retractable dew shield it's length would measure 310 mm...

Found the original thread describing your f/4 build. What is the maximum magnification you use with this scope? Are ~40x views of deep sky objects still pleasing?




She's changed a bit since then - she now has a metal focuser and a 2" diagonal which I'm hoping to convert into a prism type (it may or may not help with the CA but worth a try...). I also want to replace the soil pipe to something better.

I find the "sweet spot" for this scope is a TV 15mm plossl which gives ~27x. Exit pupil works out at 3.75mm. It also frames the Double Cluster perfectly.


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james7ca
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Re: Ultrafast ultracompact refractors? new [Re: Tamiji Homma]
      #5762628 - 03/28/13 03:53 PM

Tammy, nice pictures (of the SV50) and thanks for the feedback.

I notice that you appear to have a 2" diagonal on your (?) AT65EDQ. I assume, therefore, that with that diagonal you have no problems reaching focus with all of your eyepieces?

Also, you've apparently added a Baader Clicklock diagonal to the SV50. Nice, but (if I may) how much did that add to the total cost of the package?


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james7ca
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Re: Ultrafast ultracompact refractors? new [Re: JKoelman]
      #5762705 - 03/28/13 04:51 PM

Quote:

Quote:

The FSQs and NP's scopes are fast but not ultra-compact. They get their speed with corrective optics in the rear of the scope. The original poster was interested in a fast scope mostly to make it compact, a Traveler is compact, an NP-101 is not.




Right! Focal ratio is a proxy to "bulkiness", but a more direct measurement would be the aspect ratio: the bare tube length (dew shield retracted, diagonal removed) divided by the aperture.

My ST-80 and my EON 80 both reach an aspect ratio of 380mm/80mm = 4.75. Would the ST-80 be given a retractable dew shield, this figure could reduce to 310/80 = 3.9.

Googling for tube dimensions, it seem difficult to get significantly below this figure. The Takahashi SKY-90 has 350/90 = 3.9, and the Baby-Q 323/85 = 3.8.

The AP Stowaway achieves the same aspect ratio: 356/92.5 = 3.8.

I suspect scopes with lower aspect ratios are not feasible with current optical technology (the Pentax 100 SDUF II seems to be surprisingly long: 492 mm).

Having said this, I am sure Andy's f/4 will manage to dive below the 3.8 mark provided the dew shield is retractable.



My Astro-Tech AT72ED ratio comes out to 4.3. The Tele Vue NP127is comes in at 6.5, while the very small and thus highly portable Stellarvue SV50 is 4.7. The monster (long) Celestron 102GT f/9.8 refractor comes in at 9.2 (but very cheap -- $60 -- during the recent sale at OPT).

However, if you modify Johannes' aspect ratio formula to factor in light gathering (square of the aperture) then the Tele Vue NP127is actually comes out to be the more efficient tube design (of the three listed above, Andy's 100mm f/4 is still significantly better). However, the resulting units mm/mm*mm come out to be: units/mm (which may indicate this is something of an odd metric -- tube length per light gathering?).

In this case (tube length / aperture squared) the smaller ratio is "better":

SV50: 0.094
102GT: 0.090
AT72ED: 0.059
TV127is: 0.051
Tak Baby-Q: 0.045
Andy's Grab 'n Gasp (assuming a 310mm tube length): 0.031

By squaring the aperture you prevent something like a hypothetical 25mm aperture f/3 from looking too good:

75mm~ish / 25mm = 3 (the optical tube would likely to be even shorter to allow for back focus, so I'd expect the ratio to be below 3).

but

75mm / (25mm x 25mm) = 0.12/mm (not too good, but who would really want such a scope for visual astronomy even if it was small and squat?).

Edited by james7ca (03/28/13 05:58 PM)


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Tamiji Homma
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Re: Ultrafast ultracompact refractors? new [Re: JKoelman]
      #5762814 - 03/28/13 05:50 PM

Quote:

I suspect scopes with lower aspect ratios are not feasible with current optical technology (the Pentax 100 SDUF II seems to be surprisingly long: 492 mm).




Pentax 100 SDUF II lens hood isn't retractable. Lens cell is about 90mm from edge of lens hood. From helical focuser end to hood it is about 388mm when focuser is full racked in.



Higher resolution photo

Tammy


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Re: Ultrafast ultracompact refractors? new [Re: james7ca]
      #5763160 - 03/28/13 08:50 PM

Quote:

However, if you modify Johannes' aspect ratio formula to factor in light gathering (square of the aperture) then the Tele Vue NP127is actually comes out to be the more efficient tube design (of the three listed above, Andy's 100mm f/4 is still significantly better). However, the resulting units mm/mm*mm come out to be: units/mm (which may indicate this is something of an odd metric -- tube length per light gathering?).




It's probably best to keep this metric length divided by the aperture. My 12.5 inch F/4.06 has a tube that is about 1270mm long and a aperture of 317.5mm. The area weighted ratio is 0.000197. The 25 inch F/5 with it's 10 foot tube is 0.000069. You don't even want to know what a C14 is.

Two scopes I have that are quite compact. An old University Optics 80mm finder, it's F/3.75 with a short dew shield. I modified it using plumbing fittings to take a 2 inch diagonal. The over all length is 230mmm so the ratio is 2.875.

My ST-80 which has been fitted with a 2 inch focuser is 335mm long with the dewshield, 265mm without.

ST-80 = 4.2 /3.3.

But these short scopes without built in field flatteners, they have big troubles with astigmatism, exit pupils and field curvature. The University Optics finder does an 8 degree TFoV with the 31mm Nagler, exit pupil is a little big at 8.3mm but not so bad. What is bad is the field curvature... I calculate that the edges are out of focus by more than 2mm with the 31mm Nagler. It looks like it.

Jon


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james7ca
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Re: Ultrafast ultracompact refractors? new [Re: Jon Isaacs]
      #5763276 - 03/28/13 10:04 PM

Quote:

Quote:

However, if you modify Johannes' aspect ratio formula to factor in light gathering (square of the aperture) then the Tele Vue NP127is actually comes out to be the more efficient tube design (of the three listed above, Andy's 100mm f/4 is still significantly better). However, the resulting units mm/mm*mm come out to be: units/mm (which may indicate this is something of an odd metric -- tube length per light gathering?).




It's probably best to keep this metric length divided by the aperture. My 12.5 inch F/4.06 has a tube that is about 1270mm long and a aperture of 317.5mm. The area weighted ratio is 0.000197. The 25 inch F/5 with it's 10 foot tube is 0.000069. You don't even want to know what a C14 is...
Jon



Yes, the leading zeros are kind of hard to take but now that you've jumped from refractors to short focus reflectors and catadioptric designs it may be worth pointing out that those types of scopes are well known for providing a lot of aperture (or light gathering) in a relatively light weight or compact size and thus they offer fairly easy portability (at least in comparison to a similar aperture refractor). However, when using the original metric (length divided by aperture) these designs don't seem to offer any advantage over the currently available refractors (until you start to count catadioptric types, which in their standard configurations can't really be called "ultrafast").

I guess my quibble with the original metric is that it seems to minimize the performance advantages you'd get with a scope of larger aperture. In fact it can favor scopes of smaller aperture while the aperture weighted method seems to provide some useful feedback on that factor as long as you stay within the same design category (refractor versus refractor, rather than refractor versus catadioptric or something entirely else).

In any case, neither method seems to be perfect but both offer ways of summarizing the available candidates. Certainly, however, Johannes' original method is a little easier to handle.


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Re: Ultrafast ultracompact refractors? new [Re: james7ca]
      #5763504 - 03/29/13 01:15 AM

I would prefer a 'bulkiness' over 'light gathering' measure to be dimensionless (a bare number without units). Something like aperture squared divided by tube length divided by tube circumference. Other combinations are possible. In essence you would be measuring the scope surface area over the light gathering area.

I expect such measures to correlate strongly with he simpler tube length over aperture measure. Having said his, one could generalize this into surface area of the optical package during transport divided by light gathering area during operation. Would allow a bulkiness comparison between a Borg system and a truss tube dob. Anyone dare to compute some values?


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Re: Ultrafast ultracompact refractors? new [Re: james7ca]
      #5763539 - 03/29/13 02:01 AM

Okay, on my walk this evening as I looked to the overcast skies (as has been the case for the last several days) I had nothing better to do than ponder this question about aspect ratio versus the aperture weighted method of determining the "best" optical tube.

I'd have no issue whatsoever in using JKoelman's aspect ratio if you only compared scopes of the same aperture. So, a three inch aperture tube that had the lowest aspect ratio would be the most compact telescope in the three inch class. Etc, etc.

However, once you begin to compare scopes of different apertures I think you can run into problems. As I demonstrated before with my hypothetical 1" aperture f/3 scope where I questioned whether a telescope like that should be rated more highly than Andy's 100mm f/4.

Another problem is I don't think there is a simple, linear model that could be used to rate telescopes of different apertures (for compactness or optical tube efficiency). This is probably because as you reduce the focal length (regardless of aperture) the actual tube used to enclose the optical path gets shorter and shorter in relation to the needed focus mechanism and the required back focus. So, as you go to a very short focal length you'll eventually be limited to just a focus mechanism and the backfocus needed to use a star diagonal. This suggests to me that whatever metric you use it probably won't be a linear, straight-line function.

So, what about the aperture-weighted method? I think it probably has some value if you are comparing refractors of different aperture since by squaring the aperture you are factoring in light gathering ability (a practical measure of "goodness"). I would make one change, instead of tube length divided by the aperture squared it would probably be better to invert that and just use aperture squared divided by the tube length (i.e. don't work with fractions but instead use whole numbers with the higher values being "better").

So, here are a few of the candidates using the new aperture weighted values (the higher the value the better):

SV50: 11
102GT: 11 (seems a bit misplaced with the very small SV50, but it has twice the aperture and over four times the light grasp)
AT72ED: 17
EON 80: 17
ST-80: 17
TV127is: 20 (probably warranted since it may be one of the most compact 5" APOs that you can purchase)
Tak Baby-Q: 22
Sky 90: 23
AP Stowaway: 24
Andy's Grab 'n Gasp 100mm f/4 (assuming a 310mm tube length): 32

and a hypothetical 2" aperture f/3:

50 x 50 / 150 = 17 (same class as the AT72ED and others)

In fact, I'd expect that it could be in that class where the "tube" is almost absent and all you would have is a very tiny focuser so it might actually be more like:

50 x 50 / 75 = 33

which would be somewhat misleading in my mind but still only about as "good" as the 100mm f/4. Whereas the rating given by the simple aspect ratio would produce a value of 1.5 (seemingly insanely great) in comparison to the value of 3.1 for Andy's Grab 'n Gasp.

Note: I see that JKoelman was faster on the draw than I was, but I was actually preparing this note before I even read his reply. I like the dimensionless nature of his ideas but I'm not yet convinced that either of us has found the best metric to compare scopes across aperture size.

Okay, one more update. It seems to me that if you consider the area of the objective you are really just factoring in the flux (light) that is gathered by the scope in question. Thus, the dimensions of my aperture weighted metric could simply be called flux per mm of tube length which seems a perfectly good way to measure the efficiency of an optical tube. More flux at the same tube length results in a better design. The same amount of flux in a shorter tube is also a better design. However, if you aren't interested in light gathering ability and instead want to measure resolution then the metric reduces to aperture divided by tube length which is simply the inverse of what JKoelman originally proposed.

Edited by james7ca (03/29/13 04:11 AM)


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Re: Ultrafast ultracompact refractors? new [Re: james7ca]
      #5763715 - 03/29/13 06:21 AM

Quote:


I guess my quibble with the original metric is that it seems to minimize the performance advantages you'd get with a scope of larger aperture. In fact it can favor scopes of smaller aperture while the aperture weighted method seems to provide some useful feedback on that factor as long as you stay within the same design category (refractor versus refractor, rather than refractor versus catadioptric or something entirely else).




If you square the aperture it will always be heavily aperture weighted. This metric is a compactness metric, not a performance metric. In any event, performance is closer to linear than to the second power. Resolution and contrast are linear functions and if you think in terms of magnitudes, light gathering is relatively linear.

I think the linear method is a better fit but it's a portability metric and not a measure of the quality of the view... To get the good views with a fast refractor requires a field flattener and starting with a slow refractor...

Jon


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Re: Ultrafast ultracompact refractors? new [Re: james7ca]
      #5764575 - 03/29/13 12:45 PM

Quote:

I notice that you appear to have a 2" diagonal on your (?) AT65EDQ. I assume, therefore, that with that diagonal you have no problems reaching focus with all of your eyepieces?




Not every eyepiece that I've tried comes to focus to infinity with Astro Physics Maxbright but most of them do with Zeiss 2" prism diagonal.

Here is a thread with the topic at65edq and 2" diagonals

Quote:

Also, you've apparently added a Baader Clicklock diagonal to the SV50. Nice, but (if I may) how much did that add to the total cost of the package?




The package I bought is SV50A ED, 1.25" mirror diagonal, and soft case. I have various set of Baader adapters, diagonals for different scope to play with I think T2 Prism Star Diagonal + SCT->T2 adapter is about extra $300. You wouldn't need it if you go with 1.25" diagonal that comes with the package.

With limited clear aperture (T2 prism diagonal 34mm-ish), I can use most 2" eyepiece with SV50A ED, so TFOV is just below 6 degrees.

With SCT->2" adapter, I think I could go a little wider.

Due to weight of 2" eyepiece, I don't think I use them much with this little guy.

Early this morning, I observed Saturn with SV50A ED. I was surprised to be able to spot Cassini's Division (89x with Ethos SX 3.7, just portion of ring darken) under below average seeing. It was fun

Tammy


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james7ca
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Re: Ultrafast ultracompact refractors? new [Re: Tamiji Homma]
      #5765176 - 03/29/13 04:32 PM

Tammy, once again thanks for the feedback.

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Re: Ultrafast ultracompact refractors? new [Re: Andy Taylor]
      #5766047 - 03/30/13 01:53 AM

All-in-all, it seems that ultra-compact travel refractors are the realm of fast achro's. This limits the use to low-power rich-field viewing, but that is exactly my intended use for a backpack-transportable travel scope.

I am impressed by Andy's homemade 100mm f/4, and noticed that something similar can be purchased in the form of a 80mm f/3.75 RACI finderscope. Anyone tried the Stellarvue F80 as a rich-field telescope? This scope accepts 1.25" eyepieces, but I am a bit concerned about potential light path clipping due to the fixed Amici. Did anyone do a flashlight test on the F80 and determine it's clean aperture? Also, I am interested to hear about the size (total length) of this scope. The weight (2.2 lbs incl. prism) seems promising.

Edit: found this review, but it doesn't answer my questions.

Edited by JKoelman (03/30/13 04:03 AM)


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