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Equipment Discussions >> Refractors

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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)
*****

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
*****

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
*****

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
scholastic sledgehammer


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
scholastic sledgehammer


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
*****

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)
*****

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
*****

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

*****

Reged: 10/20/07

Loc: Birthplace James T Kirk
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
sage


Reged: 08/05/06

Loc: AZ, Scottsdale
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
sage


Reged: 05/21/11

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
Twisted, but in a Good Way
*****

Reged: 09/24/08

Loc: Epsom - UK
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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Jon Isaacs
Postmaster
*****

Reged: 06/16/04

Loc: San Diego and Boulevard, CA
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
professor emeritus


Reged: 05/16/11

Loc: Bangalore, India
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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