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Ultrafast ultracompact refractors?

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#26 Jayo

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Posted 26 March 2013 - 12:34 AM

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

#27 gezak22

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Posted 26 March 2013 - 01:02 AM

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.

#28 ManuelJ

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Posted 26 March 2013 - 01:49 AM

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.

#29 james7ca

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Posted 26 March 2013 - 02:09 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.

#30 ManuelJ

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Posted 26 March 2013 - 02:54 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.


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

#31 Tamiji Homma

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Posted 26 March 2013 - 09:48 AM

...
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

#32 John Rhodes

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Posted 27 March 2013 - 10:36 AM

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.

#33 jrcrilly

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Posted 27 March 2013 - 10:42 AM


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.

#34 james7ca

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Posted 27 March 2013 - 05:27 PM


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.

Attached Files



#35 orion61

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Posted 27 March 2013 - 08:39 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... :foreheadslap:

#36 Daud

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Posted 27 March 2013 - 09:42 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.

#37 james7ca

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Posted 28 March 2013 - 01:51 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?

#38 Andy Taylor

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Posted 28 March 2013 - 03:38 AM

My 100mm F4 achro grab 'n go.

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

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#39 Jon Isaacs

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Posted 28 March 2013 - 05:17 AM

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

#40 JKoelman

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Posted 28 March 2013 - 09:42 AM

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.

#41 Andy Taylor

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Posted 28 March 2013 - 10:44 AM

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... :grin:

#42 Tamiji Homma

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Posted 28 March 2013 - 10:55 AM

...
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:

Posted Image

Here is when it is used.
Posted Image

Tammy

#43 JKoelman

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Posted 28 March 2013 - 12:11 PM


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... :grin:


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?

#44 Andy Taylor

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Posted 28 March 2013 - 01:28 PM


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... :grin:


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.

#45 james7ca

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Posted 28 March 2013 - 02:53 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?

#46 james7ca

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Posted 28 March 2013 - 03:51 PM

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?).

#47 Tamiji Homma

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Posted 28 March 2013 - 04:50 PM

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.

Posted Image

Higher resolution photo

Tammy

#48 Jon Isaacs

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Posted 28 March 2013 - 07:50 PM

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

#49 james7ca

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Posted 28 March 2013 - 09:04 PM

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.

#50 JKoelman

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Posted 29 March 2013 - 12:15 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? :grin:






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