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I don’t understand the allure of SFT or RFT

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#1 sunrag

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Posted 03 April 2020 - 03:24 PM

I don’t understand the allure of Short Focal Length Telescopes (or Rich Field telescope?). Isn’t better to use a giant binocular instead (25x100 bino for example). What am I missing?

Whenever i am viewing through a telescope, after i find my target (for example the Orion Nebula), i just want to apply as much magnification as possible to be able to see the details. So I am only acquiring long focal length Telescopes (900mm or more). I have a 25x100 binocular to give me rich field views.

Thanks for reading! Hope to learn if i am missing anything by not getting into SFTs.
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#2 SeattleScott

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Posted 03 April 2020 - 03:35 PM

The advantage of a rich field scope is not having to also pull out binoculars. Certainly it can speed the time needed to hunt down faint fuzzies. The nice thing about my Vixen R200 is I can get a wide enough view to frame Pleiades or Beehive, or just locate targets quickly, and still apply 8” aperture to resolve galaxies and globs.

The advantage of a 4” Apo over a 4” bino is the ability to crank up magnification. Your binos are fixed 25x. A 4” Apo can zoom in on M37 or M11 or planets. Now if you are getting out a bigger scope too, then you can use that for high power work and faint fuzzies, and just use the binoculars for sweeping. But now you have to drag out two setups. The allure of the small wide field refractors is they give you wide field with the (limited) ability to zoom in on targets and resolve globulars, like a bigger scope. But if you always get out a bigger scope and binoculars, then there isn’t much advantage of a 4” F6-7 refractor. The real advantage of these scopes is grab and go observing for short sessions when you don’t want to haul out two setups or wait for a bigger scope to cool before getting good views of the Moon and planets. But for someone with a backyard observatory, with 4” binoculars and a larger obstructed scope always set up and at ambient under dark skies, there isn’t much need that I can see for a 4” refractor. Other than sticking a camera on it.

Scott

Edited by SeattleScott, 03 April 2020 - 03:47 PM.

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#3 Bonco2

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Posted 03 April 2020 - 03:44 PM

The best example that I'm familiar with is the Televue 4inch f/5 Genesis. The wide flat field of view is one of its best well known attributes. Magnification range is from 9X to 250X+. It performs well throughout that range. Great for DSO's, planets, double stars and of course the moon. Also when used on an equatorial mount it's great for photography. It's about the most flexible optic out there. Of course there are other similar telescopes that have the same abilities. I too love bino's and have 5..the biggest is a 20X80. They have their role but not the flexibility of the Genesis. If I had only one optic it would be the Genesis or something similar.
Bill

Edited by Bonco2, 03 April 2020 - 03:45 PM.

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#4 Jon Isaacs

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Posted 03 April 2020 - 03:47 PM

Some thoughts/experiences:

 

- Rich Field Telescopes need not be small aperture scopes, your 12 inch Apertura can serve as a RFT.  Sweeping the MilkyWay with a 12 inch F/5 with a 31mm Nagler is a very special treat.  

 

- Telescopes are more versatile than binoculars and can be used with filters.  You can use better eyepieces with telescopes. Your 25x100s have a 2.6 degree TFoV. Scotts 8 inch F/4 Newtonian is capable of a 3 degree field with the 31mm Nagler, 2.6 with the 21mm Ethos class eyepieces.  

 

- There are binocular-telescopes with 90 degree eyepieces that resolve the ergonomic issues of a straight-through binocular, generally they're limited to 1.25 inch eyepieces so they're competing against a telescope with 2 inch capabilities. 

 

In my mind, it comes down to one essentially perfect view with one eye or two eye views that are compromised.

 

Jon


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#5 Codbear

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Posted 03 April 2020 - 03:47 PM

For me it's about satisfying two different levels of enjoyment. My NP101is is a fast 5.4 refractor that gives me up to 4* FOV.

 

I love the way it frames globular clusters within the surrounding star field, or is able to see Venus and the Pleiades all in the same FOV with a 41mm Nagler. It's not always about the power.

 

Now when it IS about the power, that's when I use my Questar 7 or 24" SpicaEye, where M13 takes up the whole FOV, and in the 24" it's resolved to the core.


Edited by Codbear, 03 April 2020 - 07:21 PM.

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#6 aa5te

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Posted 03 April 2020 - 03:59 PM

Because a Celestron Omni XLT C150R just looks cool as heck with a Meade 5000 40mm SWA hanging off of it, OK?

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#7 vdog

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Posted 03 April 2020 - 04:02 PM

two eye views that are compromised.

Yup.  I tried different things with my 20x80s.  If I lie on my back, the weight feels like an anvil on my face.  Handholding is out as I lack the requisite Popeye-like forearms.  I tried a monopod, Seronik-style mount, tall tripod (my neck hurts just thinking about it), homemade mirror stand; none of these really worked out for me. 

 

So, now I use the 20x80s for targets from the horizon to about 45 degrees.  The Northern Jewel Box is a real treat in them, as is M6 and M7.  This summer, I think I'll take a shot at Omega Centauri (maybe from my roof). 

 

But when I want to look at anything higher up, it's nice to be able to switch to the refractor for no-hassle views of these targets.


Edited by vdog, 03 April 2020 - 04:05 PM.

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#8 Jon Isaacs

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Posted 03 April 2020 - 04:09 PM

This summer, I think I'll take a shot at Omega Centauri (maybe from my roof).

 

 

You don't want to wait too long.  I have been watching Omega Centauri these last few months when it transits in the early morning.  You don't want to wait too long and have it get lost in the twilight.

 

Jon


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#9 vdog

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Posted 03 April 2020 - 04:19 PM

You don't want to wait too long.  I have been watching Omega Centauri these last few months when it transits in the early morning.  You don't want to wait too long and have it get lost in the twilight.

 

Jon

Thanks for the tip.  I'll see if I can take a shot the next day or two before the moon becomes too much of a factor.

 

Oh, and happy birthday!
 


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#10 TOMDEY

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Posted 03 April 2020 - 04:24 PM

Yeah, the main thing is that RFT Binoculars rarely go beyond 4 inches, occasionally 6 inches. At that point, they either get prohibitively expensive or (more reasonably) we go monoscopic RFT. But, for the fanatics among us, one can go bigger.

 

My JMI RB-16 (16-inch) binoscope operates at 76x406 quite awesomely. Problem is the price tag was around $25K, with the myriad custom upgrades.

 

My 36-nch scope finally exceeded my budget for true binocular. So I settled on modest monoscope with linear binoviewer and two Gen 3 Night Vision eyepieces. That operates at 143x914. Ummm... and it cost a wee bit more than the JMIs.    Tom



#11 havasman

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Posted 03 April 2020 - 04:33 PM

Perhaps the practically perfect 282 arc' field the NP101is/31T5 allows holds no allure for you. No problem with that at all, folks have different tastes. But the attraction it may have for some observers should not be that hard to understand.

Plus, that same scope yields a 3/4o field at 146x and very usable 0.7mm exit pupil with a 3.7mm Ethos SX. That can also be quite beneficial in several observing situations.


Edited by havasman, 03 April 2020 - 08:52 PM.

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#12 kmparsons

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Posted 03 April 2020 - 04:34 PM

I love my 16 X 70 APM binoculars for wide and rich field experience. However, one of the most frequently praised instruments on this forum is the Orion 120 mm f/5 scope. It is amazingly inexpensive--actually less than many giant binoculars, and the big aperture makes the rich fields much richer. Still...it is hard to beat the experience of using both eyes with the big binoculars!


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#13 russell23

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Posted 03 April 2020 - 05:07 PM

My 102mm f/7 APO is an extremely versatile instrument - moreso than a 102mm f/10 to f/15 instrument.     It is just as capable of performing just as well at high magnifications, but can provide a wider TFOV than f/10 to 15 instruments. 

 

For most of my observing with my 102mm f/7 I do not observe at the widest TFOV, but the 3.05 deg TFOV with the 30mm APM UFF is really nice when I want it.  With a 102mm f/9.8 the 30mm APM would only give a 2.17 deg TFOV and with a 102mm f/15 it would only give a 1.45 deg TFOV.

 

Another nice thing about the shorter focal length scopes is that they are easier to mount and because of the shorter length are less prone to vibrations - which is really nice when there is a decent breeze.   Finally, the shorter length tube has less change in eyepiece height as you observer from horizon to zenith. 

 

The biggest drawback to the shorter focal length scopes is that they are tougher on eyepiece edge performance.


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#14 Codbear

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Posted 03 April 2020 - 07:48 PM

Some thoughts/experiences:

 

- Rich Field Telescopes need not be small aperture scopes, your 12 inch Apertura can serve as a RFT.  Sweeping the MilkyWay with a 12 inch F/5 with a 31mm Nagler is a very special treat.  

 

- Telescopes are more versatile than binoculars and can be used with filters.  You can use better eyepieces with telescopes. Your 25x100s have a 2.6 degree TFoV. Scotts 8 inch F/4 Newtonian is capable of a 3 degree field with the 31mm Nagler, 2.6 with the 21mm Ethos class eyepieces.  

 

- There are binocular-telescopes with 90 degree eyepieces that resolve the ergonomic issues of a straight-through binocular, generally they're limited to 1.25 inch eyepieces so they're competing against a telescope with 2 inch capabilities. 

 

In my mind, it comes down to one essentially perfect view with one eye or two eye views that are compromised.

 

Jon

To your point Jon, I actually did a shootout last night between my NP101is and TeleVue binoviewers and my APM100 ED 90* binoculars because I wanted to compare not only the image quality and brightness  between the two, but also the ergonomics of dealing with a binoviewer compared to the APM binoculars. I already knew from previous viewing that, unsurprisingly, a monoview through the 101is was virtually perfect with pinpoint stars and handily beat the sharpness of the APM ED 100s.

 

My 101is was on one side of an AZ8 mount and tripod, and my APMs were on a Manfrotto 161 tripod. I used the same set of 24mm Panoptics along with Dioptrx on each to correct for my astigmatism. 

 

My first target was M42. With a slightly gibbous Moon, it was definitely washed out, but I clearly noticed greater range of the nebula in the 101is and could clearly see all 4 stars of the Trap, which could not be said fore the APMs.

 

Next was the Moon. Again, the 101is handily beat the APMs on sharpness and detail. I just couldn't get the APMs to snap into focus, whereas with the 101is the sweetspot just jumped out at me.

 

Lastly I turned to Mizar and Alcor. Again as expected, the image was sharper and better defined in the 101is.

 

Interestingly, things got closer when I used the Baader Zoom 8-24mm eyepieces. At higher power, there wasn't as much difference between the two. Both showed all 4 stars of the Trap and nebula detail was closer. It does need to be noted, however, that Astropheric showed below average Transparency and Seeing.

 

Lastly, I did find that the APMs were more ergonomically enjoyable for me than adjusting the binoviewers, especially with the zoom eyepieces. Granted, my muscle disease makes fiddling with the zooms more difficult, but there was a very pleasing ease of use with the APMs once I set the interpupilary distance to my beady-eyed 58mm!lol.gif

 

Overall though, the fact that the 101is is a 4* FOV max compared to the APM's 2.7* FOV is what really makes the 101is more enjoyable.



#15 Allan Wade

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Posted 03 April 2020 - 09:41 PM

I find Rich Field Telescopes really come into their own under darker skies. Something like an NP101 provides for a much wider field of view than a large bino is capable of. Coupled with 2" eyepieces and narrow band filters, the large exit pupil provides wonderful observing that isn't possible in most other telescopes, let alone a binoscope or binocular.

 

So like most Astro equipment, they are tools which have different capabilities. I have RFT and binoscope options and enjoy them both. But everyone's preferences are different, and no piece of equipment is a requirement to enjoy observing.


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#16 NickWDavis

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Posted 03 April 2020 - 10:06 PM

I find Rich Field Telescopes really come into their own under darker skies. 

Yes to this. Under my skies low power, wide-field views give nothing but a slightly orange tinted gray sky. It’s not pleasant to look at.


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#17 Bomber Bob

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Posted 03 April 2020 - 10:34 PM

I'm blessed with 2 vintage ATM RFTs -- a 128mm F5 triplet refractor, and a (mostly Edscorp) 6" F4 Newtonian:

 

ATM 5x5T - Restore S25 (Ready to Mount).jpg ATM 150mm F4 Newtonian - S21 (Mizar AR1 EQ Short Pod).jpg

 

Good as the 6" mirror is, it cannot display stars as the tiny micro-dots I see in the refractor, so it's my cluster buster; conversely, the 6" reflector does better with faint fuzzies.  Both are fun & comfortable to use:  The frac on my improved old Orion VersaGo, and the Newt on the Mizar AR-1 + ShortPod (as pictured).

 

Oh...  and the 5" frac has a 3* field at 21x with a 2" 32mm Erfle, too...  1100Z:  and, I just came in from observing Jupiter, GRS, & Io shadow transit with this scope.  No trouble toting it on my VersaGo to the SE corner of our backyard to grab the morning planets.  A 5" lens @ 175x (Radian 4mm) shows a lot of detail.  So, from 15x to 200x, this SFT is very versatile -- without exotic glasses or magic crystals.


Edited by Bomber Bob, 04 April 2020 - 06:13 AM.

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#18 SpaceConqueror3

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Posted 03 April 2020 - 10:41 PM

If RFT ain't your cup of tea so be it. I personally dislike binoculars completely for astronomy, always have and always will, but that has absolutely no bearing whatsoever on those that that think it's the creme de la creme. They won't ever convince me otherwise and I doubt I'll will either. 


Edited by SpaceConqueror3, 03 April 2020 - 10:42 PM.

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#19 25585

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Posted 04 April 2020 - 01:44 AM

For me a RFT is the next step up from binoculars, rather than a rival. I use both together. My RFT is an original F5 Genesis on a TV Panoramic mount & can go from 9x with a 2" 55mm Plossl to over 100x with my Baader Morpheus 4.5mm. Taking big wide TFOV 2" eyepieces is a telescope-only thing.      


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#20 bobhen

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Posted 04 April 2020 - 06:39 AM

With my light pollution, binoculars are useless and wide-field scopes are compromised. I have owned a 102 mm F-5 achromat for 16 years but it was my travel scope and used mostly under a dark sky. UNTIL four years ago when I added an image intensifier.

 

Now objects like the Horsehead Nebula, California Nebula, North American Nebula etc. are all visible right from my front yard only 8 miles from the Philadelphia city limits.

 

Hard to do that with binoculars.

 

Bob



#21 NYJohn S

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Posted 04 April 2020 - 09:24 AM

I like binoculars but never liked mounting them so 10x50's is as far as I go. If I'm going to take out a tripod or mount then the next step up is a small refractor. 


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#22 russell23

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Posted 04 April 2020 - 09:44 AM

I like binoculars but never liked mounting them so 10x50's is as far as I go. If I'm going to take out a tripod or mount then the next step up is a small refractor. 

Yeah - I really like my Nikon Monarch 5 8x42 binoculars.  That is as large as I want to go with binoculars.   Even at that I don't use them very often - mainly for some real quick sweeping of very large clusters or the Milky Way.  I prefer to sweep with binoculars than try to hold them steady.



#23 gene 4181

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Posted 04 April 2020 - 10:13 AM

              that picture that aa5te posted ,  I could spend  an eternity looking at  'that "  .....     matter of fact I was with a 80/35Pan   and  Athlon 10x42 roof       


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#24 BillP

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Posted 04 April 2020 - 11:10 AM

I don’t understand the allure of Short Focal Length Telescopes (or Rich Field telescope?). Isn’t better to use a giant binocular instead (25x100 bino for example). What am I missing?

If you've used an RFT and do not feel it is anything special from binoculars then you are missing nothing.  Go with what works for you.  I personally find nothing endearing about RFTs either.  I generally like my telescopes to have a max TFOV capability around 2.5-3.0 degrees.  I can get much wider with my 80mm Apos but frankly prefer binoculars for scanning or target searching to a telescope in RF-mode as the hand held binoculars are more intuitive and natural.  I've also tried swing arm binocular mounts and generally do not like that process for observing either.  My favorite is a telescope with some good binoculars next to it.


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#25 sunrag

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Posted 04 April 2020 - 11:41 AM

Thanks everyone for taking the time to answer!

I am now tempted to buy or make an SFT. I definitely can see the value in being able to hold a telescope in hand and also be able to change the ep when I want to go to a higher mag.

Jon Isaacs - thanks for the tip about sweeping the AD12 with a 31mm ep. I have a 30mm and a 40mm (not a Nagler), but never took the time to do that! Will definitely do that next time I have clear skies. 


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