Telescope Reviews: Choosing an RFT refractor

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Equipment Discussions >> Refractors

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mikeinca
member

Reged: 10/05/11

Loc: North Bay Area, CA

Choosing an RFT refractor
#5320760 - 07/16/12 05:25 PM

I've been lurking for a while and enjoying the wealth of information on these forums but this is my first post. A brief introduction; I've been interested in astronomy for many years and have owned a C8 since the mid 70's but never devoted a lot of time to the hobby. About 9 months ago I decided to jump in with both feet and set up a permanent observatory. I purchased a C14 Edge HD on a CGE Pro mount and placed it on a 12" concrete pier in a Skyshed pod. This setup has been in operation for a couple of months now and having gotten the hang of using the scope and it's GOTO mount I'm beginning to really enjoy some great evenings of observing. I'm also discovering some limitations, including viewing with a scope this size at the zenith in a Pod, a problem that I'm about to resolve with a semi "roll off" solution for the dome.

Which brings me to the reason I'm posting in the refractor forum. I'd like to add an RFT to my observatory and am considering a refractor to piggyback on my C14. Also, it would be nice to have a G&G scope that I could more easily, on occasion, take to darker sky sites. I've been looking at the TV-85 and NP-101 as well as the Orion ED102T CF. I must admit being attracted to the pricier TeleVue models from the made in USA aspect as well as the fact that I've only read extremely positive things about them.

What recommendations would you make regarding these scopes (or others) for rich field use mounted on a C14? The CGE Pro mount is rated for a 90lb payload so I have plenty of margin there, but in general would the larger scopes, particularly the much more expensive NP-101 be overkill for this application? Does my desire to use this scope as a G&G significantly influence the decision? I'm currently doing only visual observing, but would like to eventually get into AP. Does that change the dynamics of the choice?

I know just enough about all of this to be dangerous so I'd really appreciate input from all of you with more experience.

Mike

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MikeBOKC
Post Laureate

Reged: 05/10/10

Loc: Oklahoma City, OK

Re: Choosing an RFT refractor new

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[Re: mikeinca]
#5320772 - 07/16/12 05:34 PM

I am visual only and piggyback a TMB92L on my CPC1100 for wide field views. It is an excellent quality scope. but if you are seeking a bit more aperture all of the choices you named are good.

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joelimite
Carpal Tunnel

Reged: 09/01/08

Loc: Fayetteville, AR

Re: Choosing an RFT refractor new

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[Re: MikeBOKC]
#5320792 - 07/16/12 05:46 PM Attachment (27 downloads)

I agree that the TMB 92L is a terrific rich-field scope. At f/5.5, it has no trouble framing huge chunks of sky. I spent an hour last night exploring the area around Sagittarius, Scorpius, and Scutum. I especially enjoyed the Lagoon Nebula and M22; it even resolved some stars in the latter. The TMB 92L is crisp across the entire field with my Naglers, and it performs well at higher powers.

Of course, the other models you're considering are excellent, too.

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Erik Bakker
Carpal Tunnel

Reged: 08/10/06

Loc: The Netherlands, Europe

Re: Choosing an RFT refractor new

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[Re: mikeinca]
#5320809 - 07/16/12 05:57 PM

Mike,

For traveling, the TV85 is more portable. As a RFT, the NP101 is the better choice. For it's flatness, wider field and especially aperture. The 101 mm just take it on the right side of brightness. A 5" could do a bit better still, but becomes much more of burden. So of the scopes you mention:NP101. It can be had used for around US $2000 or a bit less, a great value. Make sure your get a grab-n-go alt-az mount for it as well: that's a lot of fun with this type of scope!

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Jon Isaacs
Postmaster

Reged: 06/16/04

Loc: San Diego and Boulevard, CA

Re: Choosing an RFT refractor new

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[Re: joelimite]
#5320810 - 07/16/12 05:57 PM

Something to consider:

As a RFT, the NP-101 stands out because it is a Petzval and therefore has a flat focal plane. Refractors have a curved focal plane, about 1/3 the focal length of the scope. This means when viewing the widefields that eyepieces like the 31mm Nagler provide, stars at the edge of the field of cannot be in focus when the stars at the center are in focus and visa verse.

The Petzval design uses a rear element that flattens the field so that stars are in focus all the way across, even with the widest field of view eyepieces.

Used TV scopes are considerably less expensive than the new ones... One can often buy an NP-101 with the Everbright diagonal for about what an Orion 102CF costs.

Jon

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coopman
Carpal Tunnel

Reged: 04/23/06

Loc: South Louisiana

Re: Choosing an RFT refractor new

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[Re: Jon Isaacs]
#5320845 - 07/16/12 06:33 PM

I have been wondering if the curvature at the edges of the FOV in my SW120ED was normal. Now I know that it is. Thanks.

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SteveG
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Reged: 09/27/06

Loc: Seattle, WA

Re: Choosing an RFT refractor new

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[Re: coopman]
#5320853 - 07/16/12 06:40 PM

Yea - if you can afford it the NP 101 would be a perfect choice.

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Jon Isaacs
Postmaster

Reged: 06/16/04

Loc: San Diego and Boulevard, CA

Re: Choosing an RFT refractor new

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[Re: coopman]
#5320901 - 07/16/12 07:07 PM

Quote:

I have been wondering if the curvature at the edges of the FOV in my SW120ED was normal. Now I know that it is. Thanks.

Clay:

Just take a hammer and beat the objective lens flat... if you do that, you won't see any field curvature any more...

Jon

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Eddgie
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Reged: 02/01/06

Re: Choosing an RFT refractor new

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[Re: mikeinca]
#5321017 - 07/16/12 08:34 PM

I agree with Jon that the NP101 is about the best RFT refractor out there. Using eyepeieces like the 31mm Nagler in many small refractors will make the field curvature hard to manage, but in the NP101, it will not be an issue.

The Televue 101 and Genesis SDF often sell for much better prices. I don't think that the field is quite as flat with these as with the NP101, but they are pretty close, and the price is often very affordable on the used market.

I owned the TV 101 and it was a very fine telescope. I can recommend it as a great ultra-wide field telescope.

I also owned two Genesis models (non-SDF). These are also great for wide field, but not quite as sharp I think as the SDF, 101, or NP. The CA is not as well controlled in the Genesis. Unless it were sold at a big discount to the others I mentiond here, I would ante up the extra money for one of these other scopes over the Genesis.

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mikeinca
member

Reged: 10/05/11

Loc: North Bay Area, CA

Re: Choosing an RFT refractor new

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[Re: Jon Isaacs]
#5321048 - 07/16/12 09:04 PM

Thanks for all the suggestions so far. Sorry if this is obvious, but I assume that if I choose the NP-101 (or any of the other scopes mentioned) that there'd be no difficulty in mounting it to a dovetail bar on the C14. I don't see a specific adaptor for this purpose on the TeleVue site but I'm guessing there is something out there.....

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sky_dave
member

Reged: 07/10/08

Loc: Northern Colorado

Re: Choosing an RFT refractor new

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[Re: joelimite]
#5321128 - 07/16/12 10:34 PM

Joelimite,

What have you done to your Porta-Mount to get the Dovetail mount below the tube?

Dave

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joelimite
Carpal Tunnel

Reged: 09/01/08

Loc: Fayetteville, AR

Re: Choosing an RFT refractor new

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[Re: sky_dave]
#5321144 - 07/16/12 10:44 PM

It's a custom adapter made by CN forum member Manny Miles. Besides changing the orientation of the dovetail mount, it also adds a very handy panhandle. You can read more about it here:
http://www.cloudynights.com/ubbthreads/showflat.php/Cat/1,2,3,4,5,6,7,8,9,10/Number/5294539/Main/5091407

PM Manny Miles if you want one. It's a highly recommended update for Porta Mount owners!

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PhilCo126
Carpal Tunnel

Reged: 01/14/05

Loc: coastline of Belgium

Re: Choosing an RFT refractor new

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[Re: joelimite]
#5321596 - 07/17/12 09:30 AM

RFT = Rich Field Telescope ... so check:
http://www.robertmartinayers.org/RFT/

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Jon Isaacs
Postmaster

Reged: 06/16/04

Loc: San Diego and Boulevard, CA

Re: Choosing an RFT refractor new

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[Re: PhilCo126]
#5322903 - 07/18/12 12:06 AM

Quote:

RFT = Rich Field Telescope ... so check:
http://www.robertmartinayers.org/RFT/

This is Mel Bartel's RFT page:

Mel Bartels RFT

Jon

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jrbarnett
Eyepiece Hooligan

Reged: 02/28/06

Loc: Petaluma, CA

Re: Choosing an RFT refractor new

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[Re: mikeinca]
#5323459 - 07/18/12 11:27 AM

Hi Mike. Welcome to CN. Looks like we may be neighbors. You ought to swing by one of our club observing sessions (which are on the Saturday closest to the New Moon unless rained out) in Sonoma. That way you could mess around with different types of refractors.

In any case, if the RFT is going to be a refractor, will be used solely for visual observing and has a main goal of lower magnification very wide fields, I'd look for the dual blessings of aperture *and* speed. The various 6" f/5.9 achromats currently available (branded Astrotelescopes, Canadian Telescope, etc., made in each case by Kunming United Optics) would be ideal. They will gather much more light than a dinky "ED" refractor, remain capable of very wide true fields of view and, if operating at modest magnification, minimize one of the main drawbacks of a big, fast achromat, the false color.

I said "one of the main drawbacks". That means there's another. These are large telescopes. The 6ers will require a fairly beefy mount compared to the little guys if you want to use it other than on your CGE Pro.

http://www.cloudynights.com/item.php?item_id=2171

http://www.cloudynights.com/item.php?item_id=2690

There are a couple of CN reviews of this scope. The tremendous advantage in light grasp of one of these over a 4" refractor really makes DSOs in the FOV pop by comparison. On the other hand, fogettabout high magnification on bright targets (if chromatic aberration bugs you).

Regards,

Jim

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Jon Isaacs
Postmaster

Reged: 06/16/04

Loc: San Diego and Boulevard, CA

Re: Choosing an RFT refractor new

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[Re: jrbarnett]
#5323538 - 07/18/12 12:33 PM

Quote:

I said "one of the main drawbacks". That means there's another. These are large telescopes. The 6ers will require a fairly beefy mount compared to the little guys if you want to use it other than on your CGE Pro.

These are 25lb telescopes. Since the intention here is to piggy back the scope on a C-14, this could well respresent something of a challenge for the mount.

RFT is about field of view and brightness. It is pretty much independent of aperture... a fast scope with a widefield eyepiece is where it's at.

Jon

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Eddgie
Postmaster

Reged: 02/01/06

Re: Choosing an RFT refractor new

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[Re: Jon Isaacs]
#5323887 - 07/18/12 05:11 PM

Yes, the piggyback requirement kind of limits this to maybe 4" or so.

Of course the OP didn't ask, but I have tried piggybacking refractors many times on a much larger SCT, and I came to relize that the targets that these two scopes can show are sooooo different that there really wasn't any value in having it piggybacked. Most rich field observing to me is best done with a star chart, Telrad overlay (the Televue 101 for examlpe, can show s true field bigger than the Telrad 4 degree circle!!!) and the scope mounted on a simple mount.

I used to think that it would be fun to compare the view in My C11 and them my C14 to the view in the refractor, but all to often, the targets were invisible in the 4" refractor.

And very larger, rich field viewing like in Saggitarius was staggeringly good in the Televue 101 under dark skies, but here it was fun just to kind of slip the scope around to see how different groups of deep sky objects would frame up.

And using the refractor when piggybacked was almost always a pain in the behind. It seemed like it was always getting in my way when using the main scope.

In the end, I found them far better to use on a dedicated light weight mount.

But the OP didn't ask about the feasibility of using the scope in this configuration, and while I am no fan of it, I think a 4" refractor is the right size for this application.

And the Televue Genesis SDF/TV101/NP 101 are almost unbeatable as RFT type scopes. The view is brighter than 80mm type scopes, and you can actually get a wider useful field in the Televue because it is so much flatter.

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Jon Isaacs
Postmaster

Reged: 06/16/04

Loc: San Diego and Boulevard, CA

Re: Choosing an RFT refractor new

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[Re: Eddgie]
#5323910 - 07/18/12 05:33 PM

Quote:

And using the refractor when piggybacked was almost always a pain in the behind. It seemed like it was always getting in my way when using the main scope.

In the end, I found them far better to use on a dedicated light weight mount.

I totally agree. One of the reasons to own a scope like the NP-101 is that is so easy to use. From my backyard, it my most used scope because it is so easy to setup and use. Tying it to a large scope would limit it's use drastically.

Jon

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JimP
Pooh-Bah

Reged: 04/22/03

Loc: South Carolina

Re: Choosing an RFT refractor new

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[Re: Jon Isaacs]
#5323966 - 07/18/12 06:22 PM

What about astigmatism? I am speaking of the eye of the observer not the telescope objective. I had a Televue 101 and at very low powers the stars did not focus down to nice points of light because of my astigmatism. I have no problem with high power planetary observing. At low power and a large exit pupil the beautiful wide flat star fields did not materialize for me.

JimP

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Jon Isaacs
Postmaster

Reged: 06/16/04

Loc: San Diego and Boulevard, CA

Re: Choosing an RFT refractor new

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[Re: JimP]
#5323992 - 07/18/12 06:44 PM

Quote:

What about astigmatism? I am speaking of the eye of the observer not the telescope objective. I had a Televue 101 and at very low powers the stars did not focus down to nice points of light because of my astigmatism. I have no problem with high power planetary observing. At low power and a large exit pupil the beautiful wide flat star fields did not materialize for me.

JimP

Your own astigmatism can be corrected if desired. TeleVue offers the Dioptrx which offers correction for astigmatism from 0.25 diopters up to 3.5 diopters in steps of 0.25. They cost $95.

The Dioptrx does fit the 31mm Nagler so if one is desirous of the flat, 4.5 degree fields the NP-101 + 31mm Nagler provides, it is possible.

Not everyone is thrilled by the RFT views and not everyone has access to the skies where such views are best seen. And not everyone is bothered by the field curvature one sees in short focal length refractor.

For me though, it's definitely worth it.

Jon

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