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Equipment Discussions >> Reflectors

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timwetherell
sage
*****

Reged: 09/03/10

Loc: A Land Down Under
Wide field reflector new
      #5826722 - 04/27/13 04:51 PM

Hi Guys.

I'm looking at moving to an area where there is a degree of light pollution but just a few miles away is a great dark sky site. Plans are for an observatory at home but I'm also wondering about the idea of an additional very portable wide field scope for taking into the dark sky area. A 6" f5 refractor is one possibility but a fast Newtonian with a coma corrector may offer a good alternative. Anyone got any recommendations for a good and reasonably priced 8,10,12" f 3 or4 OTA ?

All the best

Tim


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Mirzam
Post Laureate
*****

Reged: 04/01/08

Loc: Lovettsville, VA
Re: Wide field reflector new [Re: timwetherell]
      #5826751 - 04/27/13 05:04 PM

Something like these perhaps:

Imaging Newtonians

JimC


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careysub
Carpal Tunnel
*****

Reged: 02/18/11

Loc: Rancho Cucamonga, CA
Re: Wide field reflector new [Re: Mirzam]
      #5826792 - 04/27/13 05:37 PM

A review of the 8" Astro-Tech:
http://www.astro.uwo.ca/~rasc/articles/at8in_review.pdf

And manual here:
http://www.optcorp.com/pdf/AstronomyTechnologies/8newt.pdf

Most websites, and the review, are curiously coy about stating the secondary size. It is 70mm, a SCT-level 35% obstruction (the 8" C8 is 34%).

Edited by careysub (04/27/13 05:40 PM)


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SeattleScott
Pooh-Bah


Reged: 10/14/11

Re: Wide field reflector new [Re: careysub]
      #5827691 - 04/28/13 02:28 AM

8 or 10 inch Meade Schmidt Newtonian. Had the 8" and loved my 24mm ES 82 deg eyepiece with it. These are discontinued but show up used now and then. If you can go smaller aperture the ES comet hunter mak newt or the vixen neoachromat refractor are both great wide field scopes. They are not cheap though.

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timwetherell
sage
*****

Reged: 09/03/10

Loc: A Land Down Under
Re: Wide field reflector new [Re: Mirzam]
      #5828744 - 04/28/13 04:53 PM

thanks for the info guys - will give me some food for thought

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plyscope
Pooh-Bah


Reged: 11/23/06

Loc: Perth, West Australia
Re: Wide field reflector new [Re: timwetherell]
      #5828815 - 04/28/13 05:44 PM

Tim if you want a wide field reflector I can't imagine you will be satisfied with anything less then something that you have created in your own special way.

Here is an article that gives food for thought;

Rich field by Mel Bartels

Mels 13 inch f3;

web page

Another interesting article;

web page


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careysub
Carpal Tunnel
*****

Reged: 02/18/11

Loc: Rancho Cucamonga, CA
Re: Wide field reflector new [Re: plyscope]
      #5829744 - 04/29/13 09:30 AM

Quote:

Tim if you want a wide field reflector I can't imagine you will be satisfied with anything less then something that you have created in your own special way.

Here is an article that gives food for thought;

Rich field by Mel Bartels

Mels 13 inch f3;

web page





I was much taken by Mel's analysis - insightful and well presented.

He does not account however for the advent of the ES100 25mm eyepiece.

When Mel wrote the page the longest FL 100 degree EP then available was the 21mm Ethos, which required a very short focal ratio to take full advantage of. Today a 100 degree EP (in a standard 2" format) is available with a 25mm FL.

This raises the optimum focal ratio from 3.0 to 3.63, probably significantly easier to make. It also suggests that a (now) quite standard F/4 mirror, used with a 25mm ES100, is doing pretty well in delivering most of what an optimum FL mirror delivers.

Edited by careysub (04/29/13 05:11 PM)


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Jon Isaacs
Postmaster
*****

Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Wide field reflector new [Re: careysub]
      #5829979 - 04/29/13 11:39 AM

Quote:


There are two reasons though to tweak his numbers - the generous pupil size he used (7mm) and the advent of the ES100 25mm eyepiece.




Carey:

I think if you look at Mel's analysis for the 13.1 inch you will see he optimized the performance for a 6mm exit pupil.

Myself, I am pretty happy with my 12.5 inch F/4.06 with the 31mm Nagler and a Paracorr as a richest field scope. It provides a 1.62 degree TFoV at 48x with a 6.6mm exit pupil. I am fortunate in that my pupil still seems to dilate to 7mm and while this is not quite the generous 1.82 degree TFoV possible with Mel's 13 inch, it's close and the value of Mel's pushing the limit depends on the usefulness of that last 18 degrees of AFoV between 82 degrees and 100 degrees.

Jon


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careysub
Carpal Tunnel
*****

Reged: 02/18/11

Loc: Rancho Cucamonga, CA
Re: Wide field reflector new [Re: Jon Isaacs]
      #5830068 - 04/29/13 12:31 PM

Quote:

Quote:


There are two reasons though to tweak his numbers - the generous pupil size he used (7mm) and the advent of the ES100 25mm eyepiece.




Carey:

I think if you look at Mel's analysis for the 13.1 inch you will see he optimized the performance for a 6mm exit pupil.

Myself, I am pretty happy with my 12.5 inch F/4.06 with the 31mm Nagler and a Paracorr as a richest field scope. It provides a 1.62 degree TFoV at 48x with a 6.6mm exit pupil. I am fortunate in that my pupil still seems to dilate to 7mm and while this is not quite the generous 1.82 degree TFoV possible with Mel's 13 inch, it's close and the value of Mel's pushing the limit depends on the usefulness of that last 18 degrees of AFoV between 82 degrees and 100 degrees.

Jon




When I wrote that I had just done a quick check and looked at the table headed:
"Mapping Exit Pupils to Eyepiece Focal Lengths" farther down on the page where the mapping of 7mm/F3/21mm relationship appears.

I did not note that it didn't include the necessary coma corrector factor (which was in the next table, and adjusts the pupil involved to 6mm).


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Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: Wide field reflector new [Re: careysub]
      #5830924 - 04/29/13 07:04 PM

Yeah, none of these short f/ratio scopes is going to be visually usable without a coma corrector.
So an f/4 scope really functions as f/4.6
Assuming a 7mm pupil, that's a longest focal length of 32.2mm.
That makes a 31 Nagler a good lowest-power eyepiece.

But, it depends who you are and what you expect.
My dark-adapted pupil is only 4.5-5mm, but I find a low power eyepiece delivering a 5.4mm exit pupil to be no problem. I tried a 40mm eyepiece (7mm in my scope), but it was noticeably darker than the shorter FL 31mm eyepiece--obviously, it was stopping the scope down because of my eye's pupil diameter.

I'd be better off with a 26mm lowest-power eyepiece, but I don't want to sacrifice the true field and besides, a little light loss at the lowest possible power in your scope isn't a real issue.

But, as a piece of personal advice, stick to f/5 for your 8" or 10" scope. The eyepiece height will be easier to use, the telescope will be easier on eyepieces (you might find you don't even need a coma corrector), and if the scope is built as a truss scope, it'll be small enough to transport in a Smart car. Heck, if you make it a travel scope, you could transport the scope in the saddlebags of a motorcycle!


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Pinbout
Postmaster
*****

Reged: 02/22/10

Loc: nj
Re: Wide field reflector new [Re: Starman1]
      #5830987 - 04/29/13 07:54 PM

More like the trunk of a motorcycle. Saddlebags are really smaller than they appear on the inside

Edited by Pinbout (04/29/13 07:56 PM)


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Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: Wide field reflector new [Re: Pinbout]
      #5831125 - 04/29/13 09:11 PM

Well, a fellow brought his 10" dob to RTMC a few years ago....in his motorcycle Panniers.

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Pinbout
Postmaster
*****

Reged: 02/22/10

Loc: nj
Re: Wide field reflector new [Re: Starman1]
      #5831695 - 04/30/13 07:48 AM

A bmw?

You know what they say sbout guys who ride bmw's


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stevecoe
"Astronomical Tourist"
*****

Reged: 04/24/04

Loc: Arizona, USA
Re: Wide field reflector new [Re: Starman1]
      #5833629 - 05/01/13 03:42 AM Attachment (26 downloads)

Tim, Don, et al;

Here is a shot of me with a 6 inch f/5 Newtonian I constructed some years ago. I was very happy with the field of view and sharp star images with this telescope. Using a 38mm Erfle that was WWII surplus (installed in the scope) the field took in the North America Nebula (NGC 7000) and some of the field around it. Things got better with a 35mm Panoptic and a 2 inch UHC filter. It also would take in all of M 24, the Small Sagittarius Star Cloud. Lots of stars in long chains and the two dark nebulae on the north side were easy. It was lots of fun to just scan along the Milky Way and see what made its way into the field of view.

I am not a mirror maker and don't wish to become one, this mirror was from the late Pierre Schwaar and it did an excellent job. I can highly recommend f/5 as plenty wide enough with views that are sharp and have excellent contrast.

Clear skies to us all;
Steve Coe

Edited by stevecoe (05/01/13 03:43 AM)


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plyscope
Pooh-Bah


Reged: 11/23/06

Loc: Perth, West Australia
Re: Wide field reflector new [Re: stevecoe]
      #5833678 - 05/01/13 05:18 AM

That's a cool scope Steve and I think your t-shirt is great too!

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Pinbout
Postmaster
*****

Reged: 02/22/10

Loc: nj
Re: Wide field reflector [Re: stevecoe]
      #5834132 - 05/01/13 11:23 AM

Quote:

Here is a shot of me with a 6 inch f/5 Newtonian I constructed some years ago




I love a 6f5 format. great size for almost a pocket travel scope, which I'm still planning on making. So all folded up it would be 7x7x4, not including the truss, maybe detach the focuser and make it 7x7x3. That would definitely be pocket size... ok tablet size.


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