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Equipment Discussions >> Reflectors

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Footbag
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Imaging Newts... What's > AT but < Powernewt? new
      #5535221 - 11/23/12 10:39 AM

So I've been considering a new fast scope for something between the 8" and the WO66. In looking at newts, I see the Astrotech's which look nice and the Boren Simon which just make me drool.

I'm wondering though, are there many other imaging newts out there that fall between these two?

Anyone know how the two BS and AT compare when it comes to optics? I'd have to assume the BS has a better mirror that lets it get to F2.8.


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David PavlichAdministrator
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Re: Imaging Newts... What's > AT but < Powernewt? new [Re: Footbag]
      #5535242 - 11/23/12 10:55 AM

The BS scope gets to f2.84 by using a corrector/reducer. Its f4 without the R/C. Two things with fast Newts: Collimation is critical and spacing between the chip and the reducer/corrector is critical. Not sort of critical, but very critical.

David


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Footbag
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Re: Imaging Newts... What's > AT but < Powernewt? new [Re: David Pavlich]
      #5535276 - 11/23/12 11:19 AM

I understand the collimation part and it is a concern, but if you want a scope this fast, there are few options.

As for camera spacing, it seems like you only have to get that right to start with and then image away.


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andysea
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Re: Imaging Newts... What's > AT but < Powernewt? new [Re: Footbag]
      #5535481 - 11/23/12 01:10 PM

Adam,
I have been researching this topic too as I would like a 6" very fast imaging scope. I think theoretically one could use the AT6in with a reducer/coma corrector. Unfortunately the only one that I could find is the ASA which comes at a premium. TS makes a SCT corrector and0.8 reducer. They say is corrects coma and field curvature but it's designed for Schmidt Cassegrain telescopes not for newtonians.

I am interested in what your finds are.
Andy


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David PavlichAdministrator
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Re: Imaging Newts... What's > AT but < Powernewt? new [Re: Footbag]
      #5535676 - 11/23/12 02:50 PM

Quote:

I understand the collimation part and it is a concern, but if you want a scope this fast, there are few options.

As for camera spacing, it seems like you only have to get that right to start with and then image away.




You're correct about both. I just mentioned the spacing because many applications can be fairly forgiving, but this one is not. It is 65mm and you want it to be 65mm with little wiggle room.

David


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careysub
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Re: Imaging Newts... What's > AT but < Powernewt? new [Re: andysea]
      #5538693 - 11/25/12 12:49 PM

I have been puzzling about the small fast Newtonians sold for imaging purposes - Orion and Astrotech offer a variety of F/4-ish Newtonians for this purposes in sizes from 4.5 inches to 8 inches.

But the these have focal lengths (and thus field curvatures) of 450 mm to 800 mm, and I calculate that the sagitta across a 12 mm sensor is larger than the depth of focus by a factor for 1.5 (8") to 3 (4.5").

It would seem that getting an in-focus image across a modest sensor chip is impossible without something to flatten the field. Paracorrs allegedly do this for scopes 1200mm and larger, but would perhaps not help much with these very short focal lengths. There also seem to be no field flattener products marketed specifically for Newtonians.


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David PavlichAdministrator
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Re: Imaging Newts... What's > AT but < Powernewt? new [Re: careysub]
      #5538928 - 11/25/12 03:12 PM

The Baader MPCC is pretty much the standard for the imagers that use the myriad f4 Newts.

David


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andysea
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Re: Imaging Newts... What's > AT but < Powernewt? [Re: careysub]
      #5539079 - 11/25/12 04:40 PM

I think a wynne or Keller corrector would do the trick.
Andy


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Footbag
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Re: Imaging Newts... What's > AT but < Powernewt? new [Re: andysea]
      #5540079 - 11/26/12 10:54 AM

I do wonder what a Keller corrector would do to an AT8IN.

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andysea
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Re: Imaging Newts... What's > AT but < Powernewt? new [Re: Footbag]
      #5541704 - 11/27/12 09:19 AM

I have been wondering about that. According to ASA their Keller corrector can be used with any newtonian between f3.5 and f5 so it should work with the AT8in. Even more interesting for me would be how it works with the at6in.
http://www.teleskop-express.de/shop/product_info.php/language/en/info/p4685_A...
Andy


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Footbag
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Re: Imaging Newts... What's > AT but < Powernewt? new [Re: andysea]
      #5541748 - 11/27/12 09:48 AM

Quote:

I have been wondering about that. According to ASA their Keller corrector can be used with any newtonian between f3.5 and f5 so it should work with the AT8in. Even more interesting for me would be how it works with the at6in.
http://www.teleskop-express.de/shop/product_info.php/language/en/info/p4685_A...
Andy




I dont think I've heard of a single report positive or negative.


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andysea
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Re: Imaging Newts... What's > AT but < Powernewt? new [Re: Footbag]
      #5541800 - 11/27/12 10:24 AM

Well given the price point it would be an expensive experiment!

Andy


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joejoe
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Re: Imaging Newts... What's > AT but < Powernewt? new [Re: andysea]
      #5543177 - 11/28/12 01:11 AM

Quote:

I don't think I've heard of a single report positive or negative.




I use a 3" ASA coma corrector in a 300mm f/4 GSO newtonian. (i.e from the same factory as both Astrotech and Powernewt). I'm 100% happy with the results. Here are some of my images:

LINK

As for the 2 inch version, the spot diagrams and vignetting graphs are available on the ASA website. Please study them closely. LINK

From what I have seen, I would expect the optical and build quality would be excellent.

The Baader MPCC would be a more cost effective option and I've seen a lot of good results from it too.

Putting the equipment element aside, you will still need to put in a lot of effort to get good images - guiding, focus & collimation come to mind.

James


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andysea
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Re: Imaging Newts... What's > AT but < Powernewt? new [Re: joejoe]
      #5543428 - 11/28/12 08:54 AM

Beautiful pictures James!
Is yours only a coma corrector or also a reducer? The 2" ASA is also a .73 reducer. That would bring the system down to f/2.92. Super fast!

Andy


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Zad
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Re: Imaging Newts... What's > AT but < Powernewt? new [Re: careysub]
      #5543553 - 11/28/12 10:20 AM

Quote:

I have been puzzling about the small fast Newtonians sold for imaging purposes - Orion and Astrotech offer a variety of F/4-ish Newtonians for this purposes in sizes from 4.5 inches to 8 inches.

But the these have focal lengths (and thus field curvatures) of 450 mm to 800 mm, and I calculate that the sagitta across a 12 mm sensor is larger than the depth of focus by a factor for 1.5 (8") to 3 (4.5").

It would seem that getting an in-focus image across a modest sensor chip is impossible without something to flatten the field. Paracorrs allegedly do this for scopes 1200mm and larger, but would perhaps not help much with these very short focal lengths. There also seem to be no field flattener products marketed specifically for Newtonians.




I am glad you brought this up, because I was confused here as well. What I found (IIRC) from doing some research a long time ago, is the following: For two telescopes of the same design the field curvature is proportional to the focal length, but that relationship does not hold true for scopes of different design. From what I remember a refractor has four times the field curvature as a newtonian reflector with the same focal length; therfore, imaging newts typically don't need a field flattener unless the focal length is very small. And that is why you very often see flatteners built for refractors, but not very often made or sold with newts.

So my question is: am I correct here in my understanding?


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Zad
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Re: Imaging Newts... What's > AT but < Powernewt? new [Re: Zad]
      #5545167 - 11/29/12 09:35 AM

I did a little more research and found out that field curvature is is very different for for two telescopes of different design and the same focal length. This link provides a table with some examples of common scopes:

http://starizona.com/acb/basics/equip_optics101_curvature.aspx

As you can see, a 4" f/10 refractor with a 1000mm focal length would have much more severe field curvature (375mm radius) than an 8" f/4 newt (800mm radius). So the refractor has much worse field curvature even though is has a longer focal length.

The main issue with an imaging newt is coma, and there are several coma correctors that can take care of that depending on how fast the scope is. The key is to get the distance to the chip correct for optimum performance.


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shams42
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Re: Imaging Newts... What's > AT but < Powernewt? new [Re: Zad]
      #5548806 - 12/01/12 01:31 PM

The paracorr works beautifully with the 8" f/4. It will produce tighter stars and sharper images than the MPCC unless your mirror is over corrected to begin with.

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andysea
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Re: Imaging Newts... What's > AT but < Powernewt? new [Re: shams42]
      #5559876 - 12/07/12 07:50 PM

So is anyone using the ASA 0.73 reducer with any of the AT f/4 Newtonians?

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shams42
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Re: Imaging Newts... What's > AT but < Powernewt? new [Re: andysea]
      #5563058 - 12/09/12 07:54 PM

I really don't think the flimsy structure of this tube is up to the task of imaging at f/2.8. I spent the better part of two years fighting with it at f/4. The tube simply won't maintain the required collimation tolerances IMO.

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David PavlichAdministrator
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Re: Imaging Newts... What's > AT but < Powernewt? new [Re: shams42]
      #5563250 - 12/09/12 10:21 PM

I'm not sure how close to perfection you need to be, but there's some fairly impressive images here. Kfir and Harel have both had their images in S&T and Astronomy. No observatory. They travel to the desert in Israel to image.

David


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