Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Reflectors & 35mm Prime Focus AP

  • Please log in to reply
7 replies to this topic

#1 ???

???

    Sputnik

  • -----
  • topic starter
  • Posts: 31
  • Joined: 24 Nov 2019

Posted 31 July 2020 - 04:48 PM

In the textbook Astrophotography for the Amateur, author Michael Covington states that Newtonians (Reflectors) normally only have 1-2cm of back focus, which would be insufficient for 35mm film (or full-frame digital) astrophotography. The work-around that the author suggests for this problem is modifying the telescope to bring the primary mirror forwards about 5cm. From what I've read online, it seems this involves either substituting the screws holding the mirror for longer ones, and/or cutting the tube itself. 

 

I'm interested in Reflectors because of their relatively fast focal ratios and modest pricing. They also seem to be one of the more often recommended types of telescopes for imaging, which makes it all the more surprising to me that they are generally unusable for prime astrophotography until they have been modified. My copy of this book is the revised edition published in 1991. Has anything changed in the design of reflectors so that they are better suited to full-frame imaging without modification? Are there any Reflectors available that are preconfigured for full-frame prime focus astrophotography? 

 



#2 jessebear

jessebear

    Mariner 2

  • *****
  • Posts: 266
  • Joined: 01 Oct 2019
  • Loc: Grand Junction, CO

Posted 31 July 2020 - 04:56 PM

There are many astrograph reflectors designed for photography currently on the market. There is also the option to install a low profile focuser in a reflector, which may get you the focus you need without any substantial modifications (though this will depend on your equipment). Another option is to use a Barlow lens, which will often move the focal plane back far enough to allow use without modification (though this option is far from ideal).
  • ??? likes this

#3 Todd N

Todd N

    Messenger

  • *****
  • Posts: 480
  • Joined: 07 Dec 2007
  • Loc: Southern California

Posted 31 July 2020 - 05:29 PM

As the previous poster stated, many newtonians are astrophotography ready so no modification is needed. This was more so a common problem from yesteryear. I have a 6" f/6 TPO and their entire line will accommodate a D/SLR with coma corrector but not off axis guider. If the scope is f/4 to f/6 be sure to buy a coma corrector and a laser collimator. You'll have to collimate everytime you set up with newts but the laser makes it a cinch.

 

Scopes like TPO and others usually have a 2" crayford focuser that will cover 35mm full-frame. Faster scopes like the smaller 6" f/4 varieties will not be as sharp and uniform across the full-frame even with a CC especially if there is light pollution, illumination is not uniform- vignetting. A longer focal length  F/6 scope is way better in this regard.

 

There is also some new super fast newt astrographs I can't recall at the moment. 6" & 8" f/2.8 with built in correctors good for 44mm image circle but again I would probably not use this scope where there is significant light pollution due to uneven illumination.


Edited by Todd N, 31 July 2020 - 05:37 PM.

  • ??? likes this

#4 ???

???

    Sputnik

  • -----
  • topic starter
  • Posts: 31
  • Joined: 24 Nov 2019

Posted 09 August 2020 - 12:17 PM

There are many astrograph reflectors designed for photography currently on the market. There is also the option to install a low profile focuser in a reflector, which may get you the focus you need without any substantial modifications (though this will depend on your equipment). Another option is to use a Barlow lens, which will often move the focal plane back far enough to allow use without modification (though this option is far from ideal).

Thank you for your response. Is there any place you would suggest looking for low-profile focusers? Would they all be 2" focusers for this purpose? I am considering a Vixen R130Sf that comes with a 1.25" focuser that I'd like to replace with one that will work for full frame.



#5 Todd N

Todd N

    Messenger

  • *****
  • Posts: 480
  • Joined: 07 Dec 2007
  • Loc: Southern California

Posted 09 August 2020 - 03:30 PM

Thank you for your response. Is there any place you would suggest looking for low-profile focusers? Would they all be 2" focusers for this purpose? I am considering a Vixen R130Sf that comes with a 1.25" focuser that I'd like to replace with one that will work for full frame.

That scope probably would not work well for full frame 35mm even if you could modify it:

- Size of the secondary is probably too small for full frame 35mm which is the case for scopes with 1 1/4" focusers for visual use.

- I don't know of any adapter mount for a 5" newt. so, it would likely have to be custom made

- You might have to modify the scope in a more drastic way by changing the position of the spider assembly with a larger seconday, more inward and cut a new hole for the light path.

- By increasing the central obstruction(Larger secondary) with fixed small mirrors size, it can have negative effects on image quality, most notable with light pollution: light donut from secondary shaddow.

 

This is all not worth it and I went down this road years ago.

 

A scope in this size range:

TPO 6" f/4 Newtonian(600mm fl)

https://www.tpoteles...g-newtonian-ota

 

The same scope can be sold under different brand names like Astro-Tech. Needs coma corrector and the scope can also be stopped down to improve off-axis performance with a an aperture mask if desired.

 

* I should have also stated trying to modify such a scope would likely cost more than buying a ready made astrograph.


Edited by Todd N, 09 August 2020 - 11:22 PM.

  • ??? likes this

#6 TxStars

TxStars

    Mercury-Atlas

  • *****
  • Posts: 2,582
  • Joined: 01 Oct 2005
  • Loc: Lost In Space

Posted 10 August 2020 - 11:08 PM

If you are not wanting to spend 4k+ then you are going to have to modify a less expensive scope to cover the image circle.

Something in the f/5 range will keep the star images from becoming too much like seagulls or comets near the frame edges with a corrector.

The TPO/GSO/Astro-Tech "Imaging newts" will come the closest to doing what you want without getting too $$..

If you do have the cash then look at the Takahashi e-180 astrograph.. or for a used e-180/e-200/e-250



#7 ???

???

    Sputnik

  • -----
  • topic starter
  • Posts: 31
  • Joined: 24 Nov 2019

Posted 11 August 2020 - 08:54 AM

Really appreciate the advice here, thank you!

 

Todd, the TPO 6" found its way to the top of my shortlist after your previous post. It seems like the most rational choice for my budget and intended purposes so I went with it. I also ordered the CEM25p which seems well suited to the long exposures required for analog, and a coma corrector and collimator. Estimated 2-3+ weeks for delivery. Now, I wait! hamsterdance.gif  

 

 


  • Todd N likes this

#8 Achernar

Achernar

    Voyager 1

  • *****
  • Posts: 12,138
  • Joined: 25 Feb 2006
  • Loc: Mobile, Alabama, USA

Posted 19 September 2020 - 03:32 PM

No, unless you buy a Newtonian astrograph with a focuser intended to accommodate a camera from the beginning. The reason why the mirror had to be moved 5 centimeters forward in the tube is because that is the depth of an average 35mm camera body from the front of the lens mount to the film plane. You will also need to use a coma corrector with 35mm film cameras, otherwise stars will be markedly degraded by coma, and the faster the focal ratio the worse coma gets. It's a lot worse at F/4 than F/5. And you'll need either a guidescope and auto-guider, or an off-axis guider with an auto-guider.

 

Taras


Edited by Achernar, 19 September 2020 - 03:34 PM.



CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics