Jump to content


Photo

Dealing with insufficient back focus

  • Please log in to reply
9 replies to this topic

#1 howard929

howard929

    Member

  • *****
  • Posts: 3502
  • Joined: 02 Jan 2011
  • Loc: Low End of High Ground

Posted 01 February 2013 - 01:05 AM

Would spacing the focuser away from the OTA on a Newtonian reflector negate the need for an optical multiplier? Not saying I would try or even could do this, just wondering If it would work.??

#2 Astrojensen

Astrojensen

    Fly Me to the Moon

  • *****
  • Posts: 5227
  • Joined: 05 Oct 2008
  • Loc: Bornholm, Denmark

Posted 01 February 2013 - 01:45 AM

Spacing it away? You need to move it CLOSER to the OTA!


Clear skies!
Thomas, Denmark

#3 Eddgie

Eddgie

    Voyager 1

  • *****
  • Posts: 12673
  • Joined: 01 Feb 2006

Posted 01 February 2013 - 09:45 AM

As Thomas has already said, the problem with most scopes is that they do not have enough inward travel to reach focus with a binoviewer.

For a Newtonian, there are two approaches. The first is to use an OCS. This is a tube with a barlow that fits inside the focuser tube. These are usually 2x, but with the Denkmeir Supersystem, you get a low power mode to bright it to 1.3x.

If you want 1x, then the bad news is that while you can do it, it requires scope modification.

There are two typical approaches. The first is to move the mirror up in the tube about 5 inches so that it is closer to the secondary. This puts the focal plane about 5" further out than it is now.

The second, for truss dob owners, is to have a second pair of shortened trusses so that the secondary mirror can be broght about 5 inches closer to the primary.

There are two issues with this approach. First, you have to be careful that you don't reduce the apeture. Most Newtonians only illuminate a fairly small area of the field, and if you move the mirrors to close, you can loose illumination even at the center of the field.

The solution is to model your scope with a Newt design program to see if you loose aperture (with will probablyl be the case) and see how big a secondary mirror you will need to put in to resotre full aperture (if it matters. Lots of people on the forums seem fine with working at less than full apeture which is a head scratcher for me).

Next, if you want to use the scope in Mono now, you will have to either put an extension on the focuser tube so your eyepeices will reach focus. Of course you could also restore the mirror spacing, but this is tedious. Screwing on a mirror extension would be much easier.

And all of this is really tedious.

This is why Dob owners like the Supersystem. It is not perfect, but it clearly is an easy approach.

And you can pick up used Standard Supersystems for $500.

#4 howard929

howard929

    Member

  • *****
  • Posts: 3502
  • Joined: 02 Jan 2011
  • Loc: Low End of High Ground

Posted 01 February 2013 - 11:03 AM

Thank you Thomas. As usual, I had that BassAckwards.

#5 howard929

howard929

    Member

  • *****
  • Posts: 3502
  • Joined: 02 Jan 2011
  • Loc: Low End of High Ground

Posted 01 February 2013 - 11:07 AM

And Thank you Eddgie as well.

I need to chew on all of this a while a see if anything lights up.

#6 howard929

howard929

    Member

  • *****
  • Posts: 3502
  • Joined: 02 Jan 2011
  • Loc: Low End of High Ground

Posted 05 February 2013 - 10:57 AM

Ok. I'm slow on the uptake. I'm not totally thrilled with the prospect of having to use my 2" 2x barlow or with having to buy a 1.7x GPC for my dob and still not able to get really low power views but I finally got it.

Thanks again.

#7 Eddgie

Eddgie

    Voyager 1

  • *****
  • Posts: 12673
  • Joined: 01 Feb 2006

Posted 05 February 2013 - 04:35 PM

You can go to something like the Denkmeier system. This will get you 1.3x, but of course you still have the small 26mm field stop limit. I don't think that is to bad in a dob though. You could still get a much wider field than something like an SCT.

#8 howard929

howard929

    Member

  • *****
  • Posts: 3502
  • Joined: 02 Jan 2011
  • Loc: Low End of High Ground

Posted 06 February 2013 - 11:39 AM

I'll stick with the MaxBright since I like Baader products, I already have it and it was highly recommended.

Or do you mean Denkmeier accessories?

#9 csrlice12

csrlice12

    Voyager 1

  • *****
  • Posts: 10817
  • Joined: 22 May 2012
  • Loc: Denver, CO

Posted 07 February 2013 - 10:22 AM

Not familiar with binoviewers, but could you use a paracorr in the chain on a newt; wouldn't this act like a barlow AND give you more inward travel (might be heavy, but....)

#10 percorell

percorell

    Lift Off

  • -----
  • Posts: 5
  • Joined: 06 Feb 2013
  • Loc: Copenhagen Dk.

Posted 08 February 2013 - 12:31 PM

As Thomas has already said, the problem with most scopes is that they do not have enough inward travel to reach focus with a binoviewer.


Sorry if this is slightly off-topic, but I have a 90X26 scope with binoviewer, where as you can see in the image, I tried placing a small 3Mp. camera ( chip is small),
This scope are made with this bino viewer and fittet with 2 26mm okulars, that can be replaced with two standard 1,25" for other magnifications than the standard 26X. But I have problems putting my Sony Nex onto one side of the bino and is planning to put the scope on the Bay becaurse I can not make even a mirrorless focus. I need maybe 3 millimeter inverts to focus as with shortest 1,25" on a C-Mount to Nex, I can not focus infinitive it can focus to round 100 meter but not further out.
How do this corrospond to what I quote, I can not see thru that ?

Posted Image

With the camera as you see in the image, it can focus but the size of the chip, cheat me to think the scope magnify 200 X, as when the small area of the chip only cover a small area of the projected image, ---- there I had hoped that a bigger chip an Aps-C would cover a bigger area and it do, but I can only focus within those limitations, and it is allmost impossible, to produce more inverts focus.






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics