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

Lumicon 2-inch diagonal Optical Light path distance?

  • Please log in to reply
9 replies to this topic

#1 HfxObserver

HfxObserver

    Apollo

  • ****-
  • topic starter
  • Posts: 1424
  • Joined: 12 Nov 2004
  • Loc: Canada

Posted 14 October 2019 - 04:49 PM

I have one of those funky angled LumiBright diagonals, I bought it ~12 years ago for the short light path and light weight, it's been great. Now I'm adapting a TSFLAT2 visual field flattener to it for the Meade Adventurescope 80mm f5 and need to install it at ~128mm from the focal point. I recall it was around the same as the AP Maxbright's 103mm. The TSFLAT2 doesn't fully thread on, but perhaps that's just the design, so I may end up purchasing a new diagonal anyway.....

 

So does anyone know the light path distance for a LumiBright 2-inch?

 

Attached Thumbnails

  • 3EF621E8-223D-4810-AC2E-1D4110B7CA03.jpeg


#2 TOMDEY

TOMDEY

    Aurora

  • *****
  • Posts: 4889
  • Joined: 10 Feb 2014
  • Loc: Springwater, NY

Posted 14 October 2019 - 05:48 PM

I have both of those make and model Star Diagonals, and measured the Lumicon to be 6mm shorter optical length than the AP. I did that with a lab test set that I designed and built. Here they are... and excerpt from my white paper, showing the little part of the procedure where the tech measures the optical length of this (or any other) optical subassembly... We measured these and tubs of others, while we were at it...

 

Depending on your exact vintage and serial # ... I guess it's possible that the differential twixt yours could be a bit different than the -6mm manifested by mine.  Tom

Attached Thumbnails

  • 141 MAXBRIGHT and Lumicon LimiBrite Star Diagonals.jpg

  • HfxObserver likes this

#3 TOMDEY

TOMDEY

    Aurora

  • *****
  • Posts: 4889
  • Joined: 10 Feb 2014
  • Loc: Springwater, NY

Posted 14 October 2019 - 05:49 PM

nother pic, the procedure... click on to be able to read ....  Tom

Attached Thumbnails

  • 140 Toms Diagonals white-paper diagonal length procedure.jpg

  • HfxObserver likes this

#4 HfxObserver

HfxObserver

    Apollo

  • ****-
  • topic starter
  • Posts: 1424
  • Joined: 12 Nov 2004
  • Loc: Canada

Posted 14 October 2019 - 11:45 PM

Wow Tom, that's incredible!

 

I really appreciate you sending along this information. So if the AP is documented as 103mm than the Lumicon is around 97mm. So I should be good to go with the Bluefireball 19.1+12.7mm extension tubes.

 

Thanks again!



#5 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 79415
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 15 October 2019 - 03:34 AM

You can measure it yourself with a pair of calipers or scale and possibly some extensions.

 

Focus the scope on an object with the diagonal in place. Measure the draw tube extension.  

 

Refocus the scope without the diagonal. Measure the draw tube extension plus any extensions. 

 

The difference is the optical path length of your diagonal.

 

Jon



#6 HfxObserver

HfxObserver

    Apollo

  • ****-
  • topic starter
  • Posts: 1424
  • Joined: 12 Nov 2004
  • Loc: Canada

Posted 17 October 2019 - 07:23 AM

Thanks Jon, 


  • Jon Isaacs likes this

#7 ji4m

ji4m

    Explorer 1

  • *****
  • Posts: 65
  • Joined: 29 Dec 2010

Posted 17 October 2019 - 06:27 PM

Wow Tom, that's incredible!

 

I really appreciate you sending along this information. So if the AP is documented as 103mm than the Lumicon is around 97mm. So I should be good to go with the Bluefireball 19.1+12.7mm extension tubes.

 

Thanks again!

Are you able to remove the nose piece of the diagonal and replace it with the TSFTLAT2 or are you threading it in on the tip of the nose piece like a filter?  The optical path length of the diagonal is measured from the shoulder not the tip of the nose piece.



#8 HfxObserver

HfxObserver

    Apollo

  • ****-
  • topic starter
  • Posts: 1424
  • Joined: 12 Nov 2004
  • Loc: Canada

Posted 17 October 2019 - 08:03 PM

I placed it on the nose piece, however, I only noticed a modest level of correction.

 

The nose piece is ~27mm. So 124mm total light path.

 

I'm hoping some sort of extender will help, so I should aim for a 15mm extender to get close to the Baader others have used to great success. 

 

Thanks again,



#9 Eddgie

Eddgie

    ISS

  • *****
  • Posts: 24842
  • Joined: 01 Feb 2006

Posted 18 October 2019 - 03:22 PM

I placed it on the nose piece, however, I only noticed a modest level of correction.

 

The nose piece is ~27mm. So 124mm total light path.

 

I'm hoping some sort of extender will help, so I should aim for a 15mm extender to get close to the Baader others have used to great success. 

 

Thanks again,

Normally the nose piece is not included in the light path.

 

This is because if you are using it in a refractor, the nose is in the focuser, and you would not count that because it adds no path at all.  The light path is from the rear of the focuser to the top of the eyepiece holder. 

 

If it is an SCT, you use the path length of the visual back because this is usually longer than the nose of the diagonal, and if you counted the nose, you would be counting that light path twice.

 

Now in the very unusual instance where the visual back will not accommodate the full length of the nose, then you would use the lenght of the nose. Otherwise, you use the visual back and from the front face of the diagonal to the top of the eyepiece holder.

 

Most 2" diagonals have a light path of 98mm to 104mm, but I have seen one model that had 110mm.  This is the longest I have ever seen. 

 

So, the nose is not included in the light path.


Edited by Eddgie, 18 October 2019 - 03:26 PM.

  • Jon Isaacs likes this

#10 HfxObserver

HfxObserver

    Apollo

  • ****-
  • topic starter
  • Posts: 1424
  • Joined: 12 Nov 2004
  • Loc: Canada

Posted 19 October 2019 - 12:04 PM

Thanks Eddgie, this helps make sense of what others wrote about using the TFLAT2. I've seen some improvement with the TFLAT2 is just seems off by a bit, it improved the inner 60-70% of the field immensely then just falls off a cliff. Another CN member using the Baader Clickstop had a light path of 112mm and found just threading the TFLAT2 onto the nosepiece provided sufficient correction with their 80mm f5 refractor. So I'm ~15mm short of their lightpath starting out. The specs on the TFLAT2 from TS Service we site are as follows:

 

The recommended distances from the M48x0.75 thread to the corrector
In principle this rule applies: the shorter the refractor´s focal length, the longer the working distance to the sensor has to be:

 

♦ focal length < 450 mm: 128 mm
♦ focal length 450-490 mm: 123 mm
♦ focal length 500-550 mm: 118 mm
♦ focal length 560-590 mm: 116 mm
♦ focal length 600-690 mm: 113 mm
♦ focal length 700-800 mm: 111 mm
♦ focal length ab 800 mm: 108 mm

 

An underrun or an overrun of the distance of up to 5% has no visible effect on the sharpness in the field.

 

In the other thread they mention that with a light path of 131mm they achieved acceptable correction in a 400mm refractor. Since, according to the chart above, the shorter the refractor focal length the greater the distance required and going from a refractor with a 450mm focal length down to 400mm the focal length might push it out another ~5mm. So for a 400mm focal length if the ideal light path is 132mm and I'm at 124mm at most I'm a little outside the 5% margin of error by a couple mm at the very least and most likely short of the ideal distance in the range of 6-15mm.  So maybe I'll just bite the bullet and spend the $16, I've likely spent $1600 of time thinking about this :)




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