Jump to content


Photo

Questar 3.5 vs Televue 85 light transmission

  • Please log in to reply
10 replies to this topic

#1 Michael Lomb

Michael Lomb

    Vostok 1

  • -----
  • Posts: 103
  • Joined: 26 Oct 2010
  • Loc: North Island New Zealand

Posted 11 September 2013 - 03:55 AM

I have read an interesting post on the Astromart site where Malcom Bird (also a contributor to this forum…thanks Malcom), was able to make a direct comparison between his 30 year old Questar and the Televue 85 refractor, on the same mount. He commented, as others have on this forum that the Questar image looked darker when compared to a similar sized refractor. This implied that the optics of the Questar disproportionally lost more light.

http://www.astromart...?article_id=909

There was another post on this forum, where a member contacted Questar and asked what the loss of light was through a Questar 3.5 with Broadband coatings. They discussed this at the shop, and the answer was 1% loss through the reflective surface of the main mirror 1% from the corrector plate. There is another 2% loss through the control box if you do not use the axial port. The central obstruction adds another 10%. In total there is 14 % loss.

Checking on the Company 7 website on the Televue 85, the transmission is 94%, so there is a 6% loss.

Now some basic math to compare the surface area of 89 mm vs 85 mm, and factor out light loss of each scope.

(8.9/2)^2 x pi = 62.21 x .86 = 53.5 for the Questar
(8.5/2)^2 x pi = 56 x .94 = 53.3 for the Televue 85

The brightness, at least for a scope with newer Broadband coatings should be the same.

#2 Erik Bakker

Erik Bakker

    Gemini

  • *****
  • Posts: 3139
  • Joined: 10 Aug 2006
  • Loc: The Netherlands, Europe

Posted 11 September 2013 - 10:00 AM

The brightness, at least for a scope with newer Broadband coatings should be the same.


No.
The way the Q forms an image in the form of an Airydisk with rings is different from the TV85. The peak intensity in the TV85 is considerably higher + it distributes less light into the diffraction rings. In the Q, peak intensity is lower and ring brightness is higher. All this contributes to lowering the visible brightness of the images in the Q when compared to a good refractor of similar light-gathering area.

Do note that the Q has a lot smaller color error than the TV85, and especially in the red end that makes itself notable while observing reddish objects like Mars at opposition.

The forte of the Q is not ultimate performance per mm of aperture, but the superbly integrated and ergonomic package in a very compact and light form. And dare I say, beautiful colors and materials used? A timeless beauty :bow:


#3 Les

Les

    Viking 1

  • *****
  • Posts: 888
  • Joined: 22 Apr 2006
  • Loc: Maryland

Posted 11 September 2013 - 12:49 PM

Eric,

Just to be clear, your argument holds for point targets like stars but not extended objects or terrestrial views as mentioned in the article.

#4 Erik Bakker

Erik Bakker

    Gemini

  • *****
  • Posts: 3139
  • Joined: 10 Aug 2006
  • Loc: The Netherlands, Europe

Posted 11 September 2013 - 04:32 PM

I am afraid it does Les. Extended objects are just a great number of points stitched together, so the effect is still there.

#5 Les

Les

    Viking 1

  • *****
  • Posts: 888
  • Joined: 22 Apr 2006
  • Loc: Maryland

Posted 11 September 2013 - 07:22 PM

Extended objects like the moon ARE collections of points, but now the diffraction rings fall on adjacent central responses making them brighter. No light is lost from the total scene, only contrast is lost. But your argument for stellar objects is perfectly true, the central response is dimmer for obstructed apertures.

#6 John F

John F

    Viking 1

  • *****
  • Posts: 619
  • Joined: 16 Feb 2004
  • Loc: Washington State

Posted 30 September 2013 - 10:39 PM

The forte of the Q is not ultimate performance per mm of aperture, but the superbly integrated and ergonomic package in a very compact and light form. And dare I say, beautiful colors and materials used? A timeless beauty:


Erik, Vey well put and I agree with your assessment. I might add that in one area the Questar really excels and that is its close focus capability. I use mine a lot for viewing terrestrial objects at powers ranging from 53x to 107x. Due to atmospheric turbulence those high powers usually don't work very well for terrestrial targets that are several miles away. However, for nature objects that are within a few hundred feet in distance (and even as close at 8 feet) it performs spectacularly well.

John Finnan

#7 Erik Bakker

Erik Bakker

    Gemini

  • *****
  • Posts: 3139
  • Joined: 10 Aug 2006
  • Loc: The Netherlands, Europe

Posted 01 October 2013 - 12:36 PM

My eyes see it differently at the eyepiece Les. In my experience, a good apo will put up a visibly brighter image at the eyepiece when compared to a similar size Mak or SCT at similar magnifications.

I did extensive comparing with my Q7 with BB en LR coatings and my FS102, as well as the C5 with special coatings and the FS102. Alongside other comparisons, these have empirically formed my opinion on this matter over the years.

Out of curiosity, your sig currently says you have a Q50th and SV90 triplet. How do these two perform for you at the eyepiece? Did you also compare them on driven equatorials at high magnifications over extended periods side-by-side? Looking forward to better understand what shaped your opinion on this matter.


#8 Les

Les

    Viking 1

  • *****
  • Posts: 888
  • Joined: 22 Apr 2006
  • Loc: Maryland

Posted 03 October 2013 - 04:44 PM

I don't doubt that you see a difference between scopes at the eyepiece. Just the optical explanation was incorrect.

The 90mm triplet mostly occupies a place in my living room looking pretty on its tripod. The Q is just so much easier to use and in my typical local viewing conditions doesn't give up much to the triplet. What good are finer diffraction rings when the central response is a boiling mass. In fact, I am looking for a buyer for the triplet even though it does look gorgeous in the living room. Ironically, its midnight blue paint job is a closer match to the old Questar purple than my current Questars.

#9 Erik Bakker

Erik Bakker

    Gemini

  • *****
  • Posts: 3139
  • Joined: 10 Aug 2006
  • Loc: The Netherlands, Europe

Posted 04 October 2013 - 12:48 AM

Thanks for the clarification Les. The little Q is unbeatable in it's ease of use.
When you say the central response is a boiling mass with a thinner ring in your SV90 triplet, is it a comparably stable mass in your Q 3 1/2, only surrounded by a fatter first ring?

#10 Les

Les

    Viking 1

  • *****
  • Posts: 888
  • Joined: 22 Apr 2006
  • Loc: Maryland

Posted 04 October 2013 - 09:11 AM

I have not had them in a side by side comparison. Theoretically, there would only be a few percent difference so I doubt that I could see the difference. Most of the viewing difference I note is just due to the 2:1 difference in focal length. But clearly, the Q has more pronounced diffraction rings on bright stars at comparable powers.

#11 azure1961p

azure1961p

    Voyager 1

  • *****
  • Posts: 10276
  • Joined: 17 Jan 2009
  • Loc: USA

Posted 08 October 2013 - 10:56 PM

I have read an interesting post on the Astromart site where Malcom Bird (also a contributor to this forum…thanks Malcom), was able to make a direct comparison between his 30 year old Questar and the Televue 85 refractor, on the same mount. He commented, as others have on this forum that the Questar image looked darker when compared to a similar sized refractor. This implied that the optics of the Questar disproportionally lost more light.

http://www.astromart...?article_id=909

There was another post on this forum, where a member contacted Questar and asked what the loss of light was through a Questar 3.5 with Broadband coatings. They discussed this at the shop, and the answer was 1% loss through the reflective surface of the main mirror 1% from the corrector plate. There is another 2% loss through the control box if you do not use the axial port. The central obstruction adds another 10%. In total there is 14 % loss.

Checking on the Company 7 website on the Televue 85, the transmission is 94%, so there is a 6% loss.

Now some basic math to compare the surface area of 89 mm vs 85 mm, and factor out light loss of each scope.

(8.9/2)^2 x pi = 62.21 x .86 = 53.5 for the Questar
(8.5/2)^2 x pi = 56 x .94 = 53.3 for the Televue 85

The brightness, at least for a scope with newer Broadband coatings should be the same.


At thirty years of age those reflective coatings are long overdue for a recoat no?

Pete






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics