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

TAK FSQ106 vs. SW ESPRIT 100

  • Please log in to reply
60 replies to this topic

#51 jerry10137

jerry10137

    Viking 1

  • *****
  • Vendors
  • topic starter
  • Posts: 783
  • Joined: 26 Jan 2013
  • Loc: Texas, USA

Posted 20 November 2014 - 08:44 PM

Thank you Jay for the kind words.  I'm no expert but I am learning very rapidly.  Been a learning experience for sure.  Feel free to message me anytime!



#52 jzeiders

jzeiders

    Viking 1

  • *****
  • Posts: 638
  • Joined: 07 Nov 2012
  • Loc: SF Bay Area

Posted 22 November 2014 - 11:50 PM

Sorry for the confusion.

 

If you think about the terms, aperture, f-ratio, and focal length, they are all referring to different things. The aperture is simply the hole that limits ow much light can pass through the system. This is usually also the maximum size of the primary objective. A lens or mirror depending on the optic being discussed. The f-ratio is only the diameter of the aperture divided into the focal length. The focal lengths simply the distance from the objective to the image plane. Zoom lenses are different and are not needed for this discussion.

 

F/ratio is important only in photographic terms. It may also be useful in giving an idea of certain qualities of a telescope. If the f number is low, <4 the scope will be relatively short for its diameter. It the number is high, >15 it may be long and thin.This obviously is not applicable to folded optical systems, though the terms f-long and f-short may apply.. A low f number or ratio in Newtonians means more attention to collimation may be required nd it usually has a larger secondary and coma will be more of an issue relative to a f/8 or f/10 Newtonian. Similarly for a refractor, a low f-ratio usually means more chromic aberration and field curvature or a more expensive and complex system such as triplets, Quads, and Quints and exotic materials.etc.

 

A 50mm f/1.4 camera lens ialways has the same focal length no matter how the aperture is set. The f-ratio changes as the iris opens or closes. So with a camera lens it is working at different apertures with a fixed focal length. In the case of the 50mm it is always 50mm no matter if you select f/1.4 or f/22. Only the hole in the iris changes. The lenses themselves are fixed.

 

Telescopes usually are fixed aperture and focal length. A 10" f/5 always has a 10" objective and a 50" focal length by definition. It makes no difference whether it is a mirror or a lens. That said you can place an aperture stop on the front of the telescope to reduce the aperture. This is exactly the same function of the iris in a camera lens. If the aperture stop is 5" the scope is now working at f/10 as the focal length is not changed, it is still 50", only the aperture has changed by means of the stop and thus the f-ratio. 

 

If you take two telescopes of any type, reflector, refractor, etc. it makes no difference, and one is a 5" f/5 and the other is a 10" f/5 and take a picture through both at the same subject the exposure will be the same for both for equal density of the image. You can try it. All you need is the two scopes and one camera.

 

If you take two telescopes of the same aperture with different focal lengths the exposures will be different. Lets take a 5" with a 25" focal length, an f/5 and a 5" with a 50" focal length a f/10 and repeat the exercise. The 50" focal length scope will need much more exposure time. Roughly 2 stops or 4 times the exposure. F/5 is 1/3 of a top from f/5.6 as is f/10 from f/11. In the list I noted above f/5.6 is 2 stops faster than f/11. So if we need to double the exposure for every stop then the f/10 scope needs 4 times the exposure relative to the f/5. (... 5.6, 8, 11 ...)

 

Aperture determines light gathering, limiting magnitude and resolution of any optical system. A 10" aperture collects more light than a 5" aperture.the 10" will show fainter stars than a 5". and the 10" will split tighter double stars.

 

Focal length determines image scale. The size of M51 is much smaller in a photo taken with a 12" (300mm) focal length as opposed to a 40" (800mm) lens focal length with the same camera.

 

Sensor size determines field of view. Whether it be an eyeball, a CCD, or a CMOS sensor. This is easily demonstrated by taking a single optic, telescope or camera lens, and take a picture of some object with a full frame DSLR and a APS DSLR. The full frame will have a wider field of view though the aperture, exposure,  and focal length do not change.

 

We are talking about the prime focus system here and have not introduced eyepieces as that would just serve to complicate the discussion at this point.

 

Sensor size, image scale and angle of view are often confused by many possibly due to the current popularity of APS sensors and people misunderstanding how it works. 

 

I hear people say that a 50mm lens, made to cover a full frame digital sensor or 35mm, becomes a 75mm lens on a 1.5x crop APS sensor. No, it is still a 50mm focal length, only the angle of view is reduced by a smaller sensor. A simile may be taking a 8x10 print and cutting off the outer portions.

 

Hope this helps,

 

Jack



#53 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 87,207
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 23 November 2014 - 04:36 AM


I was thinking the closer similarity is between the Esprit 100ED and the NP-101. 

 

-Rich

 

Rich:

 

The difference I see is that FSQ and NP series are modified Petzvals that can be used visually with a diagonal and still provide that flat field of the view. The Esprit is an apo triplet with a separate field flattener for astrophotography, used visually it is a fast triplet with field curvature, quite a different animal.

 

I suspect most NP-101 owners use them visually where their lighter weight is an advantage over the FSQ. The FSQ has that 88 mm diameter image circle which makes it the ideal astrograph..

 

I would think the appropriate comparison would between the esprit and a 4 inch triplet with dedicated flattener/reducer.

 

Jon


  • Fomalhaut and 25585 like this

#54 N1ghtSc0p3

N1ghtSc0p3

    Messenger

  • *****
  • Posts: 471
  • Joined: 19 Feb 2011
  • Loc: Texas

Posted 27 March 2015 - 11:49 AM

Just wanted to congratulate Jerry on another of his images (a set of images from this thread), and his comparison being used by SW in their latest ad in Astronomy.



#55 jerry10137

jerry10137

    Viking 1

  • *****
  • Vendors
  • topic starter
  • Posts: 783
  • Joined: 26 Jan 2013
  • Loc: Texas, USA

Posted 27 March 2015 - 12:08 PM

Thanks kindly! After I posted this comparison originally, I was emailed and SW showed interest in using the pics for an ad. So yes, the new ad for Astronomy as well as Sky and Telescope use the exact .jpgs from this thread.


  • Maryland Mike likes this

#56 gnomus

gnomus

    Explorer 1

  • -----
  • Posts: 63
  • Joined: 03 Oct 2014

Posted 14 May 2016 - 04:45 AM

I've just found these older threads.  I wanted to say thanks Jerry for this and for the comparison you did on the 150 Esprit.  Terrific.  For me the Petzval Tak is showing some of its usual distortion (in this case radial) in at least two corners.  On the other hand the Tak is revealing a bit more of the fine stuff.  


Edited by gnomus, 14 May 2016 - 04:46 AM.

  • jerry10137 likes this

#57 starbob1

starbob1

    Vanguard

  • *****
  • Posts: 2,074
  • Joined: 11 Mar 2007
  • Loc: IN

Posted 30 January 2017 - 06:06 PM

Thanks Jerry for this test. I been looking at this I find it excellent comparison. I know this is old but it still help those of us looking at buying a new scope.


  • jerry10137 likes this

#58 guyroch

guyroch

    Vendor (BackyardEOS)

  • *****
  • Vendors
  • Posts: 3,682
  • Joined: 22 Jan 2008
  • Loc: Under the clouds!

Posted 13 January 2018 - 05:48 PM

 

I can thank Backyard EOS for the ease of use.  Their software really helped in making this as simple as possible.

 

Thank you for the kind words.

 

bow.gif bow.gif bow.gif bow.gif bow.gif

 

Guylain


  • Phil Cowell likes this

#59 dpastern

dpastern

    Surveyor 1

  • -----
  • Posts: 1,614
  • Joined: 01 Jan 2009
  • Loc: Brisbane, Australia

Posted 30 August 2018 - 04:04 AM

Rich:

 

The difference I see is that FSQ and NP series are modified Petzvals that can be used visually with a diagonal and still provide that flat field of the view. The Esprit is an apo triplet with a separate field flattener for astrophotography, used visually it is a fast triplet with field curvature, quite a different animal.

 

Jon

Jon, was speaking to someone on YouTube today who has the Esprit 100, and he uses it visually with the field flattener and ES 100 degree AFOV eyepieces and says it's pretty flat and gives great views.  


  • 25585 likes this

#60 25585

25585

    Voyager 1

  • *****
  • Posts: 10,234
  • Joined: 29 Aug 2017
  • Loc: In a valley, in the UK.

Posted 24 December 2018 - 03:40 PM

Jon, was speaking to someone on YouTube today who has the Esprit 100, and he uses it visually with the field flattener and ES 100 degree AFOV eyepieces and says it's pretty flat and gives great views.  

If the same is true for 120 and 150 Esprits, with their flatteners, what fantastic rich field scopes!  


  • dpastern likes this

#61 dpastern

dpastern

    Surveyor 1

  • -----
  • Posts: 1,614
  • Joined: 01 Jan 2009
  • Loc: Brisbane, Australia

Posted 24 December 2018 - 04:59 PM

If the same is true for 120 and 150 Esprits, with their flatteners, what fantastic rich field scopes!  

I suspect so.




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