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22453RE: GTF81 focal length

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  • sberrada_99
    Oct 12, 2013
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      As an owner of the GTF81 which has a specification of F5.9 (478mm FL)


      Overall it is a good telescope with sharp images edge to edge.


      However the first time I used it, I was a little surprised that the images looked bigger than my previous ES80 (480mm) but never thought much of it.


      When I saw your email, I did the following verification\;

      - I took a single sub and ran it in astrometry.net

      - This gave me a FOV of 1.57 x 2.35 degrees

      - When I convert for the sensor I get a FL of 553mm, which equates to F6.8

      fl =(  SensorSize/2) * tan( FOV/2)

      fl = (22.3/2)*tan-(2.35/2) = 553mm


      I have a friend who took the same DSO with a new Esprit 80mm F5; and to revalidate, I measured the relative distance between 2 stars and came up with a ratio of 1.35... which again leads me to F6.7.


      So we should be asking the builder / designer why that is ?


      This makes a difference of 30 percent on a single sub exposure, to get the same light intensity.... this means that subs need to be 8 min (at F6.8) long instead of 6 min (at F5.9) for the same result




      ---In William-Optics@yahoogroups.com, <william-optics@yahoogroups.com> wrote:

      Hello - I'm new to the group here. 

      I've been an enthusiastic amateur for a long time, but in the past year and a half have become an addicted astrophotographer and have gone as "all-in" as my budget will allow. 

      I recently purchased a GTF81 from Agena to be my main imaging scope, to go with my Atik 383l+ monochrome camera.

      While the scope is beautifully made (I have never owned a scope with this caliber of craftsmanship), I have gotten some odd results from imaging that concern me.

      I use Ron Wodalski's CCD calculator and figured that based on the stated specs of the telescope (478mm focal length, f/5.9 optics) I should get an image scale of about 2.33 arcsec/pixel.

      I've done some imaging, and immediately noticed that not only is it a bit more challenging to guide this telescope because of the weight, my images seemed to have been a bit grainier, as if the signal-to-noise ratio isn't great (up until recently I had been using a Stellarvue scope working at f/5.6 - should not be a dramatic difference to f/5.9!). 

      When I plate-solve my images, I find I get an image scale of 2.05 arcsec/pixel, which corresponds to ~550 mm focal length and f/6.7!

      Has anyone else gotten similarly odd results with this scope? 


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