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Equipment Discussions >> Refractors

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droid
rocketman
*****

Reged: 08/29/04

Loc: Conneaut, Ohio
C102 HD new
      #5699362 - 02/25/13 07:12 AM

I prchased this scope some years ago, used.
I used to know it true focal length, before painting it , it had a stick on its side telling me, lol.
After painting the sticker wouldnt adhere, and has since gone missing.
In any case, I had alwasy believed since that is f/8, but I keep reading the C102s were f/10
Now I know this isnt the most scientific way to measure a lenses focal length, but when it is all up, I measured from the back of the lens to the center of the diagonal, and its 36 inches, which would make it f/9.
Im totally confused now, as its neither of what I thought it was, lol.
It wont stop me from using it, as its the best refractor , optically, Ive owned.
Just had to do away with that god aweful black it was originally. lol


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GOLGO13
Carpal Tunnel


Reged: 11/05/05

Loc: St. Louis area
Re: C102 HD new [Re: droid]
      #5699405 - 02/25/13 08:26 AM

It's a 1000 focal length...technically it's F9.8...but usually listed as F10.

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siriusandthepup
sage
*****

Reged: 02/14/06

Loc: Central Texas, USA
Re: C102 HD new [Re: GOLGO13]
      #5699515 - 02/25/13 09:49 AM

The usual way to measure would be from the back of the primary lens to the focal plane. One way to do it is to aim the telescope at the moon. Remove the eyepiece/diagonal.
Using a piece of paper see where the moon image focuses on the paper. That is the focal plane. From the back of the lens to the focused moon image is your exact focal length.

I also have one of these and Golgo is correct on the specs.


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buddyjesus
Carpal Tunnel
*****

Reged: 07/07/10

Loc: Davison, Michigan
Re: C102 HD new [Re: siriusandthepup]
      #5700564 - 02/25/13 09:01 PM

what golgo said

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Jon Isaacs
Postmaster
*****

Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: C102 HD new [Re: droid]
      #5701117 - 02/26/13 07:56 AM

Quote:

Now I know this isnt the most scientific way to measure a lenses focal length, but when it is all up, I measured from the back of the lens to the center of the diagonal, and its 36 inches, which would make it f/9.




You need to measure from the about the middle of the objective lens to the focal plane when the scope is in focus at infinity. If the diagonal is a 2 inch model, it's optical path length is about 4 inches.

But in any event, the focal length of the 102HD is 1000mm.

Jon


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csrlice12
Postmaster
*****

Reged: 05/22/12

Loc: Denver, CO
Re: C102 HD [Re: Jon Isaacs]
      #5701272 - 02/26/13 09:46 AM

and they are very well built scopes.

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droid
rocketman
*****

Reged: 08/29/04

Loc: Conneaut, Ohio
Re: C102 HD new [Re: csrlice12]
      #5701539 - 02/26/13 12:13 PM Attachment (20 downloads)

Thanks everyone... and yeah this puppy is built like a tank.
It was the first small scope to show me M51 directly, in the city with poles lights everywhere.
But I got it in my head the black had to go, and I wanted classic looking.


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GOLGO13
Carpal Tunnel


Reged: 11/05/05

Loc: St. Louis area
Re: C102 HD new [Re: droid]
      #5701543 - 02/26/13 12:14 PM

Looks pretty cool...nice!

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csrlice12
Postmaster
*****

Reged: 05/22/12

Loc: Denver, CO
Re: C102 HD new [Re: GOLGO13]
      #5701709 - 02/26/13 01:44 PM

Different color scheme, but it's basically the same thing as my 102XLT OTA, Is that an EQ4/CG5 mount?

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