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It ain't what I thought it was...

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#1 Keith Rivich

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Posted 27 September 2020 - 11:40 AM

About 10 years ago I bought an 8" f/9 reflector. Or so I thought. '70s vintage Parks Optical aluminum tube. Great little scope.

 

Yesterday I pulled the mirror cell to clean the primary. Usually I just clean the mirror while in the cell but this time I wanted to replace the felt liner that cradles the mirror's edges as it had become worn and flat and no longer keeps the mirror from sliding around. 

 

So I pulled the mirror out of the cell...

 

Lo and behold, hand written clear as day on the edge of the mirror, in big red letters, is "8" f/7  55 1/2" FL". 

 

Hah! All these years I thought it was an f/9 and used that ratio in all my calculations! 

 

I new there were extra holes toward the back end of the tube but I just thought someone in the past had moved the mirror up for some reason, imaging perhaps. 

 

Now I have to go back and edit my notes....smile.gif .


Edited by Keith Rivich, 27 September 2020 - 11:41 AM.

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#2 Couder

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Posted 27 September 2020 - 11:47 AM

funny the things you find out by accident.



#3 M11Mike

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Posted 27 September 2020 - 12:21 PM

The extra holes further back - a dead give a way.   



#4 Don H

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Posted 27 September 2020 - 01:45 PM

So all your eyepieces now produce 30% less mag and give you 30% wider field. FOV sounds like an unexpected bonus, while you can easily still bump up the mag with a shorter fl eyepiece. Win win. Maybe you can trim a foot or more off your tube, too.



#5 Keith Rivich

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Posted 27 September 2020 - 02:03 PM

So all your eyepieces now produce 30% less mag and give you 30% wider field. FOV sounds like an unexpected bonus, while you can easily still bump up the mag with a shorter fl eyepiece. Win win. Maybe you can trim a foot or more off your tube, too.

I originally bought the scope because of the f/9 mirror. It wasn't intended for wide field observing. It sits in my living room so I can take it out for solar system stuff. I probably wouldn't have bought it knowing it was f/7. I only paid $150 so I'm still happy. The scope performs really well. 

 

I like the idea of cutting the tube. Would fit in my truck a lot easier. Before I do that though I may try and find an f/9 mirror. Now I'm curious about the secondary...I need to measure it to see if it's the original mirror. I may not be getting a fully illuminated field...



#6 CHASLX200

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Posted 27 September 2020 - 04:33 PM

The extra holes further back - a dead give a way.   

Holes would have to be very way back if there was a F/9 mirror in it by around 16".


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#7 Retsub

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Posted 27 September 2020 - 05:21 PM

Years back many others thought the extra mirror cell holes were to correct mistakes in assembly !   BW 


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#8 Keith Rivich

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Posted 27 September 2020 - 06:28 PM

Holes would have to be very way back if there was a F/9 mirror in it by around 16".

Dang. You are correct. Maybe retsub is right and the extra holes are oops holes. Next time I have the scope out I'll take a look at the bottom of the tube and see if the thing was cut. The extra holes I talked about are only about 4" further down then the holes the cell now bolts to.


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#9 Jon Isaacs

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Posted 28 September 2020 - 10:34 AM

Kevin:

 

Some years ago I bought a used 6 inch F/8 Discovery Dob.  It was by far the best performing 6 inch I'd ever owned. But it didn't get much use, it just sat in a corner.

 

Someone started a thread about tube diameter versus aperture so I decided to measure my scopes.

 

The long and short of it was that this 6 inch F/8 was actually an 8 inch F/6..

 

:foreheadslap:

 

Jon


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