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Accurate Focal Length for Discovery Dob

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#1 Bert542001

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Posted 18 March 2013 - 09:06 PM

Hi folks,

Quick question regarding published f-ratio and focal length for my Discovery 8" DHQ dob. Published as an f/7, 8" objective set in 57" OT, I have always scratched my head a bit because the center of the focuser/secondary mirror is about 5" from tube end and the primary mirror is set about 2" from the bottom. Doesn't that make the fl of the scope 50" and much closer to f/6?

Not quibbling but it starts to make a difference when looking at magnification at short fl...buying a 4mm eyepiece at f/7 seems more practical than f/6 where I'll probably not get the performance under average viewing that would make it a good investment.

Thanks for any input - I'm sure I'm over thinking it. Just wanting to truth the numbers and if I see it wrong I'd like to understand why.

Clear skies!

#2 Tony~M

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Posted 18 March 2013 - 09:15 PM

Hello Bert,

Hope you are well.

The FL is the distance from the surface of the primary to the secondary plus the distance from the secondary to the focal point out the side of the tube.

Some mirrors will have the actual focal length written on the bottom of them.

Hope this helps.

Later,
Tony

#3 Bert542001

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Posted 18 March 2013 - 09:43 PM

Thanks, Tony.

Sorry for the bonehead question. I never saw it explained before to include the distance from primary focus on the secondary to the focus in the focuser. I was thinking only in terms of primary mirror to secondary.

Thanks for the education - always learning!

#4 Jon Isaacs

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Posted 18 March 2013 - 09:55 PM

Thanks, Tony.

Sorry for the bonehead question. I never saw it explained before to include the distance from primary focus on the secondary to the focus in the focuser. I was thinking only in terms of primary mirror to secondary.

Thanks for the education - always learning!


Bert:

First, hello and welcome to Cloudy Nights... :whee::jump::whee:

The focal length is the distance from the objective/mirror to the focal plane. Since the focal plane has to be near the top edge of the focuser, you do need to include the entire distance from the mirror to the top of the focuser.

A few years back, the astro-club at school built a scope. It would not focus and they asked me to look at it. They had designed it so the image focused on the secondary....

Jon

#5 wirenut

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Posted 18 March 2013 - 11:11 PM

everything you said sounds about right for a F7 8" dob,Measure mirror to mirror then add mirror thru to front of focuser for a ball park figure. I suspect there's some variation commercial and with premium less, the effect of a couple MM would be fractional in most cases.

#6 Tony~M

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Posted 18 March 2013 - 11:12 PM

Hello Bert,

No problem. Welcome to Cloudy Nights!

Later,

Tony

Thanks, Tony.

Sorry for the bonehead question. I never saw it explained before to include the distance from primary focus on the secondary to the focus in the focuser. I was thinking only in terms of primary mirror to secondary.

Thanks for the education - always learning!



#7 Bert542001

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Posted 19 March 2013 - 08:18 AM

Thanks, Jon. Looks like I'm not alone in the misunderstanding.






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