Re: [ATM] 1.25in Versus 2inch focuser.

["Marco" <marco.miglionico@btinternet.com>]

> This is a mis-interpretation of the book.
>
> A 20 inch f/5 (of which I have) yields a 57 arc minute FOV using a Nagler
> 31mm.  A real FOV of 1.7 inches is required.

Yes, but the upper cage parameters in Krieges books will start vingeting the 
FOV at around
0.5 deg see pp153 'The Dobsonian Telescope'    I thought that this had 
delibirately been set as a prctical limit due to coma.
Also see p110 Texereau. In it there is a table giving 'Permissable Field 
diameters' - set by a limit
of 100microns of the image.  - how do |I calculate this for a 20in F5 scope? 
Any help appreciated.


I realised I could open out the cage to get about 0.75in clearance either 
side giving an unvignetted
FOV of 1 deg but like I said, I thought this was too wide to go for such a 
big scope.

If you say I can go that wide, then I will fully trust your judgement!

Thanks everyone for the input:   It'll be a 2in focuser then!

Marco Miglionico


----- Original Message ----- 
From: "Mel Bartels" <mbartels@bbastrodesigns.com>
To: "'AMT LIST'" <atm@atmlist.net>
Sent: Sunday, October 08, 2006 10:44 PM
Subject: Re: [ATM] 1.25in Versus 2inch focuser.


>> However in Berry's book it is recommended that in a scope of this
>> size, parameters give a practical limt of around 0.5 degrees FOV.
>>
>> Doing some calculation tells me that I only need a 1.25in focuser
>> as a 24mm panoptic will supply me with a field of around 39 arcmin
>> which is more than ample.
>
>
> This is a mis-interpretation of the book.
>
> A 20 inch f/5 (of which I have) yields a 57 arc minute FOV using a Nagler
> 31mm.  A real FOV of 1.7 inches is required.
>
> For more, see http://www.bbastrodesigns.com/rft.html
>
> Mel Bartels
>
>
> _______________________________________________
> ATM mailing list http://www.atmlist.net/ 

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