Re: [ATM] Focal Plane Location

[John Lynch <john_lynch2005@yahoo.com>]

Don and all,

You can also take a piece of paper, 3/16th inch wide
and 1 1/2" long, bent at one end; point the eyepiece
at a distant lamp (shade off) in a semi-lit room, and
put the piece of paper (bent end first) into the
bottom of the eyepiece.  Focus the image on the paper
and measure how far up this point is in the barrel (by
marking the paper at the end of the barrel).  Then,
compute this distance relative to the eyepiece
shoulder (where the eyepiece would come to rest in the
focusing mechanism).  Sometimes this point is below
the shoulder and sometimes up inside.  I think the
convention is negative is up inside and positive is
below.  See the following TeleVue website for eyepiece
specifications.

http://www.televue.com/engine/page.asp?ID=144

If focus is internal (like some of the Nagler
eyepieces), you have to rely on the manufacturer's
specifications or ask someone who has used the
eyepiece for advice.  This method has worked for me in
the past.  Hope this helps.

John Lynch

--- donald.good@comcast.net wrote:

> 
> > -----Original Message-----
> > From: atm-bounces@atmlist.net 
> > [mailto:atm-bounces@atmlist.net] On Behalf Of
> Richard
> > Sent: Monday, November 14, 2005 10:04 AM
> > To: Gilbert Gagne
> > Subject: Re: [ATM] Focal Plane Location
> > 
> 
> > 
> > GG> 2. Eyepiece specs specify their focal length;
> what is the 
> > location 
> > GG> of the focal plane relative to the field lens?
> How can 
> > this be measured?
> > 
> > I've seen it vary from 6mm above the mounting rim
> to 50mm 
> > below the mounting rim. By 'mounting rim' I mean
> the part of 
> > the eyepiece that bottoms out on the top of the
> focuser tube. 
> > Ie, the measurements are relative to the top of
> the focuser 
> > tube with the eyepiece fully inserted. Some
> manufacturers 
> > will give that dimension and some will make a
> range of 
> > eyepieces where the dimension is the same for all
> focal 
> > lengths. Confocal(?)
> 
> Parfocal (aka parafocal) EP sets have this
> characteristic.
> 
> Mount a small frosted "Christmas tree" light bulb in
> a small box that you can set your focuser on, with a
> hole above the bulb.  Mount a small short tube in
> the hole with diameter that can fit inside the end
> of your eyepieces, long enough that the end can
> touch the first obstruction (field stop or first
> lens) inside your lenses when racked in.  Mount some
> flat object (cross hairs, slide transparency, or
> optical resolution test grating) on top of the tube.
>  Set your focuser over the tube so that the tube is
> centered (maybe tape it down).  You can now measure
> the height to the top of the focuser (mounting rim
> as described above) of all your eyepieces when you
> focus on the test object.  BUT be careful when
> racking in the focuser so as not to scratch any lens
> coatings if the tube can touch the EP lens.  A thin
> O ring can be used as a cushion if you are
> concerned.  If you want to get fancy, mount a
> vertical rod outside the focuser and attach a small
> machinist's rule to the rod so that zero is level
> with the test object.  Then when you focus, remove
> the EP and a straight edge across the top of the
> focuser to the rule will give you an accurate
> measure of mounting rim to focal plane for each EP.
> NOTE: Your measured value may not match the
> manufacturer's specs because you are measuring the
> combined effects of the EP focal length AND your own
> eye's optical effects (focal length, astigmatism,
> etc.) AND eyeglasses/contacts, if used.
> 
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> 



		
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