Re: [ATM] Focal Plane Location

[donald.good@comcast.net]

> -----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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