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using 300mm camera lens as a 'scope - 2" or 1.25" eypieces - making a focusing gizmo (a bit long) ?



 
 
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  #1  
Old December 19th 04, 06:52 PM
Hywel Davies
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Default using 300mm camera lens as a 'scope - 2" or 1.25" eypieces - making a focusing gizmo (a bit long) ?

As pre-amble I'm relatively new to all this, but technically minded, if not
necessarily that profficient. Lots of questions here, but any hints
gratefully recieved.

Rather than buy a telescope at considerable expense eg small Televue, or
excessive size e.g Newtonian, SCT etc, or seemingly naff construction e.g.
Meade ETX, it ocurred to me that I could use the wife's 300mm f4 ie 75mm
diameter (Nikon) camera lens and just need some means of fitting standard
eypieces. My first assumption is that this is a sensible idea, and that I'd
expect the quality to be pretty good as the lens has pretty sophisticated
optics by telescope standards, although it isn't apo' corrected, and I
recognise that at least some of the engineering / cost has gone into the
mechanisms, and into reducing the overall length.

The diagonal of a 35mm negative is about 2" so, I'd imagine this equates to
a pretty "wide field" for astro purposes, and I'd also imagine that I'd want
the ability to mount 2" eyepieces. Does this sound reasonable ?

The back-of-lens to film-plane distance is about 45mm. Am I right in
thinking that a given focal length eyepiece will have it's be around twice
the focal length from the focal plane of the primary (measuring from the
"optical centre" (if that's the term) of the eyepiece ?

I thought I'd buy a cheap extension tube or teleconverter in order to get a
Nikon body bayonet then turn up a suitable bit of tube to fit the eyepieces.
Whilst I have a lathe, and a comprehensive selection of engineering books,
measuring gear etc, I've not actually made that much as yet. I rarther doubt
my ability to make a rack-and-pinion mechanism, so was thinking in terms of
a push-pull focusser, then using the lens focus for final adjustement

What would be a good eyepice to purchase first ? I probably won't purchase
till I've made the gubbins so don't mind spending proper money.
What would be a reasonable maximum and minimum focal length ?
Would "wide field" eypieces be a good thing given my (possibly wrong)
assumptions earlier.

Can anyone point me towards a definition of "standard" for the 2" and / or
1.25" eyepice fittings. I'm guessing this is the just the overall diameter,
and you perhaps fit it in place with a grub-screw or somesuch.

Thanks in advance

Hywel


  #2  
Old December 19th 04, 07:12 PM
Martin Frey
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Posts: n/a
Default

"Hywel Davies" wrote:

Rather than buy a telescope at considerable expense eg small Televue, or
excessive size e.g Newtonian, SCT etc, or seemingly naff construction e.g.
Meade ETX, it ocurred to me that I could use the wife's 300mm f4 ie 75mm
diameter (Nikon) camera lens and just need some means of fitting standard
eypieces.


I have done this with a Pentax screw filling lens - I found a 2 inch
nylon castor wheel that just fits the Pentax screw and forced it on so
the Pentax cut its own thread, then drilled out the centre and screwed
a piece of wood with a 1.25 dia hole to accept the eyepiece. It works
mechanically - no problem. If you hold the lens and the eyepiece in
line you can focus on the Moon and get a good idea of the separations
required.

The problem is that their is just too much glass in a camera lens and
you are unlikely to get nice point star images - though the Moon looks
pretty good. I suspect that the optical demands for a terrestrial
photographic camera - flat focus and even brightness over a large film
area mean something must be sacrificed as compared to the minimum
amouts of glass in an astronmical telescope.

I'm glad to have done it - it didn't cost anything and it works after
a fashion and is good for demos. A 50mm lens is quite satisfying for
ultra-wide field views witha 20 or 36mm eyepiece and it makes quite a
good finder with a 10mm eyepiece. Eyepiece quality doesn't seem to be
an issue - it all seems to work just as well with £5 eyepieces from
BC&F's bumper bargain bins.

-----------------------------
Martin Frey
http://www.hadastro.org.uk
N 51 01 52.2 E 0 47 21.1
-----------------------------
  #3  
Old December 20th 04, 10:12 AM
Martin Brown
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Default

Hywel Davies wrote:

As pre-amble I'm relatively new to all this, but technically minded, if not
necessarily that profficient. Lots of questions here, but any hints
gratefully recieved.


You can do it for low magnification spotting scope, but don't expect
good planetary images. It is handy sometimes to have such a device in
your camera bag.

Rather than buy a telescope at considerable expense eg small Televue, or
excessive size e.g Newtonian, SCT etc, or seemingly naff construction e.g.
Meade ETX, it ocurred to me that I could use the wife's 300mm f4 ie 75mm
diameter (Nikon) camera lens and just need some means of fitting standard
eypieces. My first assumption is that this is a sensible idea, and that I'd
expect the quality to be pretty good as the lens has pretty sophisticated
optics by telescope standards, although it isn't apo' corrected, and I
recognise that at least some of the engineering / cost has gone into the
mechanisms, and into reducing the overall length.


It has also gone into making a flat focal plane and relatively fast
lens. A fast light cone is very demanding on eyepieces. f5.6 or f8
lenses are much easier to make scopes from. And plenty of old manual
fixed focal length lenses are in second hand photo dealers.

The diagonal of a 35mm negative is about 2" so, I'd imagine this equates to
a pretty "wide field" for astro purposes, and I'd also imagine that I'd want
the ability to mount 2" eyepieces. Does this sound reasonable ?


Cheaper to try it with 1.25" eyepieces around the 20-30mm fl mark. You
may need to use a 2x teleconverter on it to get the light cone to f8
where most cheap eyepieces will work fine.

There generally isn't enough back focus to use a diagonal.

The back-of-lens to film-plane distance is about 45mm. Am I right in
thinking that a given focal length eyepiece will have it's be around twice
the focal length from the focal plane of the primary (measuring from the
"optical centre" (if that's the term) of the eyepiece ?

I thought I'd buy a cheap extension tube or teleconverter in order to get a
Nikon body bayonet then turn up a suitable bit of tube to fit the eyepieces.


You can probably bore out the ID of the 20mm extension tube to take a
1.25" eyepiece as an interference fit. A locking screw would be nice for
visual use, but not needed for eyepiece projection.

Whilst I have a lathe, and a comprehensive selection of engineering books,
measuring gear etc, I've not actually made that much as yet.


Remember to take the aperture lever linkage out before you turn the job.
The bits tend to fly out at inconventient times.

my ability to make a rack-and-pinion mechanism, so was thinking in terms of
a push-pull focusser, then using the lens focus for final adjustement


Measure it about right and you should be able to adjust the focal point
using the lens focus mechanism (with some increase in aberrations).

What would be a good eyepice to purchase first ? I probably won't purchase
till I've made the gubbins so don't mind spending proper money.
What would be a reasonable maximum and minimum focal length ?
Would "wide field" eypieces be a good thing given my (possibly wrong)
assumptions earlier.


Low magnification would be best. You will need an expensive well
corrected eyepiece to work well at f4.

Can anyone point me towards a definition of "standard" for the 2" and / or
1.25" eyepice fittings. I'm guessing this is the just the overall diameter,
and you perhaps fit it in place with a grub-screw or somesuch.


The minimalist cheap plastic version can also be made using 1.25"
plumbing parts, a lens mount cap and some aradite.

Regards,
Martin Brown
  #4  
Old December 20th 04, 01:48 PM
Jim Easterbrook
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Posts: n/a
Default

"Hywel Davies" wrote in
:

Rather than buy a telescope at considerable expense eg small Televue,
or excessive size e.g Newtonian, SCT etc, or seemingly naff
construction e.g. Meade ETX, it ocurred to me that I could use the
wife's 300mm f4 ie 75mm diameter (Nikon) camera lens and just need
some means of fitting standard eypieces.


I've done something like this, using a 200mm f/4 lens with an M42 (Zenith
/ Pentax screw) fitting. I use it with a 40mm eyepiece as a "5X telephoto
converter" for my Nikon 4500. Only last night I was using it to image the
Pleiades:
http://www.easter-vivian.e7even.com/...N1295-1301.jpg
--
Jim Easterbrook http://astro.jim-easterbrook.me.uk/
N51.36 W0.25
 




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