RASC Hamilton Fixing an old telephoto...

[Roger Hill]

A couple of years ago, I bought an old Pentax thread mount (sometimes
called an M42 mount) 300mm f/4 lens.  The Pentax Super Multi-Coated
Takumars were excellent lenses, built like tanks, and can now be found
relatively inexpensively.

I was looking for a non-zoom lens in the 200-300mm range, around f/4.  I
found a guy in Vancouver selling a 300mm f/4 SMC Takumar lens, and
checked out its reputation...turned out to be a very good lens in its
day.  So, for just over $100, I bought it.  

When it arrived, I checked it out, and it was indeed quite sharp.  Stars
shrank as I turned the focus toward infinity, and stopped shrinking when
I could go no further.  It was definitely worth the money, and it filled
a nice niche in my collection.  Also, being at F/4, it is quite fast,
and had an effective aperture of 75mm.  

Despite being happy with it, I had the distinct feeling that it could be
better still.

I made a Bahtinov mask for it, and sure enough, it wasn't quite perfect.
I needed to get the lens a little closer to the CMOS chip in the Canon
XSi.  It was just a matter of being able to turn the focusing ring on
the lens another 2 to 5 degrees or so.

There were a couple of small metal tabs on the lens that looks as if
they would stop the lens from getting closer to the adapter needed to
fit the lens to the Canon body.

Incidentally, this is one of the great things about Canon: you can get
simple mechanical adapters that will allow the Canon camera to use a
wide variety of lenses designed for other camera bodies.  Bizarrely,
though, this does include lenses from Canons older, non-EOS series of
lenses.  For those, you need a very special adapter that includes
optical elements.

Back to the story.  

Removing the metal tabs did not do anything.  I put the project on to
the back burner for a while as I played with my 6" Ritchey-Chretien.

About a month ago, I picked up the lens again and used a 40mm eyepiece
as a magnifying glass and examined it a bit more closely.  It looked
like there were a couple of small screws that adjusted a metal collar
that might make just the difference I needed.

After all, they have to make a lens and holder accurate enough to put
the image plane to within 1/50th of a millimeter, and since they don't
make lenses with focal lengths to that accuracy, there had to be some
adjustment somewhere.

Well, there was, just not at the place where I thought it was.  Removing
this collar allowed some other parts to move that had never moved
before.

Hoping that this was a good sign, I forged on.

With the right types of twists, pulls, pushes and unscrewing, I got the
lens out of the mechanism.  Sure enough, at one point in the
disassembly, I'd had to unscrew the lens via this incredibly fine
thread...about four or five threads per millimeter, I'd estimate.  Sure
enough, there was a stop on this thread that prevented the lens from
getting any closer to the CMOS chip.  

So, I moved the stop and put the lens back together.

It took me two days.  I gave up three times.  But I hate being beaten by
a mere mechanical device, particularly one that I've taken apart.  One
of the biggest difficulties was making sure I didn't cross-thread the
adjustment.   Astonishingly for me, I did not have any parts left over.
Normally, I've very efficient at repairing stuff, using fewer parts when
I'm finished than when it left the factory!

The Bahtinov mask shows that I can now focus the lens beyond infinity,
and sure enough, in Live View, I can watch the stars shrink and then
grow slightly larger.  A quick test showed that stars are now a touch
tighter than they were before.  

So, I've modified an old set of Parallax tube rings to hold the lens
firmly and attach it to a Vixen style dovetail.  It now mounts on both
my 12" SCT and my Meade motor modified EQ5 mount.  

Once I get my modified camera back, I'll give it a good try.  It should
be perfect for things like the North American nebula and some other star
clouds in the Milky Way.  

Going to be fun!

Roger     

-----Original Message-----
From: astronomers-bounces at hamiltonrasc.ca
[mailto:astronomers-bounces at hamiltonrasc.ca] On Behalf Of Andy Blanchard
Sent: Sunday, July 01, 2012 11:31 PM
To: astronomers at hamiltonrasc.ca
Subject: Re: RASC Hamilton Getting a DSLR modified...

Roger please tell us more about you lens modification?

Andy Blanchard
905-465-2031H
905-847-7777B

On 2012-07-01, at 5:15 PM, "Roger Hill" <Roger.Hill at hiteach.net> wrote:

> Hi, Mark...
> 
> The factory default filter that Canon uses for it's regular cameras
results in a spectral response that is an odd shape.  The filter is
veryu good at blocking out UV, but letting blue through.  It has a very
sharp dividing line betrween them.  The other end of the spectrum is not
as good.
> 
> At the red end, the response gradually tails off, which results in
only a 20% respose at the Hydrogen Alpha line.  This is not good!
> 
> The Astrodon filter is a visible light only filter (until about 1200
nanometers, but by then, the CMOS chip is extremely insensitive, so it
dowsn't matter).  This filter has a very sharp cut off not only at the
UV end, but also at the IR end, as well.
> 
> At 390nm, the filter is essentially opaque, rising to 99% passthrough
at 400nm.  It remains at 98-99% transparency until 700nm and is back to
opaque at about 710nm.
> 
> I certainly hope it works with lenses...I have a 300mm f/4 SMC Takumar
I recently took apart and put back together to allow it to focus at, and
beyond, infinity. 
> 
> Roger