ATM Sea Scope

[Anthony Stillman <atmer@flash.net>]

A few years ago I designed and built a small Galilean telescope for use
underwater.  In operation the instrument's two simple lenses were separated
by a six inch space filled with sea water.  The extreme curves of the
lenses, while helping to compensate for the reduction in power associated
with immersion, significantly degraded the useful field of view.  This
field degradation effectively limited the magnification to 1.3.  It was
somewhat of a disappointment  Two weeks ago, a few days shy of a trip to
Hawaii I decided to build a second sea scope.

I modeled the first telescope in OSLO LT.  The lenses came from Edmunds.
The OTA was made of carved and sawn PVC and ABS fittings.   This time I
used the Dolland approach.  That is, I scrounged through boxes, bags and
drawers for glass, trying one combination after another in a translucent
plastic drawer filled with water, until I found something that worked.
Actually, I kept trying even after I found something that worked.  Its well
that I did, as a subsequent combination proved superior.

>From my previous effort I had a bag full of odd PVC and ABS fittings.  This
time rather than fit the plastic to the glass I did the reverse, edging the
one lens that wouldn't fit, until it did.   Normally, edging induced
de-center is a great concern.  Significant effort to minimize this effect
is often taken.  However, given the magnification and image quality
requirements, I eye balled it.

The new instrument is about three times as long and has nearly twice the
aperture of my first sea scope.   The magnification is 2.3.  As with the
previous instrument, this is image quality limited.

The small field, and low magnification of the first scope made it sort of
pointless.  With the new instrument I was able to view in detail most of a
Humu-humu-nuku-nuku-apuaa at five meters and I safely saw close up, the
inside of a couple of morays' mouths.

The greatest weakness of this telescope is its mounting.   Choppy seas
greeted us the first couple of days out.  This significantly effected
tracking, introducing a sizable periodic error.   Luckily the ocean
cooperated for the remaining sea trials.

There are three lenses in the new scope.   It is still a Galilean with a
simple double convex objective of BK7.  But the eye lens consists of two
elements, a negative meniscus, also of BK7, and a plano concave lens made
of dense leaded flint.   This last piece of glass actually caused some
annoyance.  Assembled the telescope looks a bit like a PVC pipe bomb.
Having had numerous entertaining experiences with the comedy stylings of
Airport Security, I separated the components for travel.    Well, as it
turns out, dense leaded flint is x-ray opaque.  Having my carry on bag
sniffed for explosives was really only a minor inconvenience, the hardest
part was getting it closed again.  On the trip home I carried the glass
through the metal detector in my pocket.

To use the telescope it must first be filled with water.  I do this by
pulling apart a press fit ABS tube to PVC coupling coupling, while the
instrument is submerged.  The inner meniscus lens fits loosely behind the
flint element to allow the space between those elements to fill.  Once the
air is out I reassemble the telescope for use, keeping it submerged until I
return to shore.

In use there was no noticeable longitudinal color, though there was a bit
of transverse color at the edge of the field.  It was however hardly
significant, particularly for distant objects.  A few meters of sea water
is an effective bandpass filter.

All in all I'm happy with the performance of Sea Scope Version 2.0.  Though
a bit more aperture and magnification would be nice.  And, the apparent
field of view could be larger.  It would be great if it were more compact.
Next time maybe an afocal Cassegrain with Mangin elements or a folded
Keplerian with an image inverted.  Or perhaps a.... :-)


Anthony