Use of OM-1 mirror lock for long exposures

Hi, just got an OM-1 (refurbished) for astrophotography. What are the
pros/cons of using the mirror lock for long exposure (1 min or more)
astrophotography? How is the mirror lock used (I know how to
engage/disengage it), when is it engaged etc. Thanks for your help


Chuck

 

There's a couple of factors regarding how useful mirror lockup is on
the OM-1.

One is vibration. The mirror slapping up as the shutter opens can
introduce vibration from the camera, and at telescope magnifications this
can produce a lot of image movement, sometimes even realigning the scope.
You won't know it, of course, until you close the shutter after the
exposure, or until the images come back.

The shutter can also do this too, but to a smaller extent, since the
OM-1 has cloth shutter curtains (in metal frames), very light with a low
impact factor. But in extreme cases, you not only use the mirror lock, you
lock open the shutter as well, having draped a black cloth over the end of
the telescope. Your exposure is then accomplished by removing the cloth for
the necessary amount of time to make the exposure.

Largely, these aren't too much of an issue. Mirror slap might
actually hang around as a vibration for two seconds at most - for an
exposure requiring a minute or much longer, this means an insignificant
amount of the light has hit the film in that vibration period to be
noticeable as being shaken (of course, scope realignment is another
matter). But it can be noticeable if your dim subject has a brighter
neighbor within the field of view...

But the other real benefit to the OM-1's mirror lock is that it's
strictly mechanical (and so is the shutter). So long exposures can be done
at any time, for as long as you like, without ever having to consider the
condition of the batteries. The OM-10, for instance, has an electromagnetic
mirror control, and it does not work at all without batteries (and freezes
half open, very annoying).

So using it: Line up the image where desired in the viewfinder, and
gently lock the mirror up. Wait a few seconds, then open the shutter. If
you're doing black cloth exposures like mentioned above, wait another
second or so before removing the cloth.

I have a borrowed scope, 4" reflector, and managed to make my own rig
to attach an OM-1 to it. But it has the sh*ttiest tripod imaginable, and no
tracking motor. So my astro opportunities are extremely limited. I'm
thinking I had a decent view of Jupiter the other night, including three of
the moons (!), but there was no way I was gonna be able to get the shot - I
couldn't even hold the planet in view for more than a few seconds, and once
it moved, finding it again was next to impossible with the jerky tripod...


- Al.