Per the ancient lore, part 27

grainy and ratty moon image shot through homemade telescope adapter
We be in the Space folder now, looking through squinted eyes (or at least I am) at a not-very-good photo of the moon taken through a telescope, but I include it because it’s one of the first that I took. A friend loaned me her 800mm Galilean telescope, a novice-level entry for reflector scopes, and I had endeavored to create a rig that would allow the camera to be mounted. This was the first result, which largely illustrates that it wasn’t worth the effort, but there are a lot of modifiers to be considered.

Let’s start with, the exposure time for a crescent moon is considerably longer than for a full moon, because it’s much dimmer. Seems kind of obvious, but then, you might pause and say, “Hold on there – the moon is still lit by full sunlight, so even though there’s less of it showing, it’s the same brightness, right?” Except it’s not, because the light is now hitting it obliquely and there’s far less of it reflected down towards us, while a full moon is much more of a direct reflection. And as the exposure time lengthens, so does the blur from the motion, because the Earth continues to rotate and so the moon continues to track across the sky – more or less, its own width in 180 seconds. Unless one is using a tracking motor (which this scope did not provide,) the shutter speed has to be fast enough to prevent much motion during the exposure and keep the details sharp. In this case, that meant ISO 800, which the Sony F717 did not handle well.

The author not holding still during a long exposureThen there’s light attenuation because of not just the scope itself, but whatever adapter is necessary to affix a camera. Consumer telescopes of nearly all types only have to provide an image through our tiny little eyes and are optimized for such, both in focal length and in ‘exit pupil,’ the actual size of the projected image at focus. Naturally, a 35mm film frame (which is what I originally adapted for) is a lot larger than our pupils or retinas, but it’s the same amount of light coming through the eyepiece of the scope, so it has to be spread out more to cover that frame, thus the whole image gets a lot darker. To that we add focal distance, because telescopes are optimized for the lenses of our eyes and the ‘effective’ focus of them to a couple of meters away for comfort (meaning when we focus on an image in a telescope or binoculars, we’re focusing the same as we would for an object a few meters away,) while the camera is going to be different. All of this usually means extra tubes from the eyepiece and often additional lenses too, see here in my homemade rig: the yellow tube is an old metal broomhandle the right size to lock into the eyepiece extension of the scope, glued to a rear lens cap with a hole precisely drilled into it, which it itself attached to a reversed 35mm lens, attached to the camera with a reversing ring. All of that reduces the light getting to the image.

Hold on, we’re not done yet! The Sony F-717 used in the image at top had a fixed lens that had to be accommodated, so it needed a different lens attachment – a reversed 50mm if I remember right, and a certain focal length on the camera itself. Very involved. This ended up vignetting the frame, projecting an image circle smaller than the digital sensor itself, so the edges were always dark. You don’t notice it in the top image, but can see it in the one below – which had better light and came out much nicer.

gibbous moon through telescope on morning of Venus transit
By the way, this shot was taken the morning of the first Venus transit in the past century – two occur eight years apart, followed by a gap of over a hundred years. The one in 2004 was going to be visible from Florida right at sunrise, and the dim light at sunrise (due to atmospheric scatter) could potentially allow a direct sun shot without filtration, and I was ready with the scope. Unsurprisingly, the sun was obscured by a cloud on the horizon during the transit, so I shot the moon while I was out there. Eight years later, soured by this experience and knowing the transit would occur in mid-afternoon, I wasn’t intending to get any shots at all and got extremely lucky with the conditions. You never know…

[The rig shot in the middle of the post is a self-portrait – yes, there’s too many pics of me recently, I do apologize – mostly lit by the nearly-full moon but with supplemental light from the apartment complex. I couldn’t hold completely still during the exposure time necessary, but don’t ask me what that was because it was on negative film and I never recorded the details. The camera on the scope was an Olympus OM-1, handy for this because of a manually-locking mirror, while this was taken with my trusty Canon Elan IIe. Both cameras required remote releases: the long cable release of the Olympus is visible, while the infra-red RC-1 for the Canon is out of the frame in my other hand.]

Leave a Reply

You can use these HTML tags

<a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <s> <strike> <strong>

  

  

  

This blog is kept spam free by WP-SpamFree.