Wednesday, November 12, 2014

Lens Happy

A visit to National Camera Exchange's used lenses page turned up a manual focus 200mm f/4 Olympus Zuiko OM lens. This should work with my ST-8300M, and old reviews suggest that it's got a chance of providing acceptable corner stars even when close to wide open. At a price of $40 it's worth a try. My impression is that the lens sells for $50-60 on the used market, which probably explains why it sat on NatCam's shelf for a long time at $90.

If you get the impression I've got a fixation with lenses, you're right. There have been too many objects that are a bit too large for my 422mm AT65EDQ, and I'm too impatient to make a mosaic for each of them. My telescopes currently give me a nice range of focal lengths:
  • 2350mm (C925 @ f/10)
  • 1480mm (C925 @ f/6.3)
  • 700mm (TV 102 @ f/6.9)
  • 422mm (AT65EDQ @ f/6.5)
These very roughly represent steps of two in scale. Extending these to shorter focal lengths with SLR lenses adds:
  • 135mm (Tamron OM @ f/5.6)
  • 70mm (Canon 17-70mm zoom @ f/5.6)
  • 50mm (Zuiko OM lens @ f/1.8)
  • 28mm (Tokina @ f/1.8)
The 200 mm lens fills the gap between scopes and lenses, taking the place of an old Tele-Astranar that won't reach focus with my CCD. Granted that these are a bunch of consumer-grade lenses I don't expect miracles from them.

The field of view of a 200mm lens used with an ST-8300 is huge: 5.1 x 3.9 degrees. Here are some objects that fit nicely into that field with a little room around the edges:
IC 1396, which is more extensive than Sky Tools depicts

Lagoon and Trifid Nebulae and vicinity.

Rho Ophiuchi Nebula and vicinity

Orion Nebula and vicinity

M 31 et al.

IC 1318 et al.

Rosette nebula
North America and Pelican Nebulae
The pictures represent the full field of a 200mm lens combined with an ST-8300 CCD camera as illustrated by SkyTools3.

There are more objects than these suitable for the 200mm lens, but this will make a good start!

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