Losmandy One Piece Worm Block Exposure Tests

   Numbers are a good comparison of a mounts performance but in the end the visual results count the most if you are using the mount for imaging. With the stock 2 piece block and gearbox I was able to get guide star exposures of 3.5 seconds before the effects of star elongation became visible using the 1854 mm F/L OTA and PEC enabled. I attributed this to the 76 second error which averaged 3.2 arc seconds peak-peak not being reduced using PEC because it didn't fall on a fundamental frequency and also too quick to guide out. This limited the camera rotator positions available so I could keep a guide star in the internal guider CCD.
   The following tests are done using the internal guider of the SBIG ST2000-XM camera at various exposures. The worm block is fitted with the Ruland coupler and McLennan gearbox. Each image is a 4 minute exposure to cover one worm cycle with PEC enabled.

How long can the guider exposure be set for?
Using CCDSoft the stars (marked by the +) profile RMS can be measured to give an indication of the X/Y axis differences. Ideally they should be the same under perfect conditions. Even when autoguiding these numbers vary slightly between X&Y so these unguided values are pretty good for 1856mm focal length.

(Clicking on the images will open a larger version to better see the details)

A 4 minute unguided PEC enabled image taken nights earlier.
At 1857 mm F/L and .83 arc second image scale, I'm very please at these results.
A 4 minute PEC enabled image using a 3 second guide exposure
Note the selected star profile. The less difference in X & Y the rounder the star.
A 4 minute PEC enabled image using a 4 second guide exposure Previously this was the absolute limit for a guide star exposure using the 2 piece worm block. A 4 minute PEC enabled image using a 5 second guide exposure
A 4 minute PEC enabled image using a 6 second guide exposure A 4 minute PEC enabled image using a 7 second guide exposure.
A 4 minute PEC enabled image using a 10 second guide exposure A 4 minute PEC enabled image using a 15 second guide exposure. I wouldn't go beyond this exposure out of personal preference. The first image that is unguided shows how well this mount tracks with a well trained PEC enabled.

   Imaging with a small CCD sensor limits my guider FOV so a bright guide star was essential before the upgrade. Even though the 4 minute unguided exposure looked good, I still would use a guider. The guider exposure tests were better than I expected. With longer guider exposures possible I'll be able to use dimmer guide stars especially with narrow band imaging when using the internal guider CCD.

© 2010 Michael A. Siniscalchi