Last updated: 31 August 2006
Subject: Orion 0.5x Focal Reducer Sent: Tuesday, August 29, 2006 06:32:46 From: Ted Rafferty (firstname.lastname@example.org) Orion 0.5x Focal Reducer Orion recently (summer of 2006) came out with a 0.5x Focal Reducer for their StarShoot Imaging camera. Since some of the Messier objects are a little too large for the combination DSI-I imager and LXD75 8-inch f/4 Schmidt-Newtonian I am using, I have been looking for a focal reducer. Most focal reducers are made for the Schmidt-Cassegrain telescopes that screw into the back of the telescope where the Orion focal reducer screws into the end of the 1.25-inch nose piece of the imager. Though Orion doesn't indicate that their focal reducer can be used with other imagers, I decided to get one for my DSI-I anyway. The focal reducer causes the focus for the imager to shift along the line-of-sight towards the primary. The amount of shift is dependent on how far the focal reducer is from the imager's chip (the closer the reducer is to the chip, the further in the imager much to located to focus), so one needs to have the necessary travel with their focuser to reach the focus. In my case, I had replaced the original 1.25-inch nose piece that came with my DSI-I (I didn't like the under-cut steps on the draw tube), as well as the original IR-blocking filter (the quality didn't look very good). The thread on the original nose piece is the standard one used for t-adapters so I replaced it with one that had a smooth draw tube. And I replaced the IR blocking filter with one that screwed onto the end of the draw tube of the nose piece. When I screwed the Orion focal reducer to the IR-blocking filter, I was unable to reach the focus. Switching the position of the filter and focal reducer also would not focus. Since the original nose piece for the DSI-I was a little shorter than the one I replaced it with, I put it back on the imager and was able to reach the focus if the focal reducer was screwed into the noise piece first and the filter screwed onto the end of the focal reducer. I only provide these details as an illustration that reaching the focus with the focal reducer might be difficult. Besides reducing the effective focal length, the focal reducer also decreases the f-ration of the optical system. The plus side to this is that it reduces the exposure time necessary to reach the same magnitude limit that would be required without the reducer. The negative side is that the lower f-ration makes focusing more critical. Here are two images taking of M27; one using the focal reducer and one without: Combination of ten 1-minute exposures using the DSI-I with the 0.5x Focal Reducer on a Meade LXD75 8-inch f/4 SN: Combination of eight 2-minute exposures using the DSI-I (without the 0.5x Focal Reducer) on a Meade LXD75 8-inch f/4 SN: I measure the actual scale reduction to be 0.64x instead of 0.5x, but that is just due to the differences in the dimensions of the DSI-I and StarShoot imagers. This changed the effective focal length from an f/4 to an f/2.6. Some degrading of the image quality is likely due to the additional optics of the focal reducer. I am not comfortable with recommending this reducer due to the difficulties of reaching the focus. It would likely be better to not use the focal reducer and take the necessary images to create a mosaic. Going the other way, there are a few Messier objects that are a little too small for the DSI-I/8-inch f/4 and I have tried using a Barlow to increase the effective focal length. Here are two images of M57; one using a 2x Barlow with the DSI-I and one without the Barlow. Combination of twelve 30-second exposures using the DSI-I with a 2x Barlow on a Meade LXD75 8-inch f/4 SN: Combination of thirty two 8-second exposures using the DSI-I (without the Barlow) on a Meade LXD75 8-inch f/4 SN: Measuring the scale change the 2x Barlow enlarged the field by a factor of 2.55. This changed the effective focal length from an f/4 to an f/10. Ted Rafferty Gaithersburg, Maryland
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