A view of Orion with Orion

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Introduction

I used the 12.5 inch telescope of the AAAP to guide my 35mm Nikon FG camera and an Orion 400mm f/5 telescope for this 4-minute exposure of the Orion Nebula, using Kodak MAX (800 speed color) film. I evaluated the Orion telescope as a test optic for a CCD camera under development for TASS - The Amateur Sky Survey group, a collection of amateur and professional astronomers and programmers assembled over the Internet. Their Mark III cameras, designed and assembled by Tom Droege of Fermilab are currently surveying the celestial equator (+/- 3 degrees declination), and you can see their results on the TASS Web page at http://www.tass-survey.org .

Details

The photo was taken about an hour after sunset, 7PM -7:30PM on March 16th with Orion near the meridian at about 45 degrees altitude. The 3.5" X 5" print image was scanned at 200 dpi, 24-bit color, using an AVEC II S3 flatbed color scanner. A closeup at 1000 dpi is referenced below. Film exposures and prints of 1 and 2 minutes were very similar; a 6-minute exposure showed some increased (grey) background. These were all processed at the local PhotoMoto.

The field of view is about 4 2/3 degrees wide, and 3 1/2 degrees high, as mapped out in a starmap of the region (borrowed from Uranometria 2000) (The film negative image is 26mm high by 36mm wide as is standard.) North is in the upper left corner of the image. There is a little bobble of the star images, due to some telescope drive errors, and maybe due to focus error. These errors, and vignetting, make it difficult to access the amount of coma (image distortion) present.

Photographing at prime focus for the Orion telescope required racking the scope to its maximum length: the film plane for my Nikon FG is about two inches from the lens mount (T-ring adapter), and about nine inches from where the tube meets the focusing mechanism. I did not optimize the focus for these photos: focus was originally confirmed visually using the Moon, so I simply racked it all the way out and focused the star images by eye in the camera finder.

There is vignetting in the corners of this image, where the lens does not fully illuminate the film plane. This is more notible in a contrast-enhanced flatfield photo taken at 6:30 PM of the twilight sky in the same area. This was an auto-exposure, certainly fractions of a second. Measurements of the scanned image gave me pixel values around 50 (out of 255) in the center, down to a few counts at the darkest corners. If you compare the stars in the corners of the Orion image with star magnitudes in the corresponding star map, you will see an apparent loss of magnitude. Also note the orientation of the film image on the star map, relative to the western horizon where the sun had recently set: this might account for the eastern corners of the flat appearing darker than the western corners (the flat image may be upside down).

However, given sky conditions (a gradient likely due to sunset conditions), film procssing and scanning, my quantiative measurements of the flatfield image were consistent with what is seen in the enhanced flatfield image. They suggest the lens (and internal stop) start to vignette around the "height" of the image on the film (22mm or about 3 degrees). I would need a number of flatfield images, or to make photometric comparisions of the stars in the Orion nebula image, to make a better determination.

I reviewed my findings with my good friend and colleague, Bill Murray: a former Observatory Director and now Board Director of the AAAP, and a seasoned observer and amateur astrophotographer. He readily acknowledged the limitations of the Orion: "I'm not surprised by your findings" best summarizes his reactions. "If you want to get a much better optic", he advised, "there are some available for a few thousand dollars" and he suggested a source that advertizes in the astronomy mags.

Here is a closeup scanned at higher resolution (1000 dpi interpolated) with the AVEC II S3 flatbed color scanner. This shows star images at the lower right (south) of the Orion Nebula. This scan, and the one above, was an interesting exercise in digital optics. I don't know if the chromatic abberation (color fringing) on the 1000 dpi image is due to the Orion "achromatic" 2-element 80mmm lens or the AVEC scanner! But a shot of the full moon (not shown) clearly has yellow-green and blue color fringes that are both visible and photographable.

This is a starmap of the region pictured, taken from Uranometria 2000, a popular star atlas published by Willman-Bell Inc. The Orion Nebula is roughly at RA 5h 36m, dec between -5 and -6 degrees. Note that north is toward the upper left corner of the image.

Most recent revision June 12 1998

Herb Johnson
New Jersey, USA
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