A Photographic Test of the Meade 8" f/4 Schmidt-Newton Telescope

In March 2003 I had the opportunity to take two test photographs through a Meade Model SN-8 Telescope which was loaned to me for that purpose by Astro-Experts in Wolkersdorf/Lower Austria.

At f/4, this model is a fast wide-field instrument was a respectable 8-inch aperture. I was quite interested how it would perform photographically. Theoretically it should be able to capture deep-sky objects at great speed and depth and offer relatively fine detail. At little over 800mm focal length, its primary deep-sky targets are mid-sized to large emission and reflection nebulae, galactic stellar clusters, large comets (if available) and Milky Way structure. The focal length is a bit small for all but the largest galaxies, planetary nebulae and globular clusters, although these may be portrayed to document the field surrounding them.

Description

The telescope has a clear aperture of 203mm (8") and a focal length of 812mm, yielding a focal ratio of f/4. By design the optical system is more like a Schmidt-Cassegrain, with a spherical primary mirror and a Schmidt plate (an aspherical correcting lens) at the aperture to compensate the spherical aberration of the primary, but the secondary mirror is flat and inclined 45 degrees to the tube axis, thus projecting the image to the focuser which is situated near the front end of the tube, like at a Newtonian. The Schmidt plate supports the secondary mirror, thus eliminating the need for spider vanes. The focuser is a 2" rack-and-pinion type, and the telescope comes with a 6x30mm finderscope. The coatings on primary and secondary mirror are aluminium, the correcting plate is coated with MgF2. The tube is made of enameled steel and is a bit on the heavy side with 11 kg (24 lb) including the optics.

The telescope is usually sold in combination with the Meade LXD55 German Equatorial Mount, which may be adequate for visual observations, but is overpowered when used for prime-focus astrophotography with this telescope.

Test Shots

Collimation of the Schmidt-Newton was not easy, especially the secondary mirror presented a few problems: While working one of the collimating screws, the whole secondary assembly often was subject to sudden unwanted rotation. After spending some time with trail-and-error, we were finally able to collimate the telescope to a satisfying degree during daylight. However, during a quick star-test shortly before taking the photographs, I still detected some miscollimation.

The focal length is relatively short, and so is the required exposure time, thus I decided to photograph with a guidescope. When trying to mount the guide-scope rings on top of the main telescope's mounting rings, I noticed that the screw holes are metric, not the usual 0.25" standard photo-tripod thread. As guidescope I used a 63mm (2.5") f/13.3 Zeiss Telementor refractor. At the guidescope, I employed my Pictor 216XT autoguider.

For my test shots, the telescope was mounted on a VIXEN GP-DX on top of a Baader hard-wood tripod, that combination proved barely rigid enough for the purpose, and needed a lot of counterweights. I would not recommend photographing with this setup during a windy night; get a sturdier mount if you plan using such a heavy scope - guidescope - camera - autoguider combination regularly.

The photos were taken on March 23rd at Ebenwaldhöhe, an observing site with acceptable dark sky not foo far from Vienna/Austria, where I live. The visual limiting magnitude during that night was 6.0 mag. As film I used Fuji Provia 400F in my Nikon F3 35mm camera, the exposure time was 30.5 minutes for each photograph. As targets I selected the Horsehead Nebula Region in Orion and the Seagull (or Eagle) Nebula IC 2177 at the Monoceros/Canis Major border. Both objects were relatively low in the south-western sky. The Schmidt-Newton offers enough back-focus to get the camera into focus with a standard T2-adapter. Focusing is critical at f/4, because the depth of focus is shallow.

Results

Here are the raw scans of both slides:

B33, raw scan
Horsehead Nebula Region, raw scan

IC 2177, raw scan
Seagull Nebula, raw scan

Both images suffer from uneven illumination caused by vignetting, and radial distortion of the stars near the edges, caused by field curvature and off-axis coma. There is also slight horizontal color banding visible in both images which was caused by the scanner.

The Horsehead Nebula image also suffers from multiple reflections which are caused by bright stars just outside the field-of-view, notably Epsilon Orionis at the upper left and Theta Orionis at the upper right. The Orion Nebula M42 may be responsible for the broader red reflection ring best visible right and above the image's center. The ray-like reflection at the upper left also shows a prismatic effect. Also visible are two blue-colored Schmidt ghosts, probably caused by internal refections within the Schmidt plate, they form small rings and more complicated structures at right and at the lower right of the image's center. Below, you can see enlargments of the bright reflection at the upper left, a Schmidt ghost and distorted stars at right, and of the image's center. Note that the stars are not even at the image's center perfectly round, probably due to a slight guiding error in declination.

The reason for the "spider vanes" around Alnitak, the brightest star in the Horsehead Nebula image, is not quite clear, since the Schmidt-Newton has no spider. Possibly this was caused by the mirror clips of the primary which protrude into the light path.

raw scan enlargements

raw scan
enlargements raw scan enlargements
Horsehead Nebula Region, enlargments of the raw scan

The Seagull Nebula image is free of reflections and Schmidt ghosts, probably because of the lack of bright stars near and within the imaged field.

All artifacts except the radial distortion of stars could be eliminated or at least greatly reduced during image processing in Adobe Photoshop, but some of them only with considerable effort. To reduce the extend of radial distortion, the images were simply cropped down. Click on the thumbnail previews below to see the final results after processing.

B33, processed 
image IC 2177, processed image
Processed images

Bottom Line:

This telescope is problematic in astrophotographic use. The reflections may be reduced or eliminated by painting the inside of the tube flat black and by a tube extension up front, or simply by not photographing near bright stars. Vignetting is caused by the T2 adapter and by the inner edge of the long focuser drawtube, and might be reduced by shortening the drawtube (if possible) and by using a special adapter for your camera with a wide opening. The radial distortion of the stars near the edges of the field is due to field curvature and off-axis coma which might only be corrected by a dedicated corrector which is not available for this telescope. So cropping down the image is more or less essential.

As written before, all artifacts except the radial distortion of stars can be eliminated or nearly eliminated during image processing, but most of them only with considerable effort.

Walter Koprolin, May 2003


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