A photographic evaluation of the Russian-made APM 4" f/6.5 APO

In this article I'll try a short photographic evaluation of my 4" f/6.5 (D=102mm, F=650mm) apochromatic refractor using one of the first photographs I took with this instrument, showing its strengths and weaknesses. This telescope is produced by a Russian Zeiss contractor and sold by Markus Ludes, the owner of APM Telescopes in Reifenburg/Germany. For a visual evaluation report, see Ed Ting's report on the TMB Fluorostar 4" f/8 APO.

My APM APO is an earlier 102/650 model which was clearly designed for visual observation, not astrophotography, and features a VIXEN tube, a 2" focuser and no flatfield corrector. It is built as air-spaced SDT triplett fluorophosphat apochromat.

This photograph shows the southern part of Lyra. It was taken at the backyard of a house in a small village in Lower Austria not far from Vienna under slightly light-polluted skies and was exposed 18 minutes on Fujicolor Superia 400. This is a raw scan, the only correction applied during the scan was the balancing of the colors to cover the color range of the photograph well. You can see the Ring Nebula M57 as small slightly red smudge below the center, the bright stars in the lower left corner and on the upper right are Gamma and Beta Lyrae, respectively. Field size of the 4" f/6.5 APO with 35mm negative film is 3.2 x 2.1 degrees.


Vignetting

To visualize edge vignetting the histogram of the image has been stretched:

Vignetting

This asymmetric vignetting is, however, NOT caused by the refractor, but by the off-axis guider I used as an extender to get the camera into focus, not for guiding. A simple 2-inch diameter tube covers the same distance without causing asymmetric vignetting, but some vignetting still remains.


Field curvature

Field curvature

Each image above shows a small region at the four edges of the photograph. The stars are distorted in radial direction, this is caused by the curved focal plane of the fast f/6.5 refractor. This error can only be corrected with a field flattener, which is not available for the APO. However, within 75% from the photograph's center this error is smaller than the size of the stars and can therefore be ignored.


Stellar images

Stellar images

This is the bright star Gamma Lyrae, which is situated on the lower left corner of the photograph, within the zone of noticeable field curvature. The rays extending from the star, which were also observable visually, were caused by deformed optics due to mechanical pressure on the glass. The pressure was applied by incorrectly placed pull-screw holes on the lens flange, this error has been corrected in the meanwhile. Note there is no extended blue or violet halo visible around the star, the 3-element fluorite APO is completely color-free for all photographic uses.


Depth and resolution

Detail

To determine the limiting magnitude of the photograph the area around the Ring Nebula is shown above. Note that guiding, which was done by hand, was not perfect as can be seen on the bright stars. Some of the stars visible are labeled with their magnitude taken from the Guide Star Catalog. Measured V magnitudes of these stars taken from other authors are about one magnitude fainter. Note that the limiting magnitude of this photograph exceeds the GSC. The faintest star labeled has magnitude 14.0; however, there are fainter stars visible. The limiting magnitude is about 14.5 by GSC standards or 15.5 according to the magnitudes given by other authors. Remember that this was only a single 18 minutes exposure under slightly light-polluted skies on a 400 ASA (27 DIN) film.

Resolution: Between the 13.0 and the 13.1 magnitude stars near the upper edge of the image above there is still some sky background visible. The distance between these two stars is 23.5 arcseconds. If the stars were at only half or a little less their distance they would still be resolved, so the resolution of this photograph is about 10 arcseconds or 32 microns in the focal plane. Seeing was quite bad during this exposure, so even better resolution might be achieved, especially with composite techniques. The photographic grain of the Fujicolor Superia 400 film is fine enough not to interfere.

Walter Koprolin, 04-26-99


Update February 2000

The 4" APO has seen a lot of use since I wrote this report, being my primary photographic instrument. Of the problems written above only two remain: Field curvature and vignetting.

Field Curvature: There is still no flatfield corrector available for the APM APOs, and stars towards the edges of 35mm format are imaged as radially elongated, broadened streaks, just like in the image above; the combined effect of astigmatism and out-of-focus stars because of field curvature.
Vignetting: Replacing the off-axis guider with a custom-made 2" adapter did reduce, but not eliminate, the obstructed zones towards the corners of the frame. The remaining vignetting is symmetric and most probably caused by the 2" focuser. A 2" opening is slightly too small to allow for an unvignetted 36 x 24 mm imaging area. The vignetting can for the most part be corrected with flat-fielding techniques during image processing.

New measurements of depth and resolution:

Image depth: The limiting stellar magnitude was estimated in a Veil Nebula image photographed in September 1999. It was a 2 x 60 min composite taken on a good observing site, the limiting visual stellar magnitude was 6.0 mag. Seeing was below average, and the film used was Fuji Superia 400 (not very sensitive to red light). The amount of stars visible by far exceeds the Guide Star Catalog (GSC), the faintest stars cataloged in the GSC have magnitude 15.2 and appear quite bright in the image. The limiting stellar magnitude of the composite is estimated to be approximately 17 mag (conservative guess).
Resolution: Two stars of average brightness 31.5 microns apart appear as an elongated figure "8" in the Veil Nebula image, the resolution is therefore 10 arcseconds. The diameter of the faintest (~17 mag) stars is 30 microns or 9.5 arcseconds FWHM. The diameter of average stars (~15 mag) is measured to 40 microns or 12.7 arcseconds FWHM.

Summary: The only major flaw of the APM APO is that no flatfield corrector is available. Otherwise it is a nearly perfect fast and wide-field imaging device for 35mm film. To cover medium format a flatfield corrector and a larger focuser would be necessary. The fluorite APO also provides pleasening visual views of deep-sky objects as well as being well-suited for planetary observations. It is color-corrected for visual and photographic usage, only a hardly detectable purple halo can be seen when viewing very bright objects at high magnifications (>200x).

Walter Koprolin, 02-04-00


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