QC 351 A7 no. 38 / A brief description is given of the various types of image tubes
presently used in astronomical research and a review is presented of the
past applications of image tubes to direct astronomical photography. A
detailed laboratory evaluation of the Carnegie image tube is summarized
and photographs at the telescope are presented to confirm and extend the
results obtained in the laboratory.
Iris photometry of stellar images can be carried out on Carnegie
tube photographs with about the same accuracy as is obtained by normal
photographic techniques. Compared to unaided plates the image tube typically requires about 1/15 the exposure time to record stellar images of
a specified threshold magnitude. When exposures are made to near the sky
limit, however, the Carnegie tube cannot record stars as faint as can be
recorded with an unaided plate. When exposed at a given focal length telescope, the limiting magnitude of an image tube record is about 1 magnitude brighter than that of an unaided photograph.
Primarily two characteristics of the Carnegie tube, an over-all
mottled sensitivity pattern and a light- induced background, are found to
be responsible for the loss in limiting magnitude of a Carnegie tube record. The mottle pattern is characterized by an rms variation in sensitivity of ±1.3 percent. It modulates the photographic record of the
night-sky radiation and seriously affects the signal -to -noise ratio of the threshold images. The additional background produced by the light- induced
background of the image tube generally amounts to 25 percent of the night-sky radiation on a sky-limited photograph.
In order to record the same sky-limited magnitude on a Carnegie
tube plate and an unaided plate, the image tube record must be exposed
at a longer focal length telescope. The exposure time required by the
image tube is then about 1/2 to 1/3 that of the unaided plate. Because
of the higher scale of the image tube photograph in such a case, however,
the effective gain provided by the image tube over the unaided plate is
generally somewhat larger than the relative exposure time.
The photography of extended objects is found to be particularly
affected by the nonuniformities of the image tube. Besides reducing the
over-all signal-to-noise ratio of the image tube record, the generalmottle
pattern and additional discrete patches and ripples in sensitivity of the
image tube tend to mimic low contrast features of galaxies and nebulae.
The rather subjective effects of the nonuniformities can be significantly
reduced by using telescopes with moderately long focal lengths, so that
the seeing image is then large in comparison to the nonuniformities.
The photography of astronomical sources through narrowband interference filters has been found to be a particularly promising application
of the Carnegie image tube. Preliminary tests reported in the present
study include the photography of supernova remnants, planetary nebulae,
galaxies, and reflection nebulae.
The basic quality criterion for comparing the image tube to unaided photographic emulsions is argued to be the detective quantum efficiency. Typical values of the gain over unaided emulsions provided by the Carnegie tube are calculated to be in the range 10 to 20. It is emphasized, however, that because of the variety of requirements in specific research areas and because of the several unique characteristics
of a given image tube, no single figure of merit may be defined that will
predict the usefulness of an image tube in all applications. It is suggested that the resolution of a detector should not generally be combined
into the calculation of a single figure of merit but should be considered
as a separate quality criterion.
Certain problems with the Carnegie tube (and other image tubes
as well) potentially limit its usefulness in specific research areas.
Besides the problems already mentioned, other problems include low resolution, geometrical distortion, the complexities of analyzing the final
record (as compared to an unaided photograph), and the limited field of
the image tube. Each of these characteristics can be highly significant
or entirely inconsequential in different applications.
| Identifer | oai:union.ndltd.org:arizona.edu/oai:arizona.openrepository.com:10150/621632 |
| Date | 25 April 1969 |
| Creators | Cromwell, R. H. |
| Publisher | Optical Sciences Center, University of Arizona (Tucson, Arizona) |
| Source Sets | University of Arizona |
| Language | en_US |
| Detected Language | English |
| Type | Technical Report |
| Rights | Copyright © Arizona Board of Regents |
| Relation | Optical Sciences Technical Report 38 |
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