A STAR-TRAIL TELESCOPE FOR ASTRONOMICAL SITE-TESTING - E.A. HARLAN AND MERLE F. WALKER

Sep 15, 2026

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A method for evaluating astronomical “seeing” at remote telescope sites is developed around a portable, simple instrument capable of objective image-quality estimates. The paper outlines physical requirements for field testing and motivates distinguishing good (1"–2") from excellent (≤1/2") seeing. It then investigates extracting seeing information from photographic star trails using a 6.5-inch refractor and correlating results with observations from a 120-inch reflector.

LanguageEnglish

CategoryResearch & Report

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Pages9

Reading Time23 min

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A STAR-TRAIL TELESCOPE FOR ASTRONOMICAL SITE-TESTING

STAR-TRAIL TELESCOPE

E.A. HARLAN AND MERLE F.WALKER

1965PASP***77**246H # A STAR-TRAIL TELESCOPE FOR ASTRONOMICAL SITE-TESTING* E.A.HARLAN AND MERLE F.WALKERLick Observatory,University of CaliforniaReceived May 30,1965 A number of groups are now planning to construct new largetelescopes.This fact lends considerable importance to the de-velopment of suitable test instruments that can be used at remotesites to determine what the appearance of a star image in a largetelescope would be.The physical requirements for such an instru-ment include: 1)Portability;the instrument should be as small and light-weight as possible to permit transporting it to sites inac-cessible by road. 2)Simple and rugged construction,which will permit trans-portation over rough terrain without danger of changingits calibration. 3)Rigid mounting,which will permit operating in the windin an exposed location without the necessity of construct-ing a dome or other windbreak. 4)Simplicity of operation;the instrument should be ex-tremely simple to operate and maintain so that personswith little training can be employed to make the observa-tions. 5)Minimal power requirements. Naturally,the instrument must be capable of providing anobjective estimate of the image quality or "seeing"independentof the skill or personal equation of the observer.Furthermore,even for use in only a preliminary survey of possible sites,itmust be capable of distinguishing between “good”seeing wherethe image diameter in a large telescope is 1"to 2",and “excel-lent”seeing,where the image diameter is 1/2"or less. One such device is the electronic seeing monitor developed by 1965PASP***77**246H ## STAR-TRAIL TELESCOPE Babcock¹and modified by Lynds²and Babcock.³The successfultests of this system by Babcock are of great importance sincethey demonstrate that at least under some conditions a telescopeof six-inch aperture mounted at ground level is capable of pro-viding information that can be interpreted in terms of the seeingor image size with a large(60-or 100-inch)telescope mountedperhaps 50 feet above the ground.1 The system has the disadvan-tage of requiring electrical power at the site and of employingoptical and electronic components whose adjustment and main-tenance might prove troublesome under field conditions.It ap-peared to be of interest,therefore,to inquire whether the seeinginformation from a six-inch telescope can be extracted in asimpler manner. One way of recording the information is simply that of photo-graphing star trails,magnifying the image to the point whereresolution is set by the seeing or,under the best atmosphericconditions,by the diffraction disk of the star.Star trails have,inthe past,been used by various investigators to study microseismsand the vibration of telescope mountings,⁴errors in telescopedrives,and the nature of the disturbances producing the atmos-pheric turbulence.6,7,8 The first suggestion of the use of star trailsto estimate seeing appears to have been made by King,9 althoughthis use is implicit in the work of Schlesinger.6 The technique ismentioned by Stock and Keller,10 but no details are given.Meinelhas published a design for a double-beam star-trail telescope,11but apparently this has never been constructed.In no case,sofar as we are aware,has a comparison been reported of the ap-pearance of the star trails obtained with a small telescope as afunction of the image size as seen in a large telescope. In order to investigate the use of star trails as seeing indica-tors,a visually corrected refractor objective of 61/2-inch aper-ture and 7534-inch focal length,made by Brashear,was mountedin an aluminum tube.For simplicity,and in keeping with thephysical requirements specified in the first paragraph,it wasdecided to use a fixed mounting for this telescope and to observethe star trails of Polaris to determine to what extent observationsof this one star could be used to determine the seeing conditionsat a site.A mounting,shown in Plate I,was constructed of con- 1965PASP***77**

FAQs

Why is suitable testing equipment important for planning large telescopes?

New large telescopes require instruments that can be used at remote sites to determine how a star image will appear, making site-dependent image quality measurable in advance.

What physical requirements must a field seeing-testing instrument satisfy?

It should be portable, simple and rugged, rigidly mounted for windy exposed operation, extremely simple to operate and maintain, and have minimal power requirements while providing objective seeing estimates.

How does the document attempt to estimate seeing using star trails?

It photographs the trails of Polaris with a fixed small telescope, then correlates trail appearance with seeing measured simultaneously at a larger reflector, noting that direct numeric judgments by observers may be insufficiently precise.

Sep 15, 2026

Uploaded by Cipher