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Finder Technical Information

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#1 JimK



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Posted 08 November 2012 - 05:54 PM

I have been curious about the performance of the built-in Questar finder, and looked at some tabular information in the manual (see attached excerpt). However, it did not seem to be right, and when using some standard formulas:

ExitPupil = ObjDiam * EpFL / ObjFL = EpFL / f#

I calculated an f# of 14.6 and an objective diameter of 7.33mm (0.29 inch) for the Questar finder, which is not accurate. So I made my own measurements.

A micrometer gave me a finder objective diameter of about 0.622 inch (15.8mm). This aperture seems to be unrestricted, as confirmed by the approximately equal-sized light circle in front of the objective using a Barlowed flashlight in the eyepiece holder.

Then I used a Celestron 12.5mm Microguide eyepiece to time the drift of a star across the field of view for calculating the finder focal length. I had to unscrew the eyepiece holder on the Questar and also rig a shorter barrel on the Microguide eyepiece for obtaining focus, but for this effort it worked well enough. The eyepiece has a 6mm linear scale on its reticle, so the focal length formula (per the eyepiece instructions) is:

FL = 82506 / [time(in sec for 6mm) x cos(star dec)]

I choose the double star Zeta Aquarius, which is almost on the celestial equator (making the cosine part of the equation = 1). It's also a nice binary star (magnitude 4.3/4.5, with a 2.0" separation and an almost N-S position angle) that makes it easy to identify in the telescope mode. I used an electronic stopwatch (actually, a cell phone "feature") to take 3 drift measurements, averaging for a decent time value of 794.44 sec (13 min, 14.44 sec). This evaluates to a finder focal length of 104mm (4.1 inches) for this particular Questar, which is approximately in agreement with the tabulation in the manual (107mm, 4.2 inches).

Using the measured finder aperture and focal length, the f# is about 6.58 for this small refractor finder in the Questar control box. Somewhere I have read that it uses a doublet lens.

What this analysis means is that the exit pupils identified in the manual are too small. For this particular Questar, the exit pupils are: 4.86mm (not 2.19) for the 32mm Brandon, 3.65mm (not 1.64) for the 24mm, 2.43mm (not 1.10) for the 16mm, 1.82mm (not 0.82) for the 12mm, and 1.22mm (not 0.55) for the 8mm. The magnifications and calculated (true) field of view are close enough for my purposes, but if one wishes to recalculate them please note that the 24mm Brandon has about a 53 deg apparent field of view (the 32mm probably has about 48 deg, but I have not used one), compared to the approximately 45 degree view in the others (the tabulation in the manual uses 45 degrees for all). In fact, I readily saw the "steering wheel" around Zeta Aqr (Gamma, Eta, and Pi Aqr) using the 12mm Brandon, roughly confirming the 5+ degree calculated (true) field of view. For calculations, I measured the field stop diameters as 0.850"/21.6mm (for the 24mm), 0.465"/11.8mm (for the 16mm), and 0.345"/8.76mm (for the 12mm) Brandon eyepieces.

With some eyepiece field stops wider than the finder aperture (such as the 24mm Brandon), and with such a short focal length (4.1"/104mm), it is understandable as to why the finder view is degraded (field curvature, vignetting, etc.) around the edges when using certain eyepieces.

Also, using a very simplistic yet empirical formula from Sky and Telescope, Jun'73, p 401:

telescopic limiting magnitude (TLM) = 9.7 + 5*log(ObjDiamInches)

the Questar finder TLM = ~8.7

In a numerical way, this also provides additional justification for my use of T. Taki's 8.5 Magnitude Star Atlas (available here, for free), as I discussed in another thread on CN.

In summary, the Questar finder is like an f/6.6, 4x16 or 7x16 monocular, having a typical light grasp of about 5 times that of an eyeball (based on the ratio of optical areas), and should be able to see 8.7 magnitude stars. Very interesting...
As an additional note, I sometimes attach a Rigel Quikfinder with rubber bands and a felt-covered baseplate to the dew shield for quickly positioning the Questar to the general area of interest because I like to starhop instead of using the setting circles.

Attached Files

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