||The Questar Solar Observatory system was developed by a local customer of Company Seven with the cooperation of DayStar Filter Company and of Company Seven. The following is the article
prepared by Mr. Olshausen for publication in the Company Seven "C-VII Journal" in 1990.|
"A TELESCOPE FOR LOVERS OF THE SUN"
Copyright 1990, by Michael Olshausen
Used by permission
The life-giving Sun: it's big, it's bright, it's magnificent. Jupiter-size sunspot groups, formed when the convective
upwelling of hot gasses to the photosphere temporarily becomes retarded by emerging solar magnetic flux tubes, are conspicuous
around Solar Maximum. Flares erupt, visible in H-alpha, the red light of hydrogen, near these same sunspot groups -- termed
"active regions" -- occasionally showering the earth's magnetosphere with astronaut-lethal X-rays and energetic protons.
Magnetic flux tubes loop up into the chromosphere and support enormous quiescent prominences, sometimes 100,000 miles high and
again easily visible in H-alpha. These same flux tubes provide the confined magnetic conduits for the explosive, surge
prominences that erupt from the active regions.
To share these extraordinary phenomena with friends and
students, the author assembled a personal solar telescope
satisfying four criteria: 1) high portability, 2) high precision,
3) full utilization of available aperture to preserve telescopic
resolving power, and 4) an angular field of view sufficiently
wide as to allow the whole solar disk plus prominences to be
viewed at once, at the lowest power. Constraining the telescope's design, but only in H-alpha, was the optical requirement
imposed by all narrow-band, Lyot, or (as here) etalon, interference filters, of a
nearly plane-parallel wave front: requiring, in practice, an
f/30, or slower, optical system. Also constraining the telescope's design was the aperture stop of the interference filter,
and the field stop of the lowest power (32mm) eyepiece.
Questar telescopes easily satisfy criteria 1 and 2, particularly with their new, battery-powered drives. Among the
commercially available H-alpha filters, DayStar's T-Scanner (Tilt Scanner) eliminates the need for an oven (interference
filters are temperature sensitive, varying +1.0 Angstrom over +16.80C), and so, in practice, eliminates the need for an AC
line. The T-Scanner achieves this shortcut by providing a thumbscrew-controlled mechanism for tilting the filter, thereby
compensating for ambient thermal displacements of the central wavelength of the filter's passband (called the "fringe"). You
can actually see the fringe scan across the solar image, as if passing a wand over a magic hat. Observation of H-alpha detail,
principally of filaments, plages, flares, and sunspot magnetic field alignments, over isolated regions of the solar disk is
very pleasing and easily accomplished, despite the approximately
square-wave contrast distribution. True uniformity of contrast
over the disk is possible with a T-Scanner only when the filter
element is squared-on to the incoming, convergent light beam from
the telescope objective. At only one temperature can this occur.
On the other hand, because DayStar's T-Scanners have sub-angstrom
half-bandwidths, the contrast over isolated solar regions is
excellent, regardless of temperature (especially if the telescope's effective focal ratio is increased by stopping the
objective). At additional cost, DayStar may be able to provide extra-narrow, sub-angstrom T-Scanners, for enhanced disk
observers who want to photograph the sun in H-alpha, an oven-
regulated DayStar interference filter would definitely prove
superior to the economical T-Scanner.
Questar with diaphragm/prefilter, DayStar filter body, diagonal on axial port in place
Observatory quality H-alpha interference filters work through compounding the destructive interference of polarized light. In
the case of the far more economical T-Scanner (about 1/50th the cost!), the filter will slightly pass other, narrowly as well
as widely separated wavelengths of light, and so must be preceded by a red, energy rejection prefilter (80% rejected, 20%
transmitted) over the telescope's objective, as well as accompanied by internal blocking filters. Except for f/30 refractors,
or other refractors with specially mounted negative lenses, such prefilters are generally sub-diameter, with a consequent loss
of light and resolving power.
With this in mind, the author asked DayStar to produce a
full-aperture prefilter for his 3.5" Questar. The first full-aperture prefilter specifically built for Questars, this
prefilter has been equipped (by the author) with a full-aperture iris diaphragm from Edmund Scientific Co. With the T-Scanner
linked to the Questar through the Questar swivel for easy viewing, and with all three Questar camera extension tubes placed
between the T-Scanner and Questar's formerly available (and perhaps newly available) Televid star diagonal (see photo), and
further with the iris stopped down to 2.5", the system f-ratio is f/28.3. Disk detail stands out in remarkably good contrast.
Contrast can be enhanced notably by stopping down to 1.25" off-axis (or to f/56.6 with the extension tubes in place!) with,
however, a consequent loss of resolution and light. It should be noted that, to enable Questar's screw-on, camera extension
tubes to be inserted between the T-Scanner and the Televid star diagonal, a custom-threaded male-male coupling first needed to
View of Questar front cell with diaphragm in place
It should further be noted that all centrally obstructed
telescopes (COT's) -- essentially, all non-refractors -- omit
information from the rays best suited to mimicking a planar wave-
front, namely the axial rays. Hence, no full aperture COT can
match the contrast of a refractor or coelostat-fed mirror of like
size and like effective f-ratio.
With the iris fully open to 3.5", the system f-ratio is f/20.2. Although considerable loss of contrast over the disk results
from this lowered f-ratio, the light gains and resolution gains in the prominences, especially at powers above 100, more than
justify the added expense of the full-aperture prefilter.
The author's 3.5" Questar prefilter is as large as the off-axis, sub-diameter prefilters offered for 11" Celestrons. On the
other hand, because the conventional T-Scanner has a 1.25" aperture stop, no 10"-or-larger telescope equipped with a T-Scanner
can both display the whole solar disk without vignetting the telescope's objective and also work at an f-ratio suitable for
good contrast in H-alpha (see the graph and derivation, below). Stopping-down becomes necessary. Of course, custom T-Scanner's
with larger aperture stops could easily overcome this purely geometrical problem, although, presumably, at much greater
Figure 1 demonstrates the vignetting problem in its simplest form. Let D = objective diameter; F = objective focal length; T =
filter housing thickness; and E = distance from rear of housing to eyepiece focal plane. "Stop" is the aperture stop formed by
the filter housing (1.25"). "Disk + 10%" permits prominences around the perimeter of the sun to be viewed simultaneously. The
dashed line is the optical axis. The horizontal, dotted line projects the lower edge of the "disk + 10%" diameter onto the
objective. The angle e defines the unvignetted, maximum input of information from the whole objective onto the edge of the
"Disk + 10%" image, i.e. full resolving power to the edge of the image. The distance 2x (x depends primarily on E) when added
to the size of the "Disk + 10%" image in the focal Plane must be less than 1.25", the
filter-housing aperture stop.
Note: focal length alone determines whether the "Disk + l0X" image can pass through the filter's aperture stop. A focal
length of 10 feet, regardless of aperture size, i.e. regardless of losses of resolution from vignetting, always results in a
solar disk 1.13" across (seasonally averaged).
With dimensions expressed in inches, we have:
x = (T + E) tan e
e = tan-1 [D - 1.10 (1.13/120) F] / 2F (Note: 10 ft. = 120 in.)
Solving for x and substituting into the inequality
2x + 1.10 (l.13/120) F less than or equal to 1.25
results in the following quadratic equation for F, given D.
1.10 (1.13/120) F2 - [1.10 (1.13/120) (T+E) + 1.25] F + D (T+E) = 0
The graph below displays the curve of this equation with E set
equal to value for the Questar Televid star diagonal. 1.25" star
diagonals should all measure about the same.
The point set inside the parabola intersected with the point set inside the f-ratio constraint lines, i.e. the shaded region,
yields full-resolution, full-image views of the sun in, or near, H-alpha. ("Or near," because the convergent, f/20.2 beam
shifts the central passband and significantly widens the half-bandwidth. If one recalls, however, that the passband of Lyot's
famous chronograph at Pic du Midi in 1942 was 1.5 Angstroms wide, and that Lyot's was the best in the world at the time,
DayStar's economical T-Scanner does very well, thank you.) The greater the f-ratio, of course, i.e. for lines with slopes
greater than that for the Questar at f/28.3, the greater will be the H-alpha image contrast. In fact, hands-on experience
suggests that the line at f/20.2 probably represents the lower limit for effective use of the T-Scanner, although one must
note that at this f-ratio the T-Scanner is still highly effective for prominence
observation. One can always suppress the bright red photosphere
in favor of the dimmer prominences by inserting a partial occulting disk at the edge of the eyepiece field stop. Look
particularly for bright, but as yet still "small," prominences: they are unstable and will usually reward your observation by
exploding off the limb.
It should be noted that the above geometrical results apply only to an idealized, simplest case. When internal light baffles
vignette the objective at the solar limb, the aperture is effectively stopped-down already. This is probably the case with
Questars. High-power, center-field views, however, remain unvignetted.
Experiments with a T-Scanner in combination with the full-aperture prefilter confirmed that no conventional combination of
Questar Barlows, used to boost the effective f-ratio, yields satisfactory results. Using the standard, swing-into-position
Questar Barlow, the observer will find that, in the central third of solar disk, H-alpha details are entirely washed out,
according to DayStar. Only by stopping down to 1.25" off-axis may Questar's internal star diagonal be used successfully with
an H-alpha filter. But this use defeats the purpose -- of maintaining full resolution and full light-gathering power,
especially for observation of the relatively faint solar prominences -- of the full-aperture prefilter.
Axial Questar Barlows, of which the author experimented with both the 1.5x and 2x variety, when situated close to the
interference filter produce an annoying, observationally useless, annular fringe. When these axial Barlows are separated from
the interference filter by means of the Questar swivel, the solar image produced is too large to pass through the field stop
of the lowest-power, 32mm, eyepiece. Noticeable degradation of image quality also results.
According to DayStar, a Barlow lens can, in fact, be successfully used with refractors, provided the Barlow is inserted rather
far up into the optical tube; DayStar (and Company Seven) may be able to provide such Barlows.
An important gain in contrast results from using a polarizing filter at the telescope eyepiece. This is because the light
emerging from an H-alpha interference filter is, in practice, elliptically, rather than circularly, polarized. Since filaments
on the solar disk (prominences seen in projection) are only observed because they scatter light, reduction of background glare
by analyzing away the linear polarization component enhances their visibility.
The author recommends applying a small, first-surface, circular mirror (available from Edmund) to the center of the prefilter
to eliminate differential heating of the Questar secondary. He also recommends fully stopping down the telescope at regular
intervals in order to dissipate accumulated heat from the T-Scanner, and so to maintain the filter's central wavelength close
to H-alpha. Over time, this treatment will also retard the aging of the T-Scanner's internal blocking filters, and so will
extend the filter's pre-overhaul lifetime.
One of the lovely features, incidentally, of DayStar's T-Scanner is its ability to scan on either side of H-alpha, at 6562.8
Angstroms, and thus to allow observation of Doppler shifted detail in filaments and prominences. For amateur astronomers,
there is probably no better, nor more dramatic, way of demonstrating the Doppler shift of light, a sure give-away that the
emitting prominence material has a positive or negative radial velocity.
Happy solar observing!