The instrumented 120mm-
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The
main optical instrument at the "Kolkert-Observatory" is an
instrumented "go to"-apochromatic
refractor. The following pictures give a good impression of the telescope
in its present state.
I. General/Original.
The
original outfit of the 120mm Synta-achromate OTA
consists of a Fraunhofer doublet objective mounted in
an adjustable lens cell being part of a baffled aluminium tube. The 2"
focuser is of the rack-and pinion type and contains an
aluminium draw-tube. This OTA is mounted on the CG-4 equatorial mount of my
other refractor, the C102-HD.
Two
standard eyepieces are available, a
The
focal length of the
II. Astrophotography.
Images
of sky-objects are photographed with a PC-controlled, 1-stage cooled (
III.
Optical aids and Sky-data.
Optical
options have been extended by replacing the 6x30 finder scope with a 9x50 one
and fitting a 0.1 lux CCD-videocamera
at prime focus to it. This greatly enhances finding and focusing objects up to
Magnitude 7. The
The
real-time image of this combination is depicted on a TV-monitor on which screen
a centering cross has been drawn. The circular FOV of
this combination is 2.4° and is, together with the square FOV of the
single-shot CCD-astrocamera projected at the Earth Centered Universe(ECU)-screen,
showing the relevant part of the sky. The centers of
both FOV's are made to coincide at the center cross of the TV monitor and the center
of the CCD-chip of the astrocamera. This simplifies
finding and centering sky-objects to a great extent.
General information on sky-objects is retrieved from the well-documented
REDSHIFT-3 code.
Further
on a telecompressor lens (Meade 0.63x), a Televue Powermate 5x Barlow and a
IV.
Modifications.
A.
Electromechanical.
The
CG-4 mount, of which the bearings and drives have been re-adjusted to
satisfaction, has been upgraded with:
1. an autonomous controlled stepper motor for the right
ascension axis (RA), max. 8x sidereal speed, and
The OTA
focusing device has been equipped with a variable speed DC focusing motor (JIM's
The
slew rates of the motorized mount are modest(2°/minute
max.), but suffice at the moment. Slewing to an object chosen in the real time
planetarium and telescope control program ECU, gives the following pointing
accuracy: s RA= ± 3s and s DEC= ± 49"(arc).
The
backlash in the two drives of the mount for RA- and DEC rotation at reversal is
6.6s and 51"(arc), respectively. Hysteresis is
well within these values.
The rack-and-pinion
focuser has been exchanged with a dual speed (1:10) Crayford one from Williams
Optics. It has been adapted for motorized drive and digital position read-out.
Focusing
on an object is done automatically with the motorized Crayford device. The resolution
of the DFRO ( see below), and so the accuracy to
determine the focus plane with this setup is 0.87°rotation of the
axis of the focuser, which equals
All
accessories to be used at the focuser draw-tube end are fitted with three
setting screws to facilitate alignment of the optical train, for which a
B. Optical conversion from achromate to apochromate; the
CHROMACORR.
A newly
developed item which is a leap-ahead in the reduction of the secondary spectrum
and spherical aberration, i.e. the 150mm achro-apo
converter or Chromacorr, developed at ARIES Instruments,
is also installed. This three-lens system has an over-correction in spherical
aberration of +1/7λ and was chosen as
the doublet objective of the
Both
for the original set-up and for the optical train extended with the Chromacorr, results of star-testing, Ronchi-testing
and secondary spectrum determination up to F/42 are reported here.
First
results of star images are also presented here. For the white star Altair,
images (3 sec. exposure) were made at F/8.3 up to F/42 with and without the
Chromacorr-O1 installed. Optimal collimation is mandatory in these
test and asks for some praxis.
V. Automatization.
A.
Digital Setting Circles and Focus Read-Out.
The
motorized items are connected to digital setting circles (DSC's) and to a digital
focus read out (DFRO) for which encoders have been connected directly to the
different rotation axes of the equatorial mount and to the focusing device.
These systems are home-made. Digitizing of the different encoder signals occurs
with the PCB of a modified PS2-mouse-now called the "Astro-mouse"-and
are converted into discrete counts with the software
program "encoder" which runs on an old 364 PC. With a laplink, that PC is connected to the main PC which runs the
real time planetarium and telescope control program Earth Centered
Universe (ECU). The counts of the DSC's are transformed into RA-and DEC-values
which are presented both as a (moving) cursor and a numerical window on the
PC/ECU-screen which depicts the area of the sky the telescope is pointing at.
The position of the focus plane is read-out on the 364 PC as integers with a
respective sign for inter- and intra-focus.
B. Telescope
control.
1. RA-
and DEC-motion.
Actuation
of both the DC DEC-motor and RA-stepper motor is being executed via the
STAR2000 interface module in combination with the accompanying Relay Box. The
speed of the DEC-motor can be continuously controlled manually, while their are
next to sidereal rate, two(2) speed options for the
RA-motor; 2x and 8x sidereal speed in both East- and West-direction. The lower
speeds are used for self-guiding and tracking of a chosen celestial object in
the FOV of the telescope, while the higher speeds are used for slewing
purposes.
The
STAR2000 interface module from Starlight-Xpress is
operated by either the STAR2000 software (telescope control panel) of the MX5-C
astrocamera or by the telescope control window (move
panel) of ASTROART2.0.
Finding
an object, slewing to an object and centering it is performed by combining the telescope control panel window
with the real-time local sky-chart of ECU. Choosing an object in ECU gives its
co-ordinates and deviation of the present position the telescope is pointing
at. The latter information is coming from the DSC’s on the equatorial
mount and their "Astro-mouse" interface
with ECU. By activating the relevant buttons in the control/move panel, the
deviations are zeroed and the chosen object arrives near or at the optical axis
of the telescope as observed in the continuously updated image field of the astrocamera MX5-C. For the latter situation
, the camera operates in the focus-mode under ASTROART2.0. At that
moment the position of the optical axis of the telescope and the position of
the chosen object are synchronized and centered in
ECU.
-
-
--
2. Focusing.
Focusing
is performed with the motorized Crayford focuser and the DFRO. The focus-motor
is actuated via the North- and South-buttons of the move-panel of the telescope
control window in STAR2000 and the result is real-time observed in the
regularly updated image field of the "focus"-window in ASTROART2.0.
--
--
VI.
Numerical values of the set-up.
A. FOV (Field of View).
The
calculated FOV of the ICX 055CK chip of the Starlight Express CCD-camera with
chip-dimensions of 4.9(horizontal) x 3.6(vertical) mm is for different
F-numbers:
1. F/5,3 (0.63 telecompressor),--- -f= 630 mm-------26.8' x 19.7' (arc)
2.
F/8.3 (standard),------------------- -f= 1000 mm-----16.9' x 12.4' (arc)
3.
F/16.6 (one Barlow, 2x),---------- f= 2000 mm------ 8.4'
x 6.2' (arc)
4.
F/33.2 (two Barlow's, each 2x),--f= 4000 mm--- ---4.2' x 3.1' (arc)
5.
F/41.6 (Televue Powermate
5x), f= 5000 mm-------3.4' x 2.5' (arc)
For
instance for Jupiter, this leads to a 32% PC-screen occupancy at F/42.
For the
sky-occupancy per pixel in the vertical direction of the chip of this camera,
which has 145.000 pixels (500 hor. x 290 vert.), the following values are obtained:
1.
F/5.3----------- -4.1"(arc)
2.
F/8.3---------- --2.6"(arc)
3.
F/16.6-----------1.3"(arc)
4.
F/33.2-----------0.6"(arc)
5.
F/41.6 ----------0.5"(arc)
B. Optical Resolution.
The
power to resolve individual point sources at optical wavelengths from each
other for this optical system in combination with the camera mentioned, has
been deduced, too. The dimensions of the pixels are 9.8 x 12.6 mm. For the average pixel size
of 11.2 mm and at the
maximum response wavelength (l = 0.56 mm) of the chip, the resolution per pixel at the
different F-values is;
1. resolution per pixel at F/8.3 ------------2.31" (arc)
2. resolution per pixel at F/16.6-----------1.15" (arc)
3. resolution per pixel at F/33.2-----------0.58" (arc)
4. resolution per pixel at F/41.6----------
0.46" (arc)
The
value under 4 is the so-called Nyquist-criterion for
correctly sampling the image of the observed object, for which the Airy-disk
diameter is 0.56 x F-value (in mm). Then good results are obtained with
image-processing.
As it
appears from experimentation, the chromatic aberration at F/42 is negligible
(secondary spectrum is about 0.00006 times the focal length f).
The
optimal F-number for observing planets for this system is F/40 and for deep-sky
objects it is F/20. The magnification at F/42 as seen on my PC-screen, is 790x.
STAR-TESTING of the modified
I. Without the Chromacorr installed.
A. Diffraction
patterns for Altair at full aperture.
inside focus-----------------------------------------------------------focus------------------------------------------------------------outside
focus
These
results of diffraction rings for the white star Altair (magnitude +0.79) of the
B. Summary of results.
-The
secondary spectrum (i.e. difference in focus position D f between yellowish and
purple image) at F/8.3 is
-Comparing
diffraction patterns with results of simulations (Aberration code) leads to the
conclusion that the spherical aberration of the doublet objective without Chromacorr is –0.14 l .
-Some
astigmatism and an S-zone at 30%-radius is present.
-Ronchi-testing showed straight, sharp and parallel lines
and so is to unsensitive for deducing aberrations in
this case.
II. With the Chromacorr
O-1 installed.
A. Diffraction
patterns for Altair.
1. Stopped-down aperture.
After
collimating the two elements of the objective with the optical axis of the
OTA-tube with the help of the adjustable lens cell and the
These
are the diffraction rings for Altair, obtained from star-testing with the
modified
-Summary of results.
-
Astigmatism has been removed by loosening the retaining ring pressing the
rubber O-ring onto the doublet objective in its cell.
-The
secondary spectrum Δf was improved to
less than
-It is
noticed that Δf/f=0.000059,
indicating that this 120mm OTA plus Chromacorr O-1
behaves as an excellent apochromate!
-In the
stopped-down mode, not much can be seen of the S-zone. So it's
amplitude must be smaller than 1/8λ.
-Ronchi-testing showed sharp, straight and parallel lines
all over the field of view.
2. Full aperture.
Besides
aperture stopped-down testing, star-testing was also performed with the full
aperture of the Synta
The
images above show the diffraction rings for Altair as recorded with the MX-
The
effects of the S-zone are still visible. Its effect on images of planets will
determine how severe this zoning is. Again
the magnification was about
158x.
B. Another star.
These
are the diffraction rings for a magnitude +2 light-yellow star of the
C. Effect of Vixen Deluxe
Barlow-lenses on optical performance in star-testing.
These
are the diffraction rings for Altair of the
A minor
effect of the S-zone can be seen in these images while some optical pinching of
to tightly-fastened set-screws of the Barlow-lenses can be seen, too.
The
magnification is 632x.
D. The effect of a Televue Powermate 5x Barlow on
optical performance.
These are the diffraction
rings for Rigel, a magnitude 0.18 blue star, of the 120
mm Synta doublet refractor
stopped-down to 50 mm .
The telescope was equipped with the Chromacorr-O1 and the Televue
powermate 5x. The images
were taken with the MX5-C astrocamera including an
IR-filter.
The atmosphere was rather
steady and the magnification was 790x.
The magnitude of the
S-zone (smaller than 1/8λ) is too small to be of importance for image degradation as can be seen
in the image of Saturnus(see under III).
These multi-shot composed
pictures were taken with the 120
mm Synta doublet refractor,
equipped with the Chromacorr-O1 and the Televue powermate 5x. The magnification
was 790x, while the outdoor temperature was -5° C.
IV. Mars.
This colour-converted
single-shot picture was taken at 23.46 hour on 4 September 2003 with the MX5-C
camera. The 120 mm
Synta doublet refractor was equipped with
theChromacorr-O1 and Televue Powermate
5x.
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27
September 2008