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.
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.
standard eyepieces are available, a
focal length of the
of sky-objects are photographed with a PC-controlled, 1-stage cooled (
III. Optical aids and Sky-data.
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.
on a telecompressor lens (Meade 0.63x), a Televue Powermate 5x Barlow and a
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
focusing device has been equipped with a variable speed DC focusing motor
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.
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
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.
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.
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.
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.
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)
5. F/41.6 ----------0.5"(arc)
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.
A. Diffraction patterns for Altair at full aperture.
inside focus-----------------------------------------------------------focus------------------------------------------------------------outside focus
results of diffraction rings for the white star Altair (magnitude +0.79) of 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.
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
are the diffraction rings for Altair, obtained from star-testing with the
- Astigmatism has been removed by loosening the retaining ring pressing the rubber O-ring onto the doublet objective in its cell.
secondary spectrum Δf was improved to
-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.
aperture stopped-down testing, star-testing was also performed with the full
aperture of the Synta
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.
are the diffraction rings for a magnitude +2 light-yellow star of the
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.
18 JANUARY 2010