The instrumented 120mm-APO Refractor.


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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.

Besides a description of the technical specifications of the instrumented 120mm-APO, results are presented of diffraction patterns (ie. star-tests) recorded with both the original achromatic and the modified apochromatic telescope. Furthermore pictures of planets are shown which were made with the apochromatic set-up. 

- Crayford focuser.JPG- --

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Technical specifications of the APO-modified 120mm refractor of Synta.

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 7.6 mm Plössl, series 3000 of Meade and a 20 mm one of Celestron. A Celestron polar axis finder scope with cross wire illumination is available, too. The visual finder telescope is a 6x30 Celestron type with cross wire.

The focal length of the 120 mm achromat is standard 1000 mm ( F/8.3), the light gathering power is 293, the max. magnitude at which objects can be observed is 13.1 and the optical resolution (Dawes limit) is 0.96" (arc).

II. Astrophotography.

Images of sky-objects are photographed with a PC-controlled, 1-stage cooled (-30°C) CCD-astrocamera, the 12 bits Starlight-Xpress MX5-C, which makes it possible to record repetitively single-shot multi-colour pictures. Monitoring and control is performed with the program STAR2000/MX5-C and/or ASTROART2.0 with the MX5-C plug-in. Raw images of exposures are colour-converted to LRGB images and processed with ASTROART, too.

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 0.33 inch B/W sensor of the 12V camera has 725(H)x528(V) pixels, gives 380 TV lines in horizontal and 450 lines in vertical direction, has an automatic iris and shutter speed (1/50 to 1/32000), has a 2:1 interlace scanning system and a S/N ratio better than 50dB.

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 12 mm Kellner eyepiece with illuminated double cross wire, mounted in a Flip Mirror Finder (True Technologies) equipped with an IR-filter, have been added. For testing optimized visual focusing and determination of spherical aberration of the optical train, a dummy eyepiece with a 7 lines per mm Ronchi grating is available. Maximum F-number to be obtained with the Televue Powermate is F/41.6 at a focal length of 5000 mm.

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

2. a variable speed DC declination motor (JIM's Mobile) to set/control the declination.

The OTA focusing device has been equipped with a variable speed DC focusing motor (JIM's Mobile) to focus the image plane and a digital focus read-out (DFRO) to determine its position.

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 0.059 mm axial displacement. At a 1/4 l defocusing aberration, the depth of focus is allowed to be within +1/8 and -1/8l. Then, delta-focus runs from 0.0597 mm for F/8.3 at l =0.43µm to 2.284 mm for F/42 at l =0.66µm, respectively. So, for the F/42-set-up and a resolution of ± 0.059 mm axial displacement for one count of the DFRO (see below), the defocusing aberration should be better than 1/64 l

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 Cheshire eyepiece and a TL-light source are used.

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 120 mm shows an under-correction of about –1/7λ. It is the Chromacorr-O1 and it is threaded in the forward-end of a 2-inch outer diameter extension tube with such a length that the distance in between the focal stop of an ocular or Barlow and the thread of the Chromacorr can be set from 190 mm up to 225 mm. This assembly is now centered in the draw-tube with the help of a compression ring. The Chromacorr extension tube centered in the draw-tube of the Crayford focuser and the 9x50 finder scope is shown below.

Oom Wim, 120 mm Tel


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.


Aaandrijving en DRO van Crayford Focuser           

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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.

-Oom Wim, 120 mm Tel---

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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.


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C. Tracking and Guiding.

During tracking or self-guiding of an object, a real time ECU-window shows the possible deviation in RA- and DEC-values of the actual telescope position (i.e. pointing position of it's optical axis) from the coordinates of the chosen object. Corrections can be performed both manually by actuating the hand controllers of the RA- and/or DEC motors and automatically by using the "guide"-option of ASTROART. In that case an image window shows the initial and actual position of the guide object and the STAR2000-protocol derives correction commands from the deviations of the actual position and actuates the motors to correct the position. The guiding information is updated every 0.2 seconds or at longer intervals. Control is within ±1 pixel (about 0.6"at F/40) of the CCD-chip of the astrocamera for an indefinitely period of time.

For long exposures, accurate control of the position of the object to be studied on the CCD-chip of the astrocamera is also done with the automatic self-guide option in ASTROART2.0. Control can be obtained to the 1 pixel level for an adequately picked guide-star in the FOV of the camera, which suppresses trailing on the image, completely.


Raw MX5-C images at F/40 of Upsilon-29. Left: 15 minutes exposure without self-guide. Right: 15 minutes exposure with ASTROART2.0 automatic self-guide.

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.

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STAR-TESTING of the modified 120 mm APO-refractor.

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 120 mm doublet objective (f=1000 mm) of the Synta refractor (as bought) were obtained with the Starlight MX-5C astrocamera equipped with an IR-filter. The magnification was about 158 x.

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 .62 mm; so D f/f 0.00062 for this optical system. Quality-Fraunhofer achromatic doublets have a corresponding value of about 0.0005.

-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 Cheshire eyepiece, the optical train involving the Chromacorr O-1 and the doublet objective was aligned.

These are the diffraction rings for Altair, obtained from star-testing with the modified 120 mm Synta doublet refractor stopped-down to a 50 mm aperture and equipped with the Chromacorr-O1 plus self-centering draw-tube adapter. The images were recorded with the MX-5C astrocamera plus IR-filter. The atmosphere was rather steady during these tests.

-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 0.059 mm (the resolution of the Digital Focus Read-Out, DFRO) and the spherical undercorrection was estimated to be less than 1/42l. Both parameters have been improved substantially by using the Chromacorr-O1! The magnification was 158x.

-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 120 mm refractor equipped with the Chromacorr O-1.

The images above show the diffraction rings for Altair as recorded with the MX-5C astrocamera.

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 120 mm Synta doublet refractor equipped with the Chromacorr-O1. The images were recorded with the MX-5C astrocamera. The atmosphere was rather steady and the magnification was about 158x.


C. Effect of Vixen Deluxe Barlow-lenses on optical performance in star-testing.


These are the diffraction rings for Altair of the 120 mm Synta doublet refractor plus Chromacorr-O1 and two Vixen Deluxe Barlow’s (two times each) as recorded with the MX-5C astrocamera. The Chromacorr was not optimally aligned, i.e. there is a little lateral red-blue shift.

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).

III. Saturnus and Jupiter.




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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18 JANUARY 2010