The instrumented Refractor C102-APO.

 

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The main optical instrument at the "Kolkert-Observatory" is an instrumented "goto"-apochromatic refractor. The following pictures give a good impression of the telescope in its present state.

Besides a describtion of the technical specifications of the instrumented C102-APO, results are presented of diffraction patterns (ie. star-tests) recorded with the modified apochromatic telescope. Moreover pictures of telescope focusing experiments in the achromatic modus on a bright star which stands low above the horizon are shown, too.

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Technical specifications of the APO-modified C102-HD refractor of Celestron.

I. General/Original.

The original outfit of the C102-HD achromate with equatorial mount (CG-4) consists of two eyepieces, a 7.6 mm Plössl, series 3000 of Meade and a 20 mm of Celestron, and a Celestron polar axis finder scope with cross wire illumination. The visual finder telescope is a 6x30 Celestron with a cross wire.

The focal lenght of the 102 mm achromat is standard 1000 mm ( F/9.8), the light gathering power is 212, the max. magnitude at which objects can be observed is 12.5, the FOV= 1° with the 20mm oculair, and the optical resolution (Dawes limit) is 1.14" (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 multicolour pictures. Monitoring and control is performed with the program STAR2000/MX5-C 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), two apochromatic Barlow's (Vixen Deluxe, 2x) and a 12 mm Kellner eyepiece with illuminated double crosswire, mounted in a Flip Mirror Finder (True Technologies) equiped with an IR-filter, have been added. For testing optimized visual focusing, a dummy eyepiece with a 7 lines per mm Ronchi grating is available.

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,

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

3. a variable speed DC focusing motor (JIM's Mobile) to focus the image plane.

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 focuser has beeen modified, too. The upper silicone strip has been reinforced with a metal backing, while longer set-screws have been inserted in the focuser body.

Focusing on an object is done automatically with a motorized rack and pinion 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 0.09 mm for F/10 at l =0.43µm to 2.03 mm for F/40 at l =0.66µm, respectively. So, for the F/40-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 1/32 l

All accessories to be used at the focuser draw-tube are fitted with three setting screws to facilitate alignment of the optical train, for which a Cheshire eyepiece and a TL-lightsource are used.

B. Optical conversion from achromate to apochromate; the CHROMACORR.

A newly developed item which promises a leap ahead in the reduction of the secondary spectrum, ie. the 150mm achro-apo converter - Chromacorr-developed at ARIES Instruments, is installed now. This three-lens system has an over-correction in spherical aberration of 1/7l and was chosen as the doublet objective of the C102-HD shows an under-correction of about –1/5l. 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 oculair or Barlow and the thread of the Chromacorr can be set from 190 mm up to 225 mm. The extension tube with two Barlow’s and Flip Mirror Finder is shown below.

 

For the optical train extended with the Chromacorr results of star-testing, Ronchi-testing and secondary spectrum determination up to F/40 are reported here.
First results of star images are also presented here. For the white double star Mizar in Ursa Major, images (3 sec. exposure) were made at F/39.2 with and without the Chromacorr-O1 installed. Optimal collimation is mandatory and was not yet achieved here.

 

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------------Without Chromacorr..---------------------------------------------------------With Chromacorr.

 

V. Automatization.

A. Digital Setting Circles and Focus Read-Out.

The motorized items are connected to digital setting circles (DSC's) and a digital focus read out (DFRO) for which encoders have been connected directly to the different rotation axes of the equatorial mount and 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.

 

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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. Chosing 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.

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2. Focusing.

Focusing is performed with the motorized rack-and-pinion 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 (ie. 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 camera 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 , 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/6.2 (0.63 telecompressor),----f= 630 mm------26.8' x 19.7' (arc)

2. F/9.8 (standard),------------------- f= 1000 mm-----16.9' x 12.4' (arc)

3. F/19.6 (one Barlow, 2x),----------f= 2000 mm------ 8.4' x 6.2' (arc)

4. F/39.2 (two Barlow's, each 2x), f= 4000 mm--- ---4.2' x 3.1' (arc)

For instance for Jupiter, this leads to a 11% PC-screen occupancy at F/20. That is 22% at F/40.

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/6.2------------4.1"(arc)

2. F/9.8------------2.6"(arc)

3. F/19.6-----------1.3"(arc)

4. F/39.2-----------0.6"(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 wavelenght (l = 0.56 mm) of the chip, the resolution per pixel at the different F-values is;

1. resolution per pixel at F/9.8------------2.3" (arc)

2. resolution per pixel at F/19.6-----------1.2" (arc)

3. resolution per pixel at F/39.2-----------0.6" (arc)

The value under 3. 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/40 is small (secondary spectrum is about 0.00034 times the focal lenght f, and the purple image blur is about one and a half times the diameter of the Airy-disk)!

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/40 as seen on my PC-screen, is 644x.

VII. Future projects.

A routine will be written to superimpose a window, depicting the actual position of the focus-plane of the refractor, on the ECU-screen.

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STAR-TESTING of the modified apochromate refractor C102-APO.

I. Without the Chromacorr installed.

A. Pictures of a star

Earlier star-testing at F/40 of the instrumented C102-HD achromate refractor without the Chromacorr-O1 delivered many pictures of Capella; a bright red star. A choice of images is presented here for situations inside-, outside- and in-focus.

  Capella; just inside focus, 5s exposure, 13 June 2000.

  Capella; just inside focus

  Capella; almost in focus.

  Capella; in-focus.

  Capella; almost outside focus.

  Capella; just outside focus.

  Capella; just outside focus.

B. Summary.

-The complete set of transitions as shown above took place within 1.0 mm displacement of the rack and pinion focuser. So, defocusing aberration in this series of experiments is better than 1/8l.

-The secondary spectrum (ie. difference in focus position D f between yellowish and purple image) at F/40 lies in between 1.18 and 1.53 mm; so 0.00030 <D f/f < 0.00038 for this optical system. Quality-Fraunhofer achromatic doublets have a corresponding value of about 0.0005. The longitudinal chromatic aberration is + 0.34 mm and the lateral aberration is estimated to be -0.5.

-Comparing diffraction patterns with results of simulations (Aberration code) leads to the conclusion that the spherical aberration of the doublet objective without Chromacorr is –0.18 l .

-Astigmatism and coma are absent.

-Ronchi-testing showed a spherical under-correction of about –1/5 l and some turned edge effect of the objective.

II. With the Chromacorr installed.

A. Diffraction patterns of Sirius.

After optimizing the orientation of both lenses of the doublet objective and careful alignment/collimation of the Chromacorr-O1 with the other optical components, colour pictures of the diffraction patterns of Sirius as imaged by the complete optical train at F/39.2 (achromatic doublet objective plus three-lens Chromacorr plus two apochromatic Barlow-lenses (2x each) plus flip mirror finder plus IR-filter plus MX5-C camera) were taken.

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Inside focus--------------------------------In-focus--------------------------------Outside focus

 

B. Summary.

-Estimation from diffraction patterns of Sirius of the secondary spectrum of the optical train at F/39.2 when the Chromacorr-O1 is installed, shows a longitudinal chromatic aberration of +0.03 mm and a lateral chromatic aberration of -0.10. The secondary spectrum value of 3.1x10-5 classifies the system as an excellent apochromate and is a factor 11 improvement over the C102-HD without the Chromacorr. The spherical aberration of the C102-APO is estimated to be in the order of -0.03 l !

-Defocussing aberration is about 1/32 λ.
-Ronchi-testing showed sharp, straight and parallel lines with a very small erratic curvature at one end of the central lines, both in- and outside focus. This means that the doublet objective shows a minor turned edge effect. Spherical aberration looks neutral

 

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 27 September 2008