In terms of the scientific potential of amateur studies of Mercury, CCD imaging and photometry are preferable to visual observations. This is due to the fact that the ''personal equation'' that becomes a factor for each observer performing visual inspection observations at the telescope eyepiece does not as much, or at all, influence the absolute calibration of the data.
Amateur astronomers interested in providing support observations should therefore consider using a CCD camera and photometric filters. Accurate calibration of such data is crucial and include application of flats, darks, bias frames, checks for scattered light and linearity of the chip, photometric reduction etc. To get acqainted with such techniques, the recent Willmann-Bell (2000) book The Handbook of Astronomical Image Processing by Richard Berry and James Burnell is recommended reading.
It is always recommended that the observer performs the basic calibrations to the image data before it is submitted for analysis. Photometric reduction may be more difficult to perform for the individual observer.
It is utterly important that vignetting or strong gradients in image brightness (e.g., due to dust specs on the optics) in the field of view of the CCD does not occur. This may in the worst case ruin any photometric or imaging observations. Generally a CCD camera is better than less expensive video cameras or ''WebCams'', since the electronic noise is smaller and the chip is often of higher quality (both cosmetically and in terms of uniform response to varying illumination). Color cameras are not usable for photometry.
Generally, the most valuable observations that amateurs could provide to assist professional studies are imaging of the resolved planetary disk and photometry. High-resolution imaging is difficult but requires no advanced equipment except for some kind of CCD camera. Multicolor photometry using standard passbands (Johnson, Kron-Cousins or Bessell) over as large range of phase angles of Mercury could quite possibly give new information on the optical phase reddening effect of Mercury (of which we know basically nothing today) and improve the knowledge of the phase integral and Bond albedo (which is known for the V-band only).
For imaging, red filter observations are preferable. Use of such filters will minimize the resolution-deteriorating effect of atmospheric seeing and refraction which both increase in importance towards the blue end of the spectrum, as well as simultaneously optimize the contrast relative to the bright background sky. The filter could be a photometric R or a Wratten 25 or 92. Most importantly, the filter must not have any blue red/infrared leaks which are within the response of the CCD, or else an IR-block filter should be used in combination.
The pixel scale should be selected so that the sampling criterion is fulfilled, i.e. the angular scale of two pixels should correspond to the diffraction limit in the given filter band. For 20 cm aperture, a pixel scale of 0.3 to 0.5 arc seconds is recommended. For other apertures, scale this number accordingly.
Getting sharp images is very difficult. Often the seeing is better during daytime with Mercury high in the sky than when it is lower in a darker sky. Daytime observations are usable only for imaging, as for photometric observations a calibration star has to be observable quasi-simultaneously with Mercury. The bright daytime sky makes finding and observing proper calibration stars difficult, if not impossible.
Even though directed towards photographic imaging, Jean Dragesco's book High resolution astrophotography (Cambridge Univ. Press 1995) is very valuable for CCD imagers as it details preparations of the photographic setup that any astrophotographer should consider in order to obtain good results.
Photometry can get very complicated and one should try to keep things as simple as possible. The advantage with respect to imaging is that the integrated disk intensity may be obtained even though seeing conditions are poor. Standard photometric filters must be employed. V-band filter observations are to be preferred, followed by R-band and other standard filters (I, Z, U). Unfiltered observations will not be usable for photometry, as the wavelength range will not be well defined.
To perform photometry, the sky has to be crystal clear, at least in the region of the sky traversed by Mercury and the standard star(s). Even a small cloud or haze may make the observations unusable, and therefore such observations should not be made unless skies are absolutely cloud free. Photometry requires the best astronomical conditions which occur for the shortest durations, and it is therefore not always easy to obtain high-precision data.
To obtain absolute calibration in terms of intensity, a standard star has to be observed as close as possible to the observations of Mercury. Best of all would be to intermittently switch between Mercury and the star as they both descend or ascent in the twilight sky. In any case, the star and Mercury should be as close to each other as possible. The star should be repeatedly observed through at least the same range of altitudes as those traversed by Mercury, or absolute calibration will be difficult or impossible to obtain. A list of bright standards is available in the Astronomical Almanac for each year.
For the 2002 apparitions, the following UBVRI standard stars are recommended. Observe the star closest to Mercury and/or the star passing through Mercury's altitude range in a dark sky as soon as possible before (morning, W, apparition) or after (evening, E, apparition), the Mercury observations are intitated/finished. Paranthesised stars are less suitable in terms of declination.
April-May elongation: 13 Ari, (73 Cet), (HR 753), (96 Cet), (38 Per), 18 Tau, 25 Tau, HR 1172, (46 Per), 54 Tau, 61 Tau, 77 Tau, 74 Tau, 78 Tau.
June-July elongation: The above stars plus (1 Ori), (3 Ori), 112 Tau, HR 1938, 134 Tau, 54 Ori, (12 Mon), 24 Gem, 54 Gem, 62 Gem, 78 Gem, (17 Cnc), 39 Cnc.
It is important that the calibration star(s) or Mercury do not saturate the CCD. Furthermore, the star should not be imaged with too short exposure times, as scintillation and non-linear exposure time may then cause increased photometric inaccuracy. Often the image of star has a higher surface brightness than Mercury and will therefore saturate at shorter exposure times. If so, try to defocus the stellar image and expose for a few seconds, if the chip of the camera allows it (but a shorter exposure is better than a very noisy image!). Also, to reduce problems with non-linearity of the chip (especially important for WebCams) aim, if possible, at an exposure time that will make the brightest pixel of the object fall in the middle of the dynamical range (e.g. an for an 8-bit camera the star or Mercury should have a maximum pixel brightness of 100-150 ADU:s).
Companies that carry standard photometric filter sets (Johnson, Kron-Cousins or Bessell) are Omega Optical and Optec (in the US) and Gerd Neumann (Germany). UBVRI sets with mounted 1.25-inch filters sell for about 400 USD/EUR, individual filters for about 100 USD/EUR.
Most of the time, visual observations will deliver the sharpest possible views and the highest probability of detecting the smallest planetary details. This is simply due to the fact that the eye-brain system has an unequalled dynamic and angular resolution and can remember fleeting details which appear for very short durations. In that sense, visual inspection is unbeatable, but it lacks from possibility to absolutely calibrate and to recheck the quality of the observation.
Visual observations should be made according to the standard methods by planetary observational societies, e.g. the ALPO or BAA. If possible, provide estimations of disk feature intensities according to a standard scale. All details on the observational circumstances must be recorded at the time of observation, and the finished drawing must be touched up and completed as soon as possible after the observation in order not to distort the recorded information.
Most of the time, a red or deep red filter will help to increase contrast between Mercury and the sky, and to improve seeing. Similarly, a linear polarization filter (used singularly or in combination with other filters) may be used to reduce an overly bright image and improve seeing. Any of the Wratten glass eyepiece filters from major telescope manufacturers are recommended.
We are open to different kinds of submission of data and do not restrict ourselves to specific image formats etc.We request that the observer scans visual drawings and photographs and submit them in digital format. In general, use an uncompressed standard image format available in your CCD or scanner software. For all images/drawings/photographs etc. a separate documentation file must be included, unless the image data is in FITS format.
Filenames should preferrably be setup so that observer and date are
directly indicated (e.g., nn_merc020214_1.tif, for an observer with
initials "NN" who recorded Mercury on a tiff file, number 1 in a long series, on
February 14, 2002). If this format is impossible to use with your software,
choose filenames that are unique and easily "decoded" both for yourself and
others. Please use date notation according to the international standard in the
order day-month-year.
For monochrome image data, usage of the astronomical FITS standard for data submission is recommended, as the format includes both image data and documentation.
For color images or for software not capable of saving the data in FITS
format, tiff is recommended as it is a noncompressed standard. The image
documentation should be included in a separate ASCII file with the same prefix
as the image file (e.g., mercury.tif/mercury.txt).
Correct, specific and detailed documentation of your image data is utterly important to ensure that it may be used for scientific purposes. We recommend that you fill in a data form like the following, if the data is not stored in a FITS header. This form is hopefully self-explanatory and suitable for visual, photographic or CCD/WebCam observations:
------------------------------------------------------------------------
Object :
Date (UT) :
Time (UT) :
Telescope/optics :
Eyepiece/magnific.:
Filter :
Film/Camera :
Exposure time (s) :
Weather :
Observing site :
Observer :
E-mail address :
Image filename :
Comments :
------------------------------------------------------------------------
The data form is available for download here.
Prior to submitting your initial observations, please send an e-mail to johan.warell@astro.uu.se to discuss the most convenient way of data delivery. Submitted observations will be archived and available for study on the Mercury Watch Support Observations web page. All contributing observers will be informed of the scientific use of their data and properly acknowledged in any resulting publications.
Please send questions regarding the Mercury Watch Support Observations Program to any of us. Best of luck with your observations, and thank you for sharing them!
Ann Sprague
Lunar and Planetary Laboratory
University of
Arizona
Tucson, AZ 85721-0092
USA
E-mail: sprague@lpl.arizona.edu
Phone: +1
520 621 2282
Johan Warell
Department of Astronomy and Space Physics
Uppsala
University
Box 515
SE-751 20 Uppsala
Sweden
E-mail: johan.warell@astro.uu.se
Phone:
+46 18 471 5970
Fax: +46 18 471 5999