news 2011
at MPC 476 Grange Observatory
The Grange Obs. new refractor (a Vixen NA 140 'Petzval', or double achromat) purchased in April 2009 is frequently evaluated for visual observations (up to 267 x), and is currently used as a testbed for high resolution techniques development (i.e. Registax 5 processing of Philips SPC900NC webcam MPEG footages and Nikon D 3000 digital image processing with IRIS program).
The following Moon picture was obtained with the webcam on January 16th using a 18 mm eyepiece projection, giving a 7' x 5' image (0.6 arcsec/pixel sampling, the footage best 300 frames out of 900 were stacked):
The bigger feature shown in shadow is Schickard crater (227 km) in the Moon south-eastern limb.
With an higher webcam sampling (0.32 arcsec/pixel, field 4 arcmin on frame diagonal) the Moon was imaged on April 10th with a medium seeing; in theory the smallest lunar feature resolved with the 140 mm refractor shall measure about 500 meters.
The technical details about the above images can be found in the high resolution page.
This 120 s M42 raw picture was obtained on March 6th with the D3000 digital camera (set to 3200 ISO) at the NA 140 prime focus (f/5.7); the Nikon DSLR has the possibility to do an automatic dark frame removal (option NR on), and was used with its remote control:
The limit magnitude is about 14, comparable to old photographic films standard result with a similar scope diameter; the intended use of the 10 megapixel Nikon D3000 DSLR recently purchased is high resolution imaging (CCD pixel size 6 micron) and possibly astrometric work (chip dimensions about 24 mm x 16 mm) with the NA 140 refractor, having a 2° coma corrected field covering the DSLR whole CCD array.
With a mere logarithmic stretch using IRIS software, the processed image shows increased nebulosity, looking like:
The following M13 picture was taken with the Nikon D3000 DSLR set to 1600 ISO on August 20th at the 300 mm Newton reflector prime focus (f/4, collimation problems visible); several sub-pictures were taken and summed together with IRIS (stellar registration global matching cubic and add2, log plus black commands), the total exposure time being equal to 4 minutes:
The little galaxy NGC 6207 is visible at picture lower left; the DSLR CCD sampling is equal to 1 arcsec/pixel.
The same processing layout (but 3200 ISO, 9 minutes integration time and adding an image 2x binning plus gradient removal polinomial fit command) has been used to obtain this picture of M 27 on August 24th:
The nebula central blue star is 13.8 magnitude.
The original picture limit magnitude reaches 17.9 V according to the Bruce Gary's Vul 05 variable reference star J.
With the above picture setup, the Nikon D3000 CCD limit magnitude (stars with a reasonable S/N ratio, and 48 bit images, see over) achieved with a telescope's given diameter D [mm] and integration time t [s] seems equal to:
limit magnitude = 2 ln D + ln t
where the function "ln" is the Euler's number (2.718281828) based logarithm.
At last, on September 22nd the 300 mm f/4 telescope was collimated with a dedicated tool and the Nikon D3000 DSLR full field of 68 x 45 arcmin was properly illuminated and evenly focused as shown here below:
The total integration time is 3 minutes and the limit magnitude is about 16.5 everywhere in the picture, as per the above formula; the CCD sampling with image binning 2x is equal to 2 arcsec/pixel (1950 x 1306 array), well suited for astrometry.
An automatic M27 star field recognition (with UCAC 3 astrometric catalogue) successful test was done with the latest version of Herbert Raab's Astrometrica software, being Grange Obs. a long time registered user (in 1995 the program used DOS, running on the late Windows vs 3.11).
Astrometrica works with 16 bits FITS images (Nikon DSLRs provide instead color 48 bits raw images, to be processed by IRIS); the 2x binned and "de-Bayered" images do not follow the above formula, in fact one single 60 seconds CCD exposure contains measurable reference stars (or asteroids) of about 16.5 magnitude.
The Bayer filter 2x2 sub-reticles contain two green (or "luminance" control) tiles, while the information about the color is done with the help of the remaining blue and red tiles; the camera acquisition software is apparently doing some kind of spectral balance of the stellar image (possibly not referenced).
The color interpolation used by Nikon DSLRs suggests the stars photometry is a problem with the D3000 used for astronomy purposes: in fact, the classical photometry is done using reference fully-filtered B/W 16 bits images, which potentially could be approximated doing a 2x binning of the DSLR original 48 bits raw (the photometric error should be spreaded across the image).
For what concerns the limit magnitude reached by the D3000, the result is the 48 to 16 bits image conversion appears almost tripling the camera exposure time magnitude contribution.
The above formula then becomes:
limit magnitude = 2 ln D + ln 3t
The evening of October 1st the incoming asteroid (138524) 2000 OJ8, which is expected to do a close approach with Earth on October 13th, was observed and measured using the Astrometrica program running on a Samsung N140 netbook as shown:
The 16 bits images used for measuring the asteroid had an exposure of only 30 s (compatible with the 2000 OJ8 motion of 6 arcsec/minute: the aim is keeping the asteroid motion in CCD 2x2 pixels); the limiting magnitude resulted greater than 15.
The timing was obtained keeping both the netbook and the D3000 clocks aligned with CET using the INRIM NTP primary server, afterwards transforming with IRIS the Nikon DSLR .nef raws in FITS format, whose header has informations of the start and the length of the exposure to the nearest integer second.
The typical astrometric Right Ascension residual (mainly related to timing) using the described setup seems to be about 0.23 arcsec.
The stack & track Astrometrica feature (extremely useful for recording fast or very faint objects) was tested with 2 images of 2000 OJ8 as shown below:
The astrometric and photometric reduction appeared better than that obtained with the single images:
COD 476
TEL 0.3-m f/4 Newton + CCD
ACK MPCReport file updated 2011.10.04 22.23.41
NET UCAC-3
D8524 KC2011 10 01.85854 02 55 51.68 +45 02 55.6 14.4 V 476
The weather is getting colder in the mountains at the end of October as the snow has fallen on peaks, and cloudless nights are typically rare in autumn; MPC 476 Grange Obs. has successfully resumed a preparatory program for continously following the faint (down to 20 magnitude with a 30 minutes exposure) and/or fast NEOs approaching Earth.
On the Space Weather website the NEO closest approaches are frequently updated:
Clicking on an asteroid name, a connection to the NASA JPL Horizons ephemeris calculator will indicate if the NEO fly-by is visible from a given location.
Another valuable NEO resource online is the MPC Confirmation Page where one-night observed asteroids and others are requiring further astrometry; the technique to be used is described in details here and here.
The typical magnitude of the objects exceeds 20th, but nowadays the track&stack techniques made that target achievable for observatories with limited instrumentation or for amateur astronomers remotely partecipating to observational programs using big telescopes:
On the ESA website recently appeared news about German amateur astronomers helping the Agency's Space Situational Awareness (SSA) Program by manually scanning the Canary Island's Tenerife-based OGS 1-meter telescope digital images coming from dedicated sky surveys.
The group participates to the TOTAS program with the ESA Teide's telescope shown above, whose limit magnitude appears exceeding 20th with a single 60 s exposure on a professional CCD.
Grange Obs. has contacted ESA for collaborating to the SSA Program and presenting its candidature; for instance, starting from September 1st the Pisa University's NEODyS website collecting all the MPC 476 measures of NEOs is sponsored by ESA, which is currently looking for European assets on Space Weather.
The Grange Obs. has also collaborated in the NEO Emergency Technologies and Strategies (NETS) proposal for the EU FP 7 along with the Italian INAF and the French CNRS Observatoire de la Cote d'Azur.
The tasks in the NETS proposal were Technology Transfer and Scientific Outreach; this site brings an example of Grange Obs. capabilities.
Return to Grange Obs. homepage