Plate analysis: Comet Johnson image

This is an astrometric enhanced version of the released image of the C/2015 V2 (Johnson) acquired on March 1st.

Here we have stretched the data in a different way in order to enlight faint objects and make objects more discovereable.

The first step is to solve astrometrically the image in order to get the actual relation between the pixel scale and the celestial coordinates. In order to obtain this relation we submitted the position of the stars in the image (extracted with a specific tool) to engine (here you can find the submission) then this relation (WCS headers) is attached to the data.

Now we are ready to compare our image with the stellar catalogs. Again, to make objects even more discoverable we usually invert the image. Inverting the image make faint details more contrasting wrt the black sky which is now, white. In this step, the SNR (Signal to noise ratio) is very important beacuse it is an index of the quality of the image, the more the SNR is, the more faint details we are able to detect. Obviously for that purpose it is necessarly to work with calibrated data (Dark Flat Bias corrected) in order to remove the unwanted artifacts which are added by the acquiring system (Optical imperfections, Sensor dark noise and bad pixels, dust and more).

It is important to keep in mind that the stacking procedure has the only purpose to increase the SNR and not to add more exposure time. It’s a common misconception that adding more frames will increase the total exposure time but that’s not what is really occurring because since we are averaging the data along with the noise, the result dataset (image) will keep constant signals (stars and deep sky objects) while the randomly noise will be partially suppressed but since we’re averaging the data the final result will look like the single shots in terms of exposure. What is really changing is the total integration time

Increasing SNR by stacking is a good method to detect even fainter objects but averaging data will make fast moving objects more blurred and that results in a bad position determination. So stacking is ok but be careful of fast moving objects.

Usually we image solar system objects using Orbital Tracking in order to keep the celestial body freezed at the center of the FoV and to avoid the images to be blurred by the proper motion of the celestial body. However in this case we imaged using the common sidereal tracking because of the small proper motion of this comet: only 30.5 arcsec/h.

Objects extraction

What you are seeing is the sovrapposition of the known objects of the SIMBAD (in red) astronomical database and the inverted image.

Comparing to the USNO-B catalouge we were able to detect object as faint as mag 19(V).

UGC10325a/b group


In this plate we found 430 known galaxies. The comet is very near to one of the brightest galaxies in the plate: the Radio Galaxy NVSS J161730+460530 which is shining at mag15(V) this galaxy is part of a group of galaxies (UGC10325) toghether with the near galaxy which is few pixel on the upper right. This second galaxy is caught at mag15.81.

However, the number of main galaxies th are logged in the Simbad database is 63.

The most faint galaxy analyzed is the spiral galaxy SDSS J161759.98+455704.1 found at mag18.3

Two other bright galaxies in the right side of the image


Here on the left there is a group of galaxies, the two objects marked with the an ellipse, are bight Simbad galaxies.

The left one is the 2MASX J16183389+4544330 which we detected at mag.17 and the other is the galaxy LEDA 2269387 which we caught at mag.16.45

We were able to detect most of  the galaxies in the simbad database with an high signal to noise ratio. In most of the cases we were also able to measure their brightness with photometric analysis.

Other DeepSky Objects

While most of the objects found in this image are Galaxies (which we detected often with high snr), querying the Simbad database we found other interesting deep sky objects.

DSOs found in the image querying the Simbad database

As you can see, in this image there are a lot of interesting things. The second most populous object are Quasars. These objects are one of the brightest non-transient objects in the universe but they are often very far from us (billion ligh-years) so, the brightest quasar in the night sky is shining only at mag12.8. The quasars that are in this field are extremely faint. Most of them was found using big ground based telescopes or space telescopes. With our setup (and location) we were only able to detect faint and low SNR signals from two of them which are the brightest ones at mag18-19(V). The other QSO are fainter than mag20 so, well beyond our current observing limits. Quasars are very bright active galactic nucleus. Accordingly to wikipedia, “a quasar consists of a supermassive black hole surrounded by an orbiting accretion disk of gas. As gas in the accretion disk falls toward the black hole, energy is released in the form of electromagnetic radiation

White Dwarf WD 1615+461

In the image there are also some white dwarfs, that are stellar core remnant composed mostly of electron-degenerate matter. These stars are very dense: its mass is comparable to that of the Sun, while its volume is comparable to that of Earth.

The most bright WD that we were able to detect is the WD 1615+461 (see the image). The photometric analysis of this object shows that here it is mag17.

 In this image we found also two variable stars (one of them is a RRLyr type variable) the brightness of these stars varies wrt the time so it’  s very important to provide a precise time reference while providing data on their magnitude measurement. The middle exposure time for this acquisition is 2017/03/02.013 (note that .013 stores the time information since we’re indicating that it was elapsed only 1,3% of 2017/03/02 in UTC time). 

Solar system objects

In this image is only present the comet C/2015V2 Johnson. No Asteroids or planets detected. In the inverted image it is plotted the arrow of motion of the comet.


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