While talking with people at star parties, or giving talks to schools, we are frequently asked:
"How far can you see through your telescope?”. For people not really involved in astronomy, the usual answer will be in light years perhaps, highlighting objects like a bright quasar such as 3C273, or possibly by explaining the concept of
magnitude, and relating it to telescopes like the
Hubble. The explanation of aperture rather than magnification is one that can initially be complex for beginners.
However, even professional research scientists need to know how far you can push the limit of the equipment for the work they do. This is an interesting question which we have recently tackled, as a result of a project Nick was invited to take part in with the
Lowell Observatory in imaging and refining the orbits of Kuiper Belt objects, and one which the rest of us here at
Remanzacco including Nick are now working on.
This concept of measuring the telescope´s capability in terms of how deep an image we could take, in what time, under what conditions, would give us confidence in some projects involving long period cometary bodies, TNO´s and assisting the research work of professional observatories, many of which have much larger telescopes even that the one we were testing. Recently, courtesy of our involvement in the
Lowell project and also some very deep images taken of
comet 67P in support of the European Space Agency Rosetta mission, the
Faulkes telescope team gave us the opportunity to test the capabilities of their professional-class research grade instrument, namely the 2-m Faulkes Telescopes South at Siding Spring.
The idea was to check if, through this instrument with its sensitive cooled CCD, the
C/1995 O1 (Hale-Bopp) was still visible. Comet Hale-Bopp was notable as one of the brightest and interesting comets observed in the modern era, and it's still a subject of interest for astronomers. (Nick had also recently met Thomas Bopp at the Tucson Space Fest).
According to a couple of recent works by
G. Szabò et Al. (
http://arxiv.org/abs/1104.4351 &
http://arxiv.org/abs/1210.2785) comet Hale-Bopp was observed in 2010 on Dec. 4 through the 2.2- reflector at the La Silla Observatory at around 30.7 AU solar distance (and receding), and afterwards, on 2011, Oct. 23 at 32 AU. In their papers, the authors discussed a variety of interesting scientific aims with their detection of this comet at magnitude R=23.3 on 2010, Dec. 4, and R=23.7 on 2011, Oct. 23, that have some significant consequences on our understanding of Hale-Bopp, and comets in general.
Our interest on C/1995 O1 was also raised by a report posted in the Yahoo Group - Comets-ml which alluded to the possible detection of this comet at magnitude 21.8 on 2012, Aug. 07 using a more modest telescope. Languishing now at over 33.4 AU, we knew this was going to be an exceptionally tough target. However, we decided to attempt its observation through a professional class instrument, suitably placed in the southern hemisphere (currently Hale-Bopp is located near the south celestial pole, in the constellation Octans, at 87 deg declination south). We planned a systematic follow-up campaign over the course of several nights, in order to test not only if we could image the comet, but also the instrument capabilities through different observing condition (moonlight, seeing, sky transparency, etc.).
On each night we performed a series of 180 second Bessel R-filtered exposures that were subsequently stacked through dedicated software (
Astrometrica by Herbert Raab and Maxim DL by Cyanogen). With Astrometrica we stacked along the expected proper motion of the comet (since Hale-Bopp was moving at a rate of 0.07"/min, this exposure time length would not trail the comet)
We estimated that if we were to catch this extremely faint object *(The research paper cited showed a possible light curve indicating in excess of magnitude 24) we knew we would need rather long total integration times, so we asked the collaboration of several colleagues working with the Faulkes scopes, including a work experience student, along with slots which had not been taken up on FTS. Faulkes telescope team members, including a work experience student,
Cerys Roche, also assisted with the imaging test.
We accumulated a total of five nights of follow-up from 2012, Sept.25 till Oct.9. With observing runs typically from 30 minutes up to 90 minutes in duration, depending on the scope´s availability and the local weather conditions. We carefully analyzed the resulting stacks in search of comet Hale-Bopp, whilst also measuring sky conditions, limiting magnitude and looking for other moving objects in the field.
This is the log of our observations (the reported negative detection refers to an area wide about 2-arcmin, that's several times the expected error box on Hale-Bopp ephemerides):
2012, Sept.25.5, stacking of 20 exposures, 180 seconds each, comet not seen to limiting magnitude R= 22.5.
2012, Sept.26.5, stacking of 15 exposures, 180 seconds each, comet not seen to limiting magnitude R= 21.5
2012, Oct. 4.6, stacking of 25 exposures, 180 seconds each, comet not seen to limiting magnitude R= 22.5
2012, Oct. 8.6, stacking of 20 exposures, 180 seconds each, comet not seen to limiting magnitude R= 23.0
2012, Oct. 9.6, stacking of 20 exposures, 180 seconds each, comet not seen to limiting magnitude R= 23.5
(Note. On a night, stacking some set of images, we had the impression of an possible positive detection at the very threshold limit of our exposures, however we cannot conclusively state is the comet, due to its extremely low S/N. Whilst it may indeed be Hale-Bopp, without a secure second night of detection, there is a possibility it could just be an anomaly or noise)
In this following figure we report some representative results of our image stacks , where the red square overlay indicates the expected position of comet Hale-Bopp on each of the observing sessions. Click on the image for a bigger version.
It is our conclusion that, whilst on a few instances we may possibly have detected something, we have to state that cannot claim to have a positive and verifiable detection of
C/1995 O1 even under our best observing conditions, down to limiting magnitude R about 23.5 with sub arcsecond seeing. In spite of the negative detection of the comet (which has some meaningful scientific consequences based upon the
La Silla observations anyway), this was a vital experiment for us, since we have been able to ascertained how far we can go with the Faulkes scopes, with respect to seeing and other associated conditions. We now know that we can detect moving targets on good nights down to in excess of
magnitude 23.
We also understand far more how we can process our data right at the noise level limit. All this will be very helpful in our projects like that with the Lowell Observatory, aiming at the follow-up of faint
Trans Neptunian Objects (TNOs) in need of follow-up to refine the knowledge of their orbital parameters, and also with the ESA SSA project, where detected objects can rapidly fade to limiting magnitudes which are beyond the 1m class instrument their team have.
We are grateful to those who kindly assisted us in this exciting hunt, and in particular to: P. Roche, C. Roche, A. Tripp, S. Roberts and K. Rochowicz.
by Nick Howes, Giovanni Sostero and Ernesto Guido
