The prediction of the visibility of deepsky objects

by Martin Maters

What can be more frustrating, for a beginner amateur astronomer, then going out in the dark cold night with your telescope , making sure the scope is properly aligned or if you have a GoTo it is properly initialized, only to discover that you can't get any sight of the first few objects you are trying to observe. This is what frequently happened to me about 20 years ago when I went out for a night observing with my 80mm refractor. Now that I just finished building my 12” computerized Dob (using Mel Bartels system to computerize it), and reading some articles about the visibility, I thought it would be handy to have an easy-to-use tool that at least would give me some indication about the visibility of those objects.

That is how my Deep-Sky Visibility Indicator project has started. I had to read a lot more about deep-sky visibility. For the insiders: even digging up “Blackwells” original article about “Contrast Thresholds of the Human Eye” dated back from 1946. Learning more about the Sky Background Brightness, Limiting Visibility and back-tracking some of the debate going on around this topic over the last few years. Thanks to the help of Jan van Gastel I think I more or less mastered it.

And finally here it is, and I think I can safely say it is more than just an indication tool. After finishing version 1.2 more features around the creation of observation lists where added, the BOC catalogue is added and the user-interface has been updated in this version.

DVI 1.3 or Deep-Sky Visibility Indicator 1.3 is a tool that helps the observer to determine if a deep-sky object will be visible through the eyepiece with a given telescope and night-sky conditions. Next to this the tool will also give you the optimum magnification for the selected object. One other feature of DVI 1.3 is to create and manage observation lists.

Some highlight features:
- It contains 9 catalogues [Messier, Caldwell, NGC, IC, ARP, Herschel400, UGC, Abell Planetaries, and BOC (Bright Object Catalogue     from www.AstroForum.nl)].
- It contains a database of over more than 28,000 Deep-Sky objects
- It allows you to view a selected object on-line on www.Sky-Map.org and get a view from the Sloan Digital Sky Survey.
- Create and manage observation lists, and optimizing the routing of your observation list.
- Import data from other tools
- Automatically generate Scroll-Files [for the folks using Mel Bartels system].

DVI1.3 is optimized for Excel2007, but an Excel2003 version is available as well. Anybody can freely use the tool.

Links to the two versions are here: Excel2003 and Excel2007

The user manual can be downloaded from here

For more information or feedback please mail to mmaters@zonnet.nl

Martin Maters
Bussum
The Netherlands

A footnote on Sky Background Brightness and Limiting Magnitude
Over the past couple of years there has been much debate about the Sky Background Brightness [SB0] and the translation from Limiting Magnitude [LM] to Sky Background Brightness. I believe the consensus reached in the end is that the maximum Sky Background Brightness equals 22.0 mag./arcsec2. Hence the "Visibility Surface diagram will stop at that point. Now the other relevant reference to consider is what LM will the average observer be able to see at this darkest possible point. Although I'm not convinced there is consensus here, but based on discussions and backed with extrapolated data from the Dutch Meteor Society [important to note for the sceptics amoungst you is that the data used is not only measured in The Netherlands], it is reasonable to set this at magnitude 7.0. Next then is to consider which translation formulas to use. I've looked at 3 different options:
a) SB0 = 21.58 - 5Log(10^{(1.586-LM/5)-}1) as suggested by Nils Olof Carlin
b) SB0 = 24.19 - 2.814LM + 0.3694LM2 as suggested by José Ramón Torres
c) SB0 = 22 - 5Log(10^(1.7-LM/5)-1) as suggested by Jan van Gastel, based on the initial formula by Nils Olof

Plotting those formula's against data from the Dutch Meteor Society [based on over 500 observations], it became apparent that the last formula as suggested by Jan van Gastel matched the data best. See picture below. Hence in this program this formula will be used.