Using of a software to build drawings fields of Deep Sky objects is a saving of time and considerable precision gain. For the ones who have the necessary data-processing equipment, it would be a pity to avoid it !
In our example, we chose the "Guide" software (Project Pluto). It is not free, but there are freeware on the Net like "cartes du ciel" ( "Sky charts", translated in six languages ), integrating large data bases, which should be able to give comparable results.
M 31 and M 32, 60x40 R, North is down
Creating a pre-drawn field with "Guide" can be done in several steps :
1) in the "Display/Ticks, Grids, etc" menu, notch "aperture" and enter the field diameter of your scope in degrees.
2) make adjustments which gives you the best results: according to your taste, modify the size of the stellar discs ("Display/star display"). Checking "non star" allows to have stars which truly exist, except for the zones having a non homogeneous background sky (nebulas and wide galaxies: M 8, M 33).
3) Then, it is necessary to vary the limiting magnitude of the field in function :
- a) of the visual limiting magnitude, determined from your observing experience, the instrument type and the site quality.
- b) of the number of stars which appear in the field.
The quantity of stars in the field is left with the appreciation of each one. Keep in mind you will have to check each "pre-drawn" star in the field to be sure of its existence. The more numerous they are, the more it will take time. So at beginning, don't hesitate to make tests and to prepare several fields for the same object.
In some cases, (for example dispersed open clusters without nebulous component), the software will be able to achieve a complete drawing, without almost any intervention of the observer. See the drawing of cluster NGC 4665.
IC 4665 (c) Guide
IC 4665, 60x40 R, North is down
However, this technique doesn't work well with wide nebulous objects with superimposed stars.
For example, the open cluster located in the Lagoon nebula (NGC 6530), has a different aspect on Guide than its visual appearence. It is necessary to erase the additional stars (which appear even when the "non star" option is checked) and to draw the missing stars (some are rather bright so take care !).
"Guide 7", "pre-drawn" field of M 8
(North is down, field : 55')
M 8, sketched on this star field
60x40 R
After the drawing was scanned with a PC, non-observed stars were erased.
In spite of that, this method is interesting when we envisaged to draw a certain number of objects: 27 drawings were made in four nights with a 60mm refractor at a stay in the Saint-Michel l'Observatoire Astronomy Center (France, 04). So compute and draw !