Visual Observation of Deep Sky Objects
5. Technique
Written by Toshimi Taki
Prepared on September 4, 2004
Here, I present how to make sketches of deep sky objects.
5.1 Planning
(1) Choosing Object
There are several lists of deep sky objects. See 3. Star Atlases and Catalogs.
Choose any object you like. You can choose a popular object as a Messier object or a challenging object from other catalogs. Check the brightness and size of the object.
(2) Preparing Sketch Sheet with Background Stars
You can make a sketch from scratch on a white sheet of paper. This was a popular way of sketching deep sky objects. I don't use this method these days. Instead, I prepare a sketch sheet with background stars using a star map software. I use "Hallo Northern Sky" by Han Kleijin. The software can make a negative star map up to specified magnitude of stars in any scale.
Center your object in the map and select suitable scale. I recommend you to make several sketch sheets with different scales in order that you can choose a sheet in suitable scale at the telescope. Plot background stars up to about 1 magnitude brighter than your telescope can see. This guarantees that the sketch sheet does not include stars which you can not see actually. You can draw fainter stars at the telescope. Put scale in the map if the software can. Direction also sould be put on the map.
Print the negative map (black dots on white background) on a white paper. Common copy paper is recommended. I paste the map on A4 paper in MS-PowerPoint and print on a A4 copy paper.
5.2 At Observing Site
(1) Examining Sky Condition
Before you start observing deep sky objects, examine sky condition and record it. The visibility of deep sly objects varies significantly with sky condition, transparency of the sky and light polution of the site.
The sky condition can be evaluated by finding the magnitude of the faintest star you can see in your telescope. I examine the northern polar field for this purpose. The altitude of the star field is constant at any time and in any season.
Click here to download the northern polar field with star magnitude in pdf file.
Find the faintest star and record the magnitude, the time and the magnification used.
(2) Making Sketch at the Telescope
Now, you aim your telescope to the object using your favorite star chart. I use Wil Tirion's Bright Star Atlas 2000 for bright objects and Uranometria 2000 for faint objects.
Watch the object closely. Try different magnifications as Roger N. Clark explains in "Visual Astronomy of the Deep Sky". Choose a sketch sheet with suitable scale and make a sketch with a pencil on the sketch sheet. Use the background stars already plotted on the sheet for accurate positioning and shape of the object. Draw fainter stars on the sheet if necessary. See figure 5-1 for an example of a sketch.
Record data of observation such as eyepieces used, observation date/time, observing site, etc. The sketch can be crude because it does not intend to be shown to others. Important thing is that you can reconstruct the image what you see in the telescope from the sketch. You can put memos on the sketch sheet for this purpose.
5.3 Back Indoors
(1) Making Complete Sketch
Being back indoors, make a comlete sketch.
Prepare a new sketch sheet with background stars. Draw what you see in the telescope on the new sketch sheet based on the sketch which you made at the telescope. See figure 5-2 for an example. Put a scale bar in the field.
(2) Digitalizing Sketch
This additional process enhances your sketch. You can make a positive image which is more realistic than the original negative drawing.
Using an image scanner, digitalize your sketch. Find suitable contrast setting and brightness setting. To find suitable setting of scanning, you have to do several trials. Save the file in JPEG format.
Next step is image processing. Using an image processing software, you can convert a negative image to a positive image. See figure 5-3.
Figure 5-1 Sketch of NGC6992-5, The Veil Nebula in Cyg made at the telescope on a sketch sheet with background stars
Figure 5-2 Sketch made indoors based on the previous sketch
Figure 5-3 Positive image made from the previous image (A trace of blue and green hue is added to background.)
Return to Table of Contents
Return to Home