M42 - Orion Nebula

Experiments with the Canon T1i DSLR continue.  This time I used the club's new 120x600 mm Vivitar telephoto lens.  The lens and camera were mounted "piggy back" to the 10-in LX-200 which provided a stable, tracking platform for time exposures.  

The idea of a variable focus lens is to give variable size image scales.  Some objects are quite large (cf, M31) and require wider fields (ie, shorter focal lengths) while others are quite small and require longer focal lengths.  For these tests I chose M42 - the Orion Nebula - for testing at 120 mm, 300 mm and 600 mm settings.  Some quick test images showed that the lens has quite noticeable (hence, objectionable) coma at aperture stops less than f/11.  This is too bad but there is not much we can do about it except use the f/11 setting.  The LX-200 has considerable periodic error in the RA drive which necessitates adjusting the exposure times according the the focal length used for imaging.  It is therefore necessary to take many sub-frames and later "stack" them using computer software to get the desired final exposure time.  At 120 mm and 300 mm 30 second sub-frames worked nicely.  At 600 mm (and also 2500 mm at the Cassegrain focus of the LX-200) 10 second sub-frames work nicely.

All images were taken in "S" mode (lowest resolution), saved as JPEGs, stacked and processed (slightly) in Deep Sky Stacker (which produces 16-bit FITS files), binned 3x3 and converted back to 8-bit JPEGs in MaximDL 5.  No dark frames subtracted nor flat field corrections applied, eg, they are not top quality images but serve to give a hint as to what is possible using this equipment.

Click on image(s) to see full size.

 120 mm
12x30 sec
f/11
 
 300 mm
12x30 sec
f/11
 
 600 mm
36x10 sec
f/11

 
 2500 mm
70x10 sec
f/10
 

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