Archive for DSLR

EQ6 periodic error

Posted in Astrophotography with tags , , , , , , , , , , , , , on 2011/04/06 by computerphysicslab

I took a bunch of 30 seconds shots aiming M42 nebula as regular, in order to stack them later. Polar alignment was also regular, using EQ6 polar scope, probably not perfect.

I think M42 is a good target to measure periodic error in RA movement due to its near 0 degrees declination.

In order to show the drift, I stacked the shots without drift correction using a free software called startrails that gets the brightest pixels per shot, obtaining that way the best startrail you can achieve.

To measure the length of the drift, I requested a single shot solved plate from astrometry.net. They provide an exact width in arcminutes of the field. Then, I divide the width field by the width in pixels of my DSLR camera sensor, obtaining the resolution per pixel in arcseconds (a number close to 1 arcsecond/pixel for a 1,200 mm effective focal length telescope).

Then I measure the height of RA drift pattern with my regular post-processing free software, Fitswork4, and multiply that value by the previous resolution.

That is the way I have found that my EQ6 mount drifts around 40 arcseconds in RA movement. Dividing total exposure by the number of cycles (top peak to lower peak) I got the elapsed time needed to fulfill the periodic error: around 6 minutes long.

Lyra & Hercules constellations

Posted in Astrophotography with tags , , , , , , , , , , , , on 2009/07/23 by computerphysicslab

Last night Lyra and Hercules were located near the zenith of my suburban sky, so I could make a series of 90 shots with 10 seconds of exposure each one. My equipment was the DSLR camera Canon EOS 450 (Rebel XTi). This image sequence is equivalent to a 15 minutes long single shot. After adding them with the shift-and-add technique, and substracting a dark frame (via Photoshop) that includes light pollution I got a final picture. I have aligned (using Nebulosity 2 software) the Stellarium map of the area and created an animated gif that switchs from the photo to the map continuously. It seems every star is located correctly where it should be ;-)

Globular clusters M13 and M92 are there in the photo like 2 stars.

Lyra-Hercules-photo-map-small

Lyra-Hercules-photo-map

Light Pollution

Posted in Astrophotography with tags , , , , , , , , , , on 2009/07/21 by computerphysicslab

Light pollution in suburban skies makes nearly impossible astrophotography. Nevertheless, there exists some computer techniques to make it possible. I have used a dark frame to catch the exact pattern of the light pollution in the photographed area. The way to accomplish this is not difficult. Shooting in continuous mode the DSLR camera facing the zenith in my home window, and forcing every frame to last 15 seconds I got a sequence of subframes to be processed afterward.

To get the dark frame I do some image arithmetic with Paint Shop Pro 9: I choose 3 or 4 distant subframes and compute them using “darkest” option. This way, stars become to fade until disappearing.

With Paint Shop Pro 9 and batch processor I apply a barrel lens distortion of 17 (empirical value to correct a 18mm focal lens like Canon’s) to every subframe and also to the dark frame.

Once got the dark frame DeepSkyStacker is needed to stack the single subframes and substract the dark frame. The sideral drift of the field is automatically compensated with the intelligent algorithm that DeepSkyStacker provides.

The resulting image is surprising taking into account this is an urban sky.

Light-pollution

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