Archive for Sky Watcher

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 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.

Apochromatic vs Achromatic Refractor

Posted in Astrophotography with tags , , , , , , , , , , , , on 2010/10/14 by computerphysicslab

People should think twice before purchasing an apochromatic refractor as opposed to an achromatic telescope. There is a huge difference in price and a small difference in performance according to an experiment I made recently.

In doing astrophotography, people use to spend a lot of money in equipment. It is important not to fall into the elitist apochromatic myth if you want to save some money.

I took a single 30 seconds shot to Pleiades open cluster (M45) using my 6-inch Sky-Watcher achromatic refractor that costs around 1,000 US$. Then I used a 6-inch apochromatic Takahashi TOA-150 that costs around 10,000 US$ to take an equivalent shot on the same field.

Resulting images speak by themselves. Obviously achromatic refractor shows a blue halo around stars, due to chromatic aberration. Nevertheless, a simple post-processing technique can remove the halo, obtaining a similar image to apochromatic’s.

Now, the question is: Is it worth paying the extra 9,000 US$?

Saturn with a 6 inches refractor

Posted in Astrophotography with tags , , , , , , , , , , on 2010/04/10 by computerphysicslab

This image of Saturn has been obtained after processing a 4 minutes video took with a Casio Exilim digicam. In order to avoid refractor chromatic aberration a Fringe Killer filter was used in combination with a 14mm eyepiece and a Barlow lens.