Ring Nebula ST-9 image analysis

    *note click here for another depth study                             go to home

A few days ago, I tested out the new ST-9 camera using my JMI NGT-18 telescope.  I hauled the scope out onto the driveway and polar aligned it (sort of), and then snapped a 10 second image of the ring.

I got a few moments today, and decided to try to figure out just how deep 10 seconds will go in a suburban setting using an 18 inch Galaxy mirror.

For analysis software, I used software Bisque's CCDSoft, and TheSky.  The sky provides a feature called Image-Link that aligns CCD images with it's planetarium generated virtual sky.

I first loaded the ring.st9 file into CCDSoft.  Then I executed Image|Background, and then clicked on the <Auto> button.  This produced a balanced but ugly image shown below.

Note the light triangle on the lower left of the image.  This is thanks to a pole light out by the road.  The NGT-18 is an open truss scope, and needs help shutting out light streaming in from the side.

Then I clicked on Edit|copy to place a copy of the bitmap in the paste buffer.

I then started TheSky, and performed the image-link procedure, linking to 82 stars.   The result of this is shown below.

If you think about this, it is a pretty fantastic thing to be able to take a lousy looking image and skewer it through 82 points perfectly.  I clicked on the dimmest stars with holes in them (pegged stars) and found that the dimmest ones were at 16th magnitude.  Not bad for 10 seconds of exposure.  Note that not all of the stars were punctured.  This means that many of these stars are even dimmer than 16th magnitude.

During this process, I learned that my image scale was

Scale: 1.89 arc-seconds/pixel
North angle: 102.08°
RA: 18h 53m 22.2s Dec: +33°09'24" (Epoch 2000) at the center of the field.

Using this data, I went back to CCDSoft to do some more analysis.

Using Astrometry|Auto-Astrometry, and the information provided above, I got:

This means that 62 of the 82 stars identified by TheSky could be used to do precise astrometric calculations.  The table shown below is organized by brightest star first, and ends with the dimmest star.  The dimmest star found suitable for use by astrometric calculations was 14.3.  Again not bad for a 10 second exposure.

What this means:  If I can get a high quality 14.3 magnitude star in 10 seconds, I should be able to get a 15.3 star in about 2.5 times this length of time, or something like 25 seconds.  The 2.5 number is an approximation of the brightness difference between the magnitudes.  From this I compute an exposure table as follows:

                Exposure             depth

                10                         14.3
                25                         15.3
                62.5                      16.3
               157      (2.6min)     17.3
               391      (6.5min)     18.3
               976.5   (16min)      19.3
               2441 (41min)        20.3
               6103 (101min)      21.3

But just what does all this mean?  I probably won't be going to 21st magnitude here in my yard in Rockville.  It is foolish even to think that that might be possible given the level of light pollution here.  Additionally, if I have a hope of capturing a burst, I will need to do it within the first three hours.  It would take two hours to get to a dark site, and then another hour to set up.

What this data really means is that if I am home when the burst trigger goes off, that I can capture stars down to at least 16th magnitude with a 10 second exposure and have enough stars available to do a fairly accurate astrometric determination.  I can also easily stack images using track-and-accumulate to get a 2 minute exposure which would take me to perhaps 19th magnitude.  Remember the depth chart given above was for stars of sufficient accuracy to be used for measurement stars.  The photograph went about 3 magnitudes deeper.

This should be sufficient to handle bursts out to about 4 hours given the transient decay curve found elsewhere on this site.