Saturday, October 19, 2013

Thursday, October 17, 2013

First View of Film from Calstar 2013

I shot film at Calstar 

Yes, it's old technology.  But it works very well for what it can do.

Here are some images from when I got the film back from K&S


Acros filmstrip

Acros single frame

Wednesday, October 16, 2013

Continuing work on the Calstar images

I'm tweaking my calibration process with the images from Calstar.  As I've noticed in my final images, because the flexure is so much under control now, hot pixels are being a problem.  Thus, I have to do a better job of calibration to remove them from the source.

Friday, October 11, 2013

More Calibration from Calstar 2013

With the past week, I've been working on calibrating the data from the starparty.  I've run into a few roadblocks.  Either there's not enough data, not enough good data, or bad data was gathered in the first place.

As noted in the earlier post, there are some star flares that appear and make for havoc in subexposures.  Removing these flares requires more good data -- there's just not enough good frames to balance out the bad ones.  For IC59, this means that I'll need to get up to Montebello and image at least 6 frames on the target. 

For other targets, like the horsehead, there's no content other than the flared data.  This means that at least 9 or more frames will be needed to fix it.  I've worked on the HH before, so I might toss this data in with that set for refinement. 

For targets like the Triple Cave, there are hot pixels that have made it past the calibration phase.  This is probably because the bad pixel map isn't robust enough to catch them.  I've tried to boost the strength of the DSS detection to catch them but it sucks out the inside of bright stars instead.  Now that I've fixed flexure to the level where it is now, hot pixels are being reinforced and without dithering it's hard to control them automatically.  I believe that the only way to remove them at this point is with a stronger bad pixel map during calibration.

And then there's focus.  Some pictures are so out of focus that they are unusable.  Temperatures were dropping so quickly during the evening that focus didn't settle.  Not much I can do about that problem!

Tuesday, October 8, 2013

Calstar 2013 - Calibration and Stacking

I've run through a basic calibration using the data that I currently have in my library of darks.  Some stacks are very thin with only 3 subexposures available.  Others are more robust and are ready to use.

Temperatures for the camera were a comfortable mid-teens C in the early evening, quickly dropping as the night wore on.  The problem I face now is that there were temps in the 5C range that are out of the scope of what I have in the library with the current cooler.  I could reference earlier libraries and may do that when recalibrating.

Also, when I removed the IDAS filter, there was one day where flats made were overexposed by 2 stops.  These flats may be unusable and I may have to use flats from another night.  This error seems to show up as banding across the field.

I have noticed strong star flares on certain images, mostly where the IDAS filter was in place.  I'd like to look in more detail at some stacks where the IDAS filter was removed.

Interestingly, when working with the IC 59 stack, I was playing with Kappa settings and found that the flare caused by Navi would erode based on lower Kappa values.  With Sigma Clipping, one flare would disappear and another would remain strong.  With Sigma Median, both flares would erode but not disappear. 

In the above example, note how the flares are shown differently.  The left side has Median Kappa-Sigma (Kappa = 0.50, Iterations = 20) and the right has Median Kappa-Sigma (Kappa = 1.00, Iterations = 10).  In looking at this again, I like the appearance of the higher Kappa value.

I think that the course of action here - barring getting more data - could be to clone data from the Clipping stack to the Median stack.  The area that has the flare needs more unflared data to balance it.

I've also seen new flare types showing up from stars that are even further from the field of view.  For example, when imaging the Monkey Head nebula, the star Propus causes lines of flare and an odd ring pattern.  The ring pattern will be fairly easy to crop out yet the lines reach nearly to the center of the frame.  I think I'll have to look at the flocking again and see what is causing the glinting effect.

Also, I do see diffraction spikes around the brightest of stars.  These are likely because of edges of the flocking paper entering the light cone.  Will have to trim these offending bits for future work, but can't do much about them with the existing data set.

You can see these spikes appearing in the below example of the Horsehead.  Also in the view are a strange series of circular bands.  These are not visible in the sub exposures before or after calibration.  I've tried stacking with Average, Median, Sigma Clipping, and Sigma Median.  I think there's just too little data to use for the stack with only 3 subframes.  Interesting that I haven't seen the error before with other examples that only had 2 frames (the Pelican image from earlier this summer was an example of this kind of work).

For what it's worth, the IDAS filter was in place for this shot.  I think the strong star flares are bouncing off the IDAS and onto other elements.  When taking other pictures without the filter, the flare disappears.

Calstar 2013 First Report

First report after the event:

Dark Sky Map of the Central Coast

This was my second Calstar star party.  For reference, it is the end-of-season star party held at Lake San Antonio, near Lockwood CA.  The skies are dark and there is little light pollution.

Getting there is not so hard, it's mostly 101 south through the Salinas valley.  Traffic is agriculture support.  Being a coastal region, the road temperatures are not very hot - nothing like driving I5.