Friday, 10 November 2017

Hypercam 183c - A Work In Progress

Hypercam 183c - A Work In Progress

A few weeks have gone by, and I've had the opportunity to get out with the Altair Hypercam 183c for a decent amount of time under a reasonable sky.  I think I can now finally say that I have the process of setting up and data collection quite well refined.  I have found that as long as I stick to the same principles and spend time on the polar alignment routines, I don't really need to engage guiding from PHD2.  In my testing so far, I have been using 60 second exposures with very little, if any, signs of star trailing.  This helps me keep to the principles of keeping this as simple as I can when it comes to equipment.
In my last post, I gave some results and thoughts on the process of using SharpCap or AltairCapture as the method of choice for data collection.  Since that post, I have been quite busy away from the telescope.  Firstly, I went to an Astrophotography Course where I was able to pick up some fantastic tips when it comes to use DSLR and single shot colour CMOS cameras like the 183c.  But, perhaps the most interesting part of the course was regarding the post processing of the data.  I was introduced to Pixinsight.  An application designed specifically for astrophotography, it performs calibration, stacking, alignment and a whole host of post processing tasks.  From the demonstration I watched, I was quite impressed, so since then, I have downloaded the free 45 day full trial of the software and have since got stuck into using it.
All set up, back on the 27th October, I relocated to the garden and into the shed with the computer and all the toys on a mission to start collecting data.  I chose 3 targets for the night, and following the tips picked up from the course, and some of my own experience, I decided to use the same camera settings for all 3 targets.  They were:

  • To use SharpCap to connect to the Hypercam and collect the data.  I now think SharpCap will give the better results, especially with all the new features coming on line with SharpCap Pro.
  • To use a gain of 2000.
  • To use an exposure time of 60 seconds per light frame.
  • To collect approximately 30 bias frames.
  • To collect 10 dark frames.
  • To collect 10 flat frames. (More info on that to follow).
So far, so good, but not all was plain sailing.  In fact, after collecting all that data, and then processing in pixinsight, it was soon apparent that I had a problem.  Firstly, these are the 3 results.

M27

M45

C19
The results for M27 and M45, though show promise, are overall quite poor.  Initially, C19 also had this halo effect, but I found that it was removed to an extent in processing.  Looking at the light frames, and the flat frames, the feature was apparent in each of them to one extent or another.  Of course, the act of stacking all these frames, though good for detail of the target itself, also amplifies this bright anomaly.  This had me stumped.  I couldn't figure out how this has happened, so I started to go through the process of what I had done.  I kept drawing a blank, so I decided to open it up to a couple of forums on line and ask for advice.  This is the beauty of astronomy.  I've always found that there are always people out there willing to help.  The suggestions were quick in coming and now I have a list of things to try out.  Indications are that this could be down to one of two possibilities.  Either there is excess light getting into the light train, or, a type of lens flare is happening.
Excess light may have be caused from local light pollution, and though I don't think it was particularity worse than normal, I have to remember that the camera that I am now using is much more sensitive.  A simple thing to try will be to extend the dew shield at the front of the telescope with a piece of card or similar and prevent any light coming into the OTA from the side.
A second possibility was suggested, and that was it might be down to the x0.8 focal reducer that I used in the light train.  It was suggested that perhaps the extra lenses in the reducer combined with the doublet in the telescope cause this effect at the point of focus.  This is also an easy thing to test by simply using the same process of collecting data, but leaving out the focal reducer.  This will change the field of view of the telescope, but results should be evident within the first few frames.
I have also picked up another two tips from the forum and the course I attended.  So far, I haven't used any filters in any of my imaging.  It's been suggested that a standard light pollution filter is almost mandatory, and it stands to reason I suppose.  Especially under the light polluted UK sky.  The second suggested filter is a more specific Infrared/Ultra Violet filter.  Both of which are now on the Christmas list!

Further processing.

Amongst the various posts on the forums, someone kindly took my image of M27 and processed it further themselves.  They managed to process out nearly all of the feature, although it was quite a technical procedure by the sound of it.  Here is what the image looked like afterwards.

M27 processed further by a fellow forum member on the Stargazers Lounge forum, wimvb.
So I know that my data does contain the information I want.  I also know that it is possible to cut out a majority of the error, but I shouldn't need to carry out these additional steps.  More testing to come!

Flats.

Finally, flats.  Over the last couple of years, I have tried two main ways of collecting flats data.  Both utilise a white t-shirt stretched over the end of the telescope.  Previously I have used a white image on the laptop screen held over the end of the scope.  Now though, I have bought a USB powered LED tracing box which is light weight and easier to hold up over the end of the scope.  When I first saw the problems in the images above, one of the things I wanted to rule out was the quality of the flat frames I took within SharpCap.  I was fortunate enough to be put in touch with the developer of SharpCap Pro who was able to provide me with the optimum mean values I should be looking for when creating my flat images.  Fortunately, following the initial evening of data gathering, I had left the camera connected to the telescope at the same focus and orientation.  So, I was able to experiment and take further flats to introduce into the calibration process in pixinsight.  I've now been able to rule out the flat calibration files.

So, onward I go, and more trials are in the pipeline.  The forecast is reasonable for Sunday evening, so it will be a good opportunity to try out the dew shield extension and omitting the focal reducer from the light train.  Both will be able to provide fairly instant results as I should only need to take a couple of light frames.  I should be able to do a full histogram stretch fairly quickly to see if the issue is still present or not.  Keep an eye out for my results!
Thanks for reading.

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