Mini Series Part 2 -Achieving Focus, Sensor Analysis and Smart Histogram

Welcome to part 2 in this mini series of blog posts looking at the changes I need to make to my imaging set up following upgrading my camera to an Altair Hypercam 26c.  In part 1, I looked at how it fitted to me existing rig and the changes I needed to make to the imaging train to make the best of this fantastic new camera.

In this post, I will look at the next steps I needed to take after introducing the changes to the imaging train to accommodate the new camera.  I'll be looking at focussing, running a sensor analysis and why that is important before finally talking about how I can take the guesswork out of what settings I need to use when imaging with the 26c.

It's Sharpcap all the way!

As readers of my previous posts might pick up, I am a Sharpcap user through and through.  Back when I got my first Altair Hypercam (the 183c) I also got a free 1 year license for Sharpcap.  When that license expired, I had learned so much, and it had helped me a great deal that I decided to purchase a lifetime license for the software.  I'm glad I did because at the time of writing, the lifetime licenses are no longer available and it is now an annual subscription.

Sharpcap has a great set of tools to help an amateur astrophotographers get the best out of their equipment.  It's constantly under development and works really well with the Altair range of cameras.

Sensor Analysis

Every camera is different.  It's my understanding that even if you get two camera's the same, the sensors in them may not be absolutely identical.  To be honest, it's slightly irrelevant in my case.  I have a new and different camera, and to make the most out of it, and Sharpcap's features, I needed to run a sensor analysis on it.  For all the complexity in the maths and bits and bobs that happen as part of the analysis, it is incredibly easy to do.  But why do we need to do it?  Running a sensor analysis helps Sharpcap learn all about the sensor in your specific camera.  It takes a wide variety of measurements and metrics to eventually produce an analysis of noise and gain measurements for the sensor in your camera.  

Available via the Tools menu in Sharpcap, it's best practice to run the analysis twice.  Once in RAW8 and once in RAW16.  This helps give Sharpcap the full range of data needed for the analysis.  It's worth noting that the analysis needs ideally to be carried out during the day, or, at least with an artificial light source at the end of the telescope.  In my case, I had a nice day to do it, so I just put a white t shirt over the end of the telescope and started the analysis.  I've even read of people doing the sensor analysis before putting the camera on the telescope.  It doesn't matter which way you do it.  The important thing to remember is that it is the sensor that is being analysed, so it doesn't matter if there is focus, or if the camera is in-situ or sitting on a table.  

The whole process is wizard driven and you will be prompted to expose the camera to light, then to cover up the sensor (or as I did, put the cap on the end of the telescope) etc.  At the end of it all, you get a nice pretty graph! 😁

Great, so I have a nice graph with some crosses on it.  How does that help me then?  Well, unless you're particularly geeky and strive to understand all the data, it doesn't really.  But what it does do is help Sharpcap help you further down the line.  As I said earlier, you only need to run the analysis process twice.  Once in RAW8 and once in RAW16 in my case.  Sharpcap then saves the analysis data in some config files on your computer and refers back to them later.  For now, that is all I need.

Focus, Focus, Focus

It's night time, and the stars are out.  It's time to take another step close to that all important first imaging run, or, First Light.  As yet, I don't have an automatic method of focussing, so I need to do it manually at the start of each imaging run.  To help me, I use a bhatinov mask, which is quite a low tech and easy way of focussing.  The mask consists of a piece of plastic which fits over the end dew shield of the telescope.  In the plastic, there are a series of strips cut out of it.  Once that is in place, I slewed the telescope to point to a reasonably bright star in the vicinity of the target I want to image.  

The Bhatinov Mask

When the target star is in sight, it's time to crank up the gain and set exposure time to a few seconds a frame in Sharpcap.  Once the mask is in place, I zoom in to around 150% in the live view.  The star when viewed through the bhatinov mask appears as a set of straight lines.  "How can you tell that you have a good focus?" I hear you say.  Well, using the focus knob on the telescope, the trick is to get the lines in the image equally spaced.  If you are out of focus, the line striking through the middle of the star will appear to be over to one side (left or right).  If you have good focus, it will appear to strike directly through the middle of the star like the image below.  Once good focus is achieved, the focus knob on the telescope is locked into place.  Job done!

The star light shining through the slits in the Bhatinov mask gives this set of cross hairs which can be used to achieve a good level of focus.

The Sharpcap Smart Histogram

So now I have my camera in place, an analysed sensor and a good focus.  Time to go imaging?  Not quite yet.  But this is where Sharpcap steps up and provides even more help.  In forums up and down internet land and in social media groups, I often see people asking questions along the line of "I've just bought a lovely brand new camera and put it on my telescope and it looks lovely.  Can someone tell me the best settings I need to use so I can do my imaging?"  Well, if there was ever a single answer to that question, everyone would know it, and they would be set as default behind what I imagine would be a big green button that says "Start imaging now" around it.  No need for any other settings or sliders or boxes.  Just one button.

Of course, it's not that simple and the full answer to that question is well beyond my comprehension.  As an amateur astronomer, I want to understand things, but also, help to get things right.  Sharpcap's Smart Histogram is the tool I turned to to help with this conundrum.  It takes the guess work out of imaging.

It's a "Pro" feature in Sharpcap meaning that you need to have a license for it, but this alone is probably worth the license fee in my opinion.  The Smart Histogram tool is opened from the Tools drop down menu.  Once you can see the histogram, there is a "brain" button to the top left.  This is where the magic happens!.  All I needed to do in this tool is tell Sharpcap how much imaging time I have, the shortest exposures I am willing to use, and the longest exposures I am willing to use.  That's more or less it.  Then it's just a case of setting the process off.  Using the sensor analysis data obtained earlier, Sharpcap then works through a whole process of scripted changes of gain, exposure length, etc.  You just leave it to do it's thing.  The whole process can take from just a couple of minutes through to 10 or 15 minutes or longer, depending on the parameters you enter at the start of the process.

For me, I try to keep my imaging runs to around 3 hours.  That gives me plenty of time in the winter to do one, two or even 3 targets in a night, but also squeeze in a session in late spring before we lose astronomical darkness.  So with that set, Sharpcap took it's measurements and made it's recommendations.  At the end of that process, it gives you a nice "Apply" button which when pressed, put all the recommendations into place.  How easy is that!? 😄

Sharpcap's calculations gave me Exposure, Analogue Gain, Auto Exp Target and Black Level recommendations.

That brings me to the end of part 2 of this mini series.  At last, I have everything I need to start taking my first image.  Thanks for reading.  In the third and final part of this mini series, I'll share some surprising results.  Clear skies!