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Afocal Astrophotography Tutorial

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george7378    36

I thought I'd make a little tutorial on how I capture and process pictures of the planets and the Moon afocally, so others can hopefully learn from it. This isn't a 'best way to capture afocal images' tutorial, because I am not really an expert, and I don't know what most of the tools and buttons in the processing programs do!!

This tutorial should hopefully let you get from this video frame:


...to this processed and stacked image:


Anyway, here we go.


- Sky-Watcher 250PX telescope (but any scope will do)

- Panasonic Lumix DMC-ZX1 compact camera (again, any other compact will be OK as long as it can capture video)

- Afocal bracket (there are a lot of these available - mine is a PH047 standard mount)

- Computer


- Registax (I used version 5 - RegiStax Download)

- RAD video tools (http://www.radgametools.com/down/Bink/RADTools.exe)

- GIMP (GIMP - Downloads)


OK, now we should be ready to take a video of the object you are imaging. This is why the technique will not work for dim objects - your camera has to be able to show it up on film. If you are filming a planet, the best way to capture the 10 or so seconds of video you need (assuming you don't have a tracking mount - if you do, you can take more) is to align the telescope with the planet, then attach the camera so that it is zoomed in sufficiently to see the details you want to capture. I would recommend using a wide field eyepiece, and using the camera's optical zoom to get closer.

Once you are ready, start filming when the planet is just entering the field of view. Let it drift naturally across, and stop filming when it leaves the eyepiece. You don't even need to try and manually track it - just try to get it nice and focussed and leave it to naturally drift through the view, so that it is nice and steady. Congrats, you now have your video!

If you are filming the Moon, do the same, but you are going to have to try and push the scope to track the Moon in a basic way, or you will not have the same features in all your video frames. I just push the scope until about 10 seconds has passed, then stop filming.

OK, you should now have a video file on your camera, which shows the object in a clear, focussed image, for about 10 seconds. In my camera, this gives us 300 frames to work with.

Processing - choosing the final cut:

Now comes the first part of processing. Choosing the frames you want to use.

To do this, put the video file somewhere on your computer, and then open it up to watch it. Before you convert it, you will have to choose the part of it you want to put forward into processing. My videos are typically a couple of minutes long, and I will let the object pass through the view about 10 times, just to experiment. Then, I view the video, and choose the best 10 seconds of it to convert.

To do this, all you have to do is find the best bits using your video player program, then calculate the frames that you need. This is simple - for example, if the best 10 seconds of my video are from 1:10 to 1:20, and my camera's frame rate is 30fps, then my start frame will be (70 seconds x 30 fps) = 2100 and the end frame will be (80 seconds x 30 fps) = 2400. Keep these values for later, as we will need to enter them into the convenient RAD video tools program.

Processing - conversion:

Now that you have the numerical locations of your bit of film, open RAD video tools. We will convert the relevant parts of the movie file on your computer into uncompressed frames of AVI video, for the Registax program.

You will see a navigator window - navigate to the video file on your computer, and then single click to highlight it. Then press 'convert a file', and you should see a new window. All you need to do here is find the 'Frame range' boxes, and enter your start and end values from previously into the 'start' and 'end' boxes. Then, press the 'Output Type' button at the top, and choose 'AVI'. Then press 'convert'. Finally, in the box that comes up, choose 'Full frames (Uncompressed)' and press OK.

Your conversion will finish, and you now have a Registax ready AVI file of your requested frames!

Processing - Registax

Now open up Registax. Then, at the top, choose 'Select' and import your AVI video file. Now comes the important part. You should see your object in the window, and when you move your mouse over it, a box should surround your pointer. This is the 'alignbox', and you now have some options. You need to choose the right size of alignbox using the 'alighbox size' options on the left. You can use 'user set alignbox' to choose a custom size:

- For a planet, choose a size that completely surrounds the planet, so that none of the disc is outside the box.

- For the Moon, select a prominent feature like a crater or sea, and choose a box that fits around that object.

Now, move the alignbox over the desired object, and left click to place it.

Now that the alignment feature is selected, you can tell it to choose the best X% frames using the 'Lowest quality' control on the left. I usually use 75 or 80. Doesn't really matter.

Now press 'Align' and let it finish.

When it is 100% aligned, press 'Limit' and then 'Optimize and stack'. Now all you have to do is sit back and let it do the work. When the final stacking is complete (you can follow its progress using the bar at the bottom left) you will be presented with a (probably blurred) image. Use the layer sliders on the left to experiment until the detail has come out, but be careful about the noise - don't pull them too far.

When you are happy with it (don't overdo the processing here - you will use GIMP to add the final touches like colouration, sharpness, etc... - all you need to edit here is the sliders, and possibly the 'noise reduction' option on the right), you can press 'do all', and then 'save' and save your master image file. I save it as a PNG because this is a nice flexible format with high quality.

Processing - GIMP:

Now you can open GIMP and load your image into it. You are now presented with a lot of processing tools. I don't even know what most of them do, so I can only recommend these:

- Colours --> Curves. This is a fantastic way to bring out colour and detail. Just slide the curve around and watch your image change.

- Colours --> Levels. Similar to Curves, but you can edit the colours in different ways. I usually use the levels sliders after 'curves' to get the final touches right.

- Colours --> Brightness/Contrast. The Contrast slider is worth a play around with.

- Colours --> Colour balance. Pull the 3 colour sliders about until your colouration looks a bit nicer.

- Filters --> Enhance --> Sharpen. Great on Moon pictures.

Just have a play around, and see what you get. The final quality and result is subjective, so don't take my word for what to do.

You can now save your final image, and it should hopefully look a lot nicer and sharper than the single frames or images taken with a compact camera.

I'd love to see your results and efforts, so please let me know how you get on!

All of the close-up images on the site gkastro.tk were taken using this technique - take a look at the ones in the 'Moon' section of astro-photography for another flavour of what Registax can do.


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JohnnyD333    81

Great tutorial, I would just add that the bracket is pretty important because in the freezing cold it is hard to keep the camera steady if you hold it up to the lens. Also, seems people (along with myself) are having issues with Registax Version 6 so stick with version 5 as you mentioned until it gets fixed.

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