AstroMatt - Capturing and Processing Mosaics

Capturing and Processing Mosaics

Part I
The Plan

Part V
Assembling the Mosaic in Photoshop

Part I - The Plan

The first thing that I do is figure out what area of the sky I want to image and what I'm going to image with. Based on the field-of-view (FOV) of the telescope and camera, I compute how many frames I will need to cover the area of sky I'm interested in. It is possible to do this by hand, but TheSky offers a great tool in planning a mosaic.

I use TheSky6 to plan the mosaic. I already have a field-of-view indicator (FOVI) setup in TheSky that matches my camera and telescope combination. I have confirmed the FOVI parameters by pasting a full-frame image into TheSky and executing an Image Link. With both the FOVI and the image visible, it is easy to see if the FOVI needs adjustment.

The first thing to do is center the area of interest on the virtual sky. TheSky will use the center of your screen as the center of your mosaic. You can center the mosaic on any set of coordinates, but it is simpler to center it on the virtual sky.

Now launch TheSky's mosaic tool by selecting the Tools->Mosaic menu.

TheSky's mosaic tool is very easy to use. I enter the parameters in this window from the bottom up.

First I select my defined FOVI from the drop-down list and I click the "Get Geometry..." button. TheSky fills the X, Y, and Position angle fields from this information. Be careful here. If the FOVI does not match the actual rotation of the camera on the telescope, the Mosaic function will not work properly. An Image Link will set the Position angle correctly.

Next click the "Get From Virtual Sky" button to pull the center coordinates into the window. Of course you can enter these manually if you desire, but centering the target area and clicking a button is a lot easier!

Finally enter the desired size of the mosaic. You might need to experiment around with the number of rows and columns you need to cover your target. Enter a good guess for starters.

The value for "Percent overlap" is important. It should be large enough that you have a significant enough overlap between each frame, and small enough so that you are not wasting time with too many panels. The amount of overlap will also affect how you can process your images. If you don't have enough overlap, you will not be able to generate a smooth transition between frames. So, as a general rule, I've found that having 100-200 pixels is enough to allow for a smooth transition between frames. For my camera 10% is 120 or 160 pixels. Start with a similar overlap for your first mosaic. Wait to play with this value until after you have completed processing your first mosaic.

Finally check the "Visible" check box and click the "Apply" button. This will create your defined mosaic on the virtual sky.

If something isn't quite how you like it, you can then adjust the settings and click the "Apply" button again. If the mosaic doesn't cover your entire target, change the size and try again. If you need to reposition the mosaic, move the virtual sky around and click the "Get From Virtual Sky" again.

Once you are happy with the layout of the mosaic, click the "Close" button.

Now lets examine what TheSky created for us.

Above you can see my recent 9-frame mosaic of the Virgo Cluster. Notice there is a red caption in the middle of each of these frames. The captions read "A1" through "A9". TheSky assigns a tag to each frame and makes each frame a click-able object. Click on one of the tags for your mosaic.

You can see I clicked on the "A5" tag. Notice the title, "Mosaic A5". This is now a defined object in TheSky. You can Go To this object. You can search for this object. You can do just about anything you can do with a star, a galaxy, or a planet in TheSky. This can be very important during your acquisition!

If you are using a self-guided camera, the last thing to consider in the planning stage is your guide star availability.

Check each frame to ensure that you will be able to get a suitable guide star. If there are only dim stars available for a particular frame, check for 180 degrees opposite rotation. If there are still only dim stars available, strongly consider using longer guide exposures. Ideally you do not want to deviate from the planned mosaic positions. The more you deviate from the planned positions, the more difficult imaging the mosaic will be.

If you are self-guiding, I believe the best approach is to increase your guide exposure until one of the stars in the FOV is suitable to guide on.

Previous: Introduction

Next: Part II - Acquisition

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