Accurate framing for astrophotography

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anguslau
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Accurate framing for astrophotography

文章 anguslau » 週三 21 10月, 2009 16:07

There are at least two important reasons to achieve accurate framing for astrophotography:
1) to obtain an optimal composition for aesthetic reasons
2) to minimize wastage of valuable image area while stacking a larger number of frames, particularly when some of these are taken over multiple sessions

To achieve accurate framing via trial-an-error is a very time consuming and frustrating process. It also wastes much of our valuable imaging time under a dark sky. Fortunately, with the help of some widely used software, highly accurate framing is not difficult to obtain with moderate effort. I am sure there are many different ways to do it. Just want to share with you how I do it at present with my tools on hand. Hope this may be useful to some of you.

In order to maximize our imaging opportunities, it is best to plan our imaging sessions before hand at home instead of under the dark sky. Here are the steps I followed:
1) identify an object to image
2) determine equipment (scope, camera, reducer/extender etc) to use
3) create FOV indicator in TheSky
4) center the object within the FOV
5) drag FOV indicator around until desired framing is achieved; more on this below
6) at the center of the FOV indicator, ctrl-click the screen to create a user-defined reference point
7) press Alt-U to bring up the user-defined data dialog box
8 ) check the coordinates and other settings and adjust if necessary; enter a meaningful label for the marker

During the imaging session, I follow the steps below:
1) firstly slew the scope to near the desired object
2) entered the correct parameters for the scope and the camera in MaxIm and connect MaxIm to the scope; this will ensure that the correct coordinates and image scale will be attached to all images captured
3) using MaxIm, capture a quick image of the sky; use several seconds exposures such that some stars can be clearly identified
4) now select Analyze-Pinpoint in MaxIm to solve the exact coordinates that the scope is actually pointing at
5) go to the Telescope tab in MaxIm and sync the mount with the solved coordinates
6) press Alt-U in TheSky to bring up the user-defined data dialog box; select the appropriate reference point saved above; click the center button on the dialog box
7) the screen will now be centered on the reference point; right click on the reference point and select Slew; the scope will be slewed to the reference point, accurately framing your object as desired

It is sometimes very difficult to determine the appropriate framing for an object because the atlas on the screen may not show the deep sky object. For example, we may be working in blind trying to frame the Barnard’s Loop on the star atlas. One way is to import a DSS image and superimpose it on the screen. Apart from being an expansive option which needs to be purchased separately, the quality of some DDS images are not very high. It would be much better to search for an appropriate image on the web for the purpose.

After an appropriate image has been downloaded from the web, there are several things we need to know in order to be able to put it on the screen:
1) the center coordinate of the image; in TheSky, this must be the same as the linked object
2) the image scale
3) the rotation angle of the image

Unfortunately there is no one-click utility for you to determine these. Here is a not too complicated method I used:
1) display the atlas on the screen with the object centered and a FOV indicator displayed, centering on the object; the FOV indicator should be sized to include the entire web image to be displayed
2) zoom in or out the screen to ensure that the FOV indicator is quite large appearing on the screen and includes a reasonable amount of bright stars within it
3) press Alt-PrtScr to capture an image of the screen
4) Go to Photoshop and paste the captured screen as a new document; crop the image so that only the FOV indicator rectangle is left; save the image
5) Bring up RegiStar and register the web image to the FOV indicator screen capture; the resultant image should be “match” of the web image to the FOV; save the resultant image
6) With the knowledge of the image size of the FOV image and its true FOV, we can calculate the image scale; the resultant image will also have the same image scale
7) The rotation angle of the resultant image will be the same as the FOV indicator, which we can conveniently set to 0
8 ) Now we can add the image to TheSky using the Data->Image Manager function.
9) We can further fine tune the positioning and the transparency of the image via the “Place Image” tab

With an object image superimposed on the atlas, I can make much better framing for my imaging. The above procedures assumed TheSky, MaxIm, and RegiStar. These are the tools I currently use. It should not be difficult to adapt the procedures to other tools if you happen to use something else.

For instance, I have used a slightly different procedure to import an image into SkyTools, which assisted me in framing the Barnard’s Loop.
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Wah!
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文章 Wah! » 週三 21 10月, 2009 16:19

文章已置頂 :)

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noodle
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文章 noodle » 週三 21 10月, 2009 17:23

非常有用 ,多謝分享 [跪拜禮] [跪拜禮] [跪拜禮]

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MANDII
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文章 MANDII » 週三 21 10月, 2009 17:27

Thank you !
Roughly can understand it .

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sn-10
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文章 sn-10 » 週三 21 10月, 2009 17:34

I would like to add one missing point, use binning to increase sensivity (thus decrease exposuretime) when framing. I usually do 4x4 binning or even 6x6 for very faint objects.

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