Last updated: 17 April 2007
Subject: Drizzle Alignment: an update Sent: Tuesday, April 17, 2007 11:14:12 From: Ted Rafferty (firstname.lastname@example.org) Drizzle Alignment: an update Back on September 28, 2006, I wrote a description of Drizzle Alignment using the Meade AutoStar Image Processor (following this update). In December 2006 I replaced my color DSI with a DSI Pro II and have found that the frames can be enlarged even more for the Drizzle alignment. In the case of the color DSI, I doubled the frame size from 648 by 488 to 1296 by 976 before doing a two-star alignment. For the DSI Pro II, I found that the frame size could be tripled and still yield good results. The following three images all used the same eleven 2-minute exposure frames of M51 taken on April 13, 2007 using a Meade LXD75 8-inch f/4 SN. Using the original sized 748 by 577 frames and doing a two-star alignment with the Meade Image Processor, I came up with the following image (which was cropped to show an enlarged section): On the above image, you can see the pixel structure on each of the star images. Doubling the frame sizes to 1496 by 1154 (using the "Resample" function to change the sizes of the frames) and then doing a two-star alignment yields the following: The pixel structure on each star is reduced, but less apparent than on the first image. By tripling the frame size to 2244 by 1731 and doing a two-star alignment, the following image is produced: Now the pixel structure is no longer apparent in the stars. Ted Rafferty Gaithersburg, Maryland
Sent: Thursday, September 28, 2006 08:53:30 From: Ted Rafferty (email@example.com) Drizzle Alignment Meade has made a process called "Drizzle" one of the main features of its AutoStar Suite Update 4. Drizzle is a process developed for the Hubble Space Telescope to improve its resolution and is likely one of the most useful tools for improving the quality of astro-imaging. Drizzle is a rather simple form of aligning frames, though Meade has grouped it with a method of collecting the frames, which might cause some to be unaware of its benefits. Here I will only describe the Drizzle alignment method. When doing the common form of aligning, frames that are, for example, 648 by 488 pixels in size, can only be shifted in units of a full pixel because the reference array is also 648 by 488 pixels in size. If a "one-star alignment" is used, the shifts are in the only x and y directions. If a "two-star alignment" is used, besides shifts in the x and y directions, a rotation adjustment can also be made. But still the adjustments can only be done in units of a full pixel. The first step of the Drizzle process is to enlarge the original frames, often doubling the size, for example, from an array of 648 by 488 to 1296 by 976. This change of size does nothing to improve the appearance of the original frames. The only difference is that each pixel of the original frames is an area of two pixels by two pixels on the enlarged frames. When the enlarged frames are aligned, the frames can, once again, only be shifted in units of a full pixel as done with the common form of alignment, but the full pixel shift of the enlarge frames is at the half pixel level of the original frames. As a result, this causing an improvement of the resolution when the frames are combined. But to take full advantage of the Drizzle alignment, the stars in each of the frames need to be shifted slightly so they aren't covering the exact same pixels on each frame. If the stars on each frame covered the exact same pixels on each frame, no alignment is necessary and the Drizzle process would provide no improvement. In most cases, having slight shifts between frames is the norm. The periodic error in the RA gearing causes some shift, even when using an auto guider. Even the flexure between the primary scope and the guide scope can introduce a desirable shift in a long series of frames that the Drizzle process can take advantage of. And if one finds that these accidental shifts are not sufficient, the telescope could be moved slightly between exposures. Here are examples of combined images using the same eleven of frames of M51, with one using the common form of alignment and the other the Drizzle alignment. The images have been cropped and enlarged to better show their differences. Combined image from eleven 648 by 488 frames using a two-star alignment: Combined image from the same eleven frames except they were enlarged to 1296 by 976 before using a two-star alignment (i.e. Drizzle): Here is a combined image of the eleven frames with no alignment to show the amount of shift in the series of frames: Each frame was a 4-minute exposure, and the shift shown is most likely due to flexure between the primary telescope and the guide scope over the 44-minute period it took to take the eleven frames. I used the Meade AutoStar Suite Image Processor to create these images (using the "Resample" function to change the sizes of the frames), but the Drizzle method could be done using other image processors provided they have the means to change the sizes of the frames and can do a two-star alignment. Ted Rafferty Gaithersburg, Maryland
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