DRIZZLE ALIGNMENT Last updated: 17 April 2007

Subject:	Drizzle Alignment: an update
Sent:	Tuesday, April 17, 2007 11:14:12
From:	Ted Rafferty (raff650@erols.com)
 
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 (raff650@erols.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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