Re: [APML] How to build a mosaic?

["Bobby Middleton" <bobm@koyote.com>]

Hi Nicola.
Mosaics can be a super way of showing a larger piece of the sky, but there
are many problem associated with doing them. You have just touched upon two
of the problems.
The first problem is the color balance (and also differences in brightness
which are similarly fixed). It is best to shoot the adjacent frames while
they are close to the zenith. Use the same film and try to do the different
frames the same night. This will minimize any color shifts due to elevation,
light pollution, and other variables. But there will still be some
differences in color and brightness. These have to be equalized as best as
possible in Photoshop using curves and levels adjustments. I adjust the
frames before I combine them. After combining it is still possible to
equalize the different frames by using selective adjustments and feathering
with the lasso tool. The seems may still need blending with the clone tool.
The second problem is harder to deal with and had up to a point limited the
size of my mosaics. That is the problem of curved fields which will cause
different frames to warp when joined. Axel Mellinger has a program that he
used to fix this for his milky way mosaic, but I have since learned that the
different frames can be aligned in programs like RegiStar on top of a single
wide-field image of the area to be mosaiced. This is what I now do and the
technique has allowed me to add as many as ten frames from a 2200mm focal
length newtonian without any major problems from frame warping.
With constellation mosaics you may have difficulty in getting a wide-field
image to use for registration. Perhaps there are some large-sized web images
that might be used with the author's permission to align your different
frames. And it may be possible to use certain star mapping programs to make
an image to register your frames with.
Here is my 10 frame Rosette mosaic:
http://www.koyote.com/users/bobm/rosette2.htm
Good luck! Mosaics are difficult but a good one is well worth the effort.
Bobby Middleton

----- Original Message -----
From: "Nicola Montecchiari" <nmontec@libero.it>
To: <astro-photo@seds.org>
Sent: Thursday, September 26, 2002 2:11 PM
Subject: [APML] How to build a mosaic?


> Hello all,
>
> I've got some mily way shots I would like to put together in a mosaic of
> pictures.
> The problem is: the colours are different from one shot to another and
with
> photoshop it seems impossible to mathc them, or at least i don't know how
to
> do that.
> And also, stars look like they are in slightly different position like it
is
> not possible to overlap them.
> Is there any program that cna help me with all those problems above? Or a
> web page where it is explained how to do it?
>
> Thank a lot!
> Nicola
>
>
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