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<p>Thanks for the feedback Warren,</p>
<p>I'm a little unclear on your concern about the usefulness of the
calculator. But a large part of Dr Glover's presentations seems
to be directed to getting astro-photographers to consider using
shorter sub-exposures and larger stacks. <br>
</p>
<p>If you believe this to be incorrect, or less than "optimal",
maybe we can work together to come up with an alternate
user-selectable calculation model that can be added to this tool.
I would just need this to be described in such a way that I can
implement.</p>
<p>I'd also be curious to see if folks would run this calculator to
compare their experiences. <br>
</p>
<p>Here's a process that might be helpful to determine the value of
the calculator.<br>
</p>
<p>Pick one of your images, or just a channel used in an image that
you consider to be good quality. <br>
</p>
<p>Set up the calculator with equipment, the conditions and the gain
setting that you used for the imaging. <br>
</p>
<p>Try to adjust the noise increase % so that calculator exposure
time is close to the sub-exposure that you used for the image. (It
might be tough to get a perfect match, close is good enough).<br>
</p>
<p>How where did the noise increase % value end up? Very far from
the default 5%?<br>
</p>
<p>Look at the stack grid to find the closest exposure count to what
you used in the stack. What is the Ratio on that line?<br>
</p>
<p>---</p>
<p>So here's a long description and details from my learning
experience a few years ago that lead to my research into
sub-exposure calculations. (Keep in mind that I still consider
myself to be very much a novice in this hobby.) <br>
</p>
<p>As I was first learning in my backyard using a one shot color
ASI-071MC, with an f/5.5 refractor. (I typically set the camera
gain at 50). I tried imaging at the 3 to 4 minute exposure times
that I saw recommended on forums. The results were awful, and
very noisy. I then purchased both a multi-band filter (Optolong
l-Enhance for nebulae) and a light pollution filter (Optolong
l-Pro for galaxies). But even after weeks of trial and error, I
found that using the l-Pro filter for example, I still had to
reduce my exposure times to about 60 seconds with no moon, and
about 30 seconds with a 1/2 moon. In these conditions, to get an
image that I considered acceptable required about 6 hours for the
stack.<br>
</p>
<p>There's a darker site in the mountains about 90 minutes drive
from my home. I only make that trip around a new moon. My trial
and error process there included exposures up to 10 minutes, but
even at 5 to 6 minute subs there was excess noise. I settled on
exposures that were 3 to 4 minutes; and I could get a result that
was good enough to show to my friends and family, on a stack with
just 2 to 3 hours of imaging. <br>
</p>
<p>This experience triggered the research which lead me to Dr
Glover's presentations. I used Dr Glover's equations initially on
spreadsheet and later in a Java app. The sub-exposure time from
those computations matched my experience fairly closely. <br>
</p>
<p>I've since measured the SQM in my backyard on a new moon night as
19.3, and about 18.5 with a half moon. So lets look at what the
calculation says for these conditions using the l-Pro filter (I
estimate that the l-Pro is passing about 165nm), and I'm leaving
the noise increase % at the default 5% recommended by Dr Glover:<br>
</p>
<p>In my backyard with a new moon, the calculated exposure is 69
seconds; just slightly higher than the 60 seconds I found with
trial and error in these conditions. <br>
</p>
<p><img src="cid:part1.5OMzf1vV.Bp4vuyzP@sbcglobal.net" alt=""></p>
<p>Back then I was still employed with limited available time, so I
had been limiting my stacks to what I could get in a single
night. With 6 hours of imaging the calculator shows a ratio
(quality) of about 80. That a ratio of 80 was good enough for me
to share with my friends and family. But in looking at the
stacking data I see that the quality is still climbing well; going
to a 7th hour would improve the quality by 8%, that might have
have been worth doing. <br>
</p>
<p>But at some point we have to weigh the cost in time vs benefits
of longer stacks. The quality improvement at 20 to 21 hours is
not so great; the gain in quality would only be 2.4% for that
added hour, and it would be a stack of nearly 1100 images. <br>
</p>
Then with a half-moon in my backyard: the calculated exposure
matches the 30 seconds that I found I needed with trial and error.
But to be honest I was never able to get a very good galaxy image
around a half-moon from my backyard. But now it's clear from the
calculator that I would need about 14 hours in these conditions to
reach a ratio of just 80. <br>
<p> <img src="cid:part2.UAO6NQE9.9DyZ6FBz@sbcglobal.net" alt=""></p>
<p>Now at the darker site near my home, (I've not yet measured the
SQM at this site, but a light pollution map says it is 20.5):</p>
<p>The calculation shows a sub-exposure of 221 seconds, that is
right in the middle of the 3 to 4 minute range I found with trial
and error. And with just 3 hours stacking the ratio (quality)
shows 101. I was really happy with images from that site with
just 3 hours of stacking.<br>
</p>
<p><img src="cid:part3.GixN0Ecz.VAIDFdV8@sbcglobal.net" alt=""></p>
<p>So let's run one more calculation for a very dark sky, SQM 21.96
right on the margin of Bortle 1 & 2. <br>
</p>
<p>I have not yet experienced such a site, so I cannot make any
comments about the calculator's result. But it is showing an
optimal sub-exposure of about 14 minutes. It also shows that a
stack of just 1 hour, (5 exposures), would easily exceed the
quality that I find acceptable to share with my friends and
family.</p>
<p><img src="cid:part4.afu0RS8L.T6SsP0bY@sbcglobal.net" alt=""></p>
<p>I also think it's very interesting to see that quality
improvement of adding just a second hour in these conditions; a
34% improvement in quality to go from 1 to 2 hours of imaging in
these conditions! But the diminishing improvements of larger
stack are still evident; at the 20 to 21 hour time-frame the
quality improvement is only 2.2%, (but that is at a ratio of over
500, so it mat not be possible to recognize any noise in this
image). <br>
</p>
<p><br>
</p>
On 5/18/23 13:28, Warren wrote:<br>
<blockquote type="cite"
cite="mid:CALeJgM98Z_Yo_KJuanbX5D_+tJx0PEaX3xsu_0Yv=BHyo8T1Xw@mail.gmail.com">
<meta http-equiv="content-type" content="text/html; charset=UTF-8">
<div dir="auto">I think a fundamental problem with this approach
is that it tells you the *minimum* acceptable exposure duration,
which is long enough for some other noise source (likely
skyglow) to greatly exceed your sensor’s read noise.</div>
<div dir="auto"><br>
</div>
<div dir="auto">This is useful information, but mostly when you’re
shooting from a Bortle 1-2, where your sensor’s read noise is
potentially the limiting noise source — where 60 minute
narrowband subs make sense.</div>
<div dir="auto"><br>
</div>
<div dir="auto">For folks in urban and suburban environments, with
modern low-noise cameras, any realistic exposure duration (e.g.
60-300 seconds) is sufficient for skyglow shot noise to greatly
exceed sensor read noise.</div>
<div><br>
<div class="gmail_quote">
<div dir="ltr" class="gmail_attr">On Wed, May 17, 2023 at
10:55 PM Hy Murveit <<a href="mailto:murveit@gmail.com"
moz-do-not-send="true" class="moz-txt-link-freetext">murveit@gmail.com</a>>
wrote:<br>
</div>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">
<div dir="ltr">Joseph,
<div><br>
</div>
<div>Thanks so much for getting the exposure calculator up
and running in KStars. Impressive accomplishment!</div>
<div><br>
</div>
<div>I just tried using it, and have some
questions/comments I was hoping you could address.</div>
<div><br>
</div>
<div>Here's a screenshot, with questions below:</div>
<div><img src="cid:part5.WSNtRTen.iess0jGq@sbcglobal.net"
alt="Screenshot 2023-05-17 at 10.13.06 PM.png"
style="width:683px;max-width:100%" class=""></div>
<div>
<ul>
<li>I think I filled in the boxes appropriately above,
though not sure, please let me know. I tried these
values: sky quality 19 (about what I've measured at
my house), f/8 reflector, full bandwidth (300nm), my
ZWO ASI1600mm camera at gain 75 (I assume it wants
the gain I use for the 1600, but I tried other
values too), 20 total hours of exposure time
desired, default noise increase of 5%. It seems to
be telling me to take 5956 images each 12.09 seconds
long, which is obviously not a good answer. Am I
doing something wrong?</li>
<li>Not sure what Stack Time, Stack Noise, and Ratio
mean. Are shot noise and total noise in electrons?
(Need tooltips to help)</li>
<li>I was able to get it to give me a reasonable
exposure time (e.g. about a 2-minutes) if I set
Noise Increase % to 0.4, but I really didn't know
what to put in there, and so used the default was
5%. Do you know, is 5% a good default for the noise
increase? Can we give more guidance on what noise
increase people should start with?</li>
<li>The tool needs better tooltips for pretty much
each value that needs to be entered. Most tooltips
say "An implementation of Dr Robin Glover's exposure
calculation." We can give credit elsewhere (e.g.
usually done in "About KStars"), but the tooltips
should be informative. For instance, is gain the
actual gain values one enters for the camera, or do
you mean something like quantum efficiency? Assuming
it's the value entered to the camera's driver, you
should say that "Gain value used for your camera".
Filter Bandwidth should include units (e.g. nm in
this case.). </li>
<li>Don't need 3 decimal places for Sky Quality (make
it one or two decimals). Ditto for focal ratio. </li>
<li>Is there some documentation on use somewhere? E.g.
can a section be added to the handbook? Also,
please start a forum thread describing this new tool
and how you recommend users use it.</li>
</ul>
</div>
<div>Thanks again,</div>
<div>Hy</div>
<div><br>
</div>
<div><br>
</div>
</div>
</blockquote>
</div>
</div>
</blockquote>
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