Meade DSI: Deep Sky Astrophotos “Your First Night

Transcription

Copyright (C) 2004 Cloudy Nights Telescope Reviews
Meade DSI: Deep Sky Astrophotos “Your First Night Out”?
by Suk Lee
An imaging revolution from Meade?
“Take Pictures Just Like These Your First Night Out” proclaims Meade’s beautiful full-page color ads
for their new Deep Sky Imager (DSI) CCD camera. The ads show impressive pictures of M20, M51,
NGC891, etc, all for the VERY tempting price of only $299. The fine print below the pictures says “All
images were taken with a Meade 8” LX200 GPS using Meade’s Deep Sky Imager.”
How capable is the camera and can you really accomplish picture like those the first night out? I’ll cut
the suspense – the answer is definitely “yes”, but there’s a little fine print of my own. Read on…
What's in the box?
The DSI comes with (clockwise from top left):
- DSI camera body
- RJ to DB9 adapter
- eyepiece parfocalizing ring
- Autostar Suite CDROM
- Quickstart instructions
- 4 wire RJ cable
- 6 wire RJ cable
- USB cable
Everything you need to use the DSI is supplied in the
box, including a built-in IR filter. The camera body is a
nicely machined block of metal with a standard 1.25”
nosepiece, threaded for 1.25” filters.
The nosepiece unscrews to reveal the unusual IR
filter sitting below the nosepiece. Unlike a regular
1.25” or 2” filter, it’s a flat disk of glass with a
rectangular IR block area that fits over the CCD
sensor. The quickstart instructions warn to remove
the nosepiece carefully, as the filter is loose. If you
plan on taking the filter in and out a lot, you might
want to invest in a regular 1.25” screw-in filter for the
front of the nosepiece.
The specifications say that the sensor is a “High
Sensitivity Sony Super HAD Color CCD”, and
specifies the pixels as 9.6x7.5 microns, in an array of
510 x 492 pixels. The use of the HAD CCD is good
news as they are known to have good sensitivity with low dark current (thermal noise). The pixel size
is larger than the ones in the ubiquitous Philips TouCam Pro, so it’s not the same sensor, although
from the same family of sensors.
Exposure time capability is listed as being from 1/10,000s to 1 hour.
One of the benefits of the Meade ground-up design, rather than modifying an existing webcam, is that
long-exposure control does not necessitate a separate parallel cable – everything is controlled
through one convenient USB cable.
Autostar Suite Overview
The Autostar Suite software package that comes with the DSI is a combination
Planetarium/Telescope Control/Imaging Acquisition/Image Processing system. As I do not have a
Meade telescope, I only evaluated Autostar Suite's image acquisition and processing capabilities.
Software installation is straightforward. Pay attention to the numerous warnings not to re-boot your
computer until the very end of the installation. Also noted in various on-line forums was some
difficulties with USB ports, etc. Immediately after installing the software (Dec 2004) I installed the
latest patch from Meade, obtained from their website, and experienced no problems.
After installation of the software you attach the camera to your USB port with the supplied cable
(which is a little on the short side) to complete the hardware driver install process. Launching the
software brings up the following screen:
Clicking on Image -> DSI Imaging brings up the image acquisition window:
Going clockwise from upper left the major functional areas are:
- camera controls for exposure and gain control
- image acquisition control area controlling how pictures are acquired and initially processed
- image preview/viewing screen
- Magic Eye Focus assist window
- image contrast/brightness & histogram window
Autostar Suite - Capturing your first terrestrial image
Acquainting yourself with the DSI and Autostar Suite is easiest in the daytime where you have a
bright stationary object. Select a terrestrial object far away enough from your scope that you can
reach focus and select an object that has a bright glint on it. A reflection off of a street lamp, for
example, is perfect. Focus with your eyepiece then remove the eyepiece and insert the DSI. They
likely will not be parfocal so expect to have to refocus. The purpose of selecting an object with a
bright glint is to be able to see something to focus on.
Make sure "LIVE" and “Terrestrial” are
selected and click on "Auto Adj" to get the
camera rolling. Autostar Suite will set the
exposure to 1 millisecond then gradually
increase exposure until you have a
reasonable image and then stop. This can
take a little time so be patient
Ignore the "Magic Eye" at this point and
manually focus until you have a crisp image. I
found the default image a little dim and
manually increased the exposure time until I
got a brighter picture. I also reduced the gain
so that the brighter areas of the picture didn’t
burn out.
At this point you’re set to head out under the night sky and focus on a star.
Autostar Suite Magic Eye focus assist
In my review of the Meade LPI, I covered the Magic Eye focus assist and didn’t find it to be useful,
recommending instead a Hartmann mask. Here’s the section on using a Hartmann mask from that
review. Note that the screen shots are slightly different because they’re showing the LPI window, but
the principles are the same.
Make
a
Hartmann
mask,
basically
an
aperture
cover with
two holes
cut in it.
As
you
can see
from the
picture it
doesn't
have to be
particularly
precise or
pretty. For
my larger
telescopes
I’ve made
permanent
masks out
of
foamcore.
Start up
Autostar
Suite and
get close
to focus
and then
put
the
mask on
your
telescope.
Instead of
one star
you now
have two.
As you get
closer to
focus the
images
will
approach
each
other.
As you get
closer to
focus the
images
will also
get
brighter,
making it
hard
to
judge
exactly
when they
overlap.
Manually
reduce
exposure
to reduce
the
brightness
of
the
images
and then
bring them
into
overlap.
You are in
focus.
Once you've achieve critical focus on a star you can swing over to your object of interest.
First Light – M1 with Mewlon 250 and 0.375x reducer
As the MSI is advertised as being suitable for an 8” SCT, I used a configuration which would give a
similar focal length. I selected M1 as a reasonable first small DSO. I used a Mewlon 250 (3000mm
prime focal length) and 0.375x reducer, the SBIG FR237. The FR237 is a 1.25” filter thread lens
which conveniently screws into the nosepiece of the DSI. Equivalent reducers designed specifically
for webcams are available from various manufacturers. The combination leads to a focal length of
1100mm, which is similar in focal length and therefore field-of-view to an 8” SCT with a 0.63x reducer.
Compare FOV with and without the reducer:
Step 1: Focus
I selected Capella as a nice bright star and focused. Note that the LIVE window is all that was
necessary with an exposure of 1 second, making it very easy to locate and focus on the star:
Step 2: Take dark frames
Every CCD camera generates spurious electrical signal due to heat in the CCD. Fortunately, the
accumulation of signal is very predictable, so it’s possible to take a set of “dark frames” (with the
telescope capped) which are just the spurious signal and then subtract the spurious signal out of your
images. In order for this process to work precisely, the CCD must be temperature controlled. The DSI
is not, however if the ambient temperature is fairly constant, then a good approximation can be made
by turning on the camera, letting it equalize to ambient temperature, then taking a set of dark frames
just before image acquisition.
In Autostar Suite select “Take Darks” under the “Image Process” pulldown:
The window to the right will change to show the range of darks that will be taken by Autostar Suite
and the total time, 8.5 minutes in this case. I left it to the defaults it came up with, and clicked on
“Start”.
The software then thoughtfully reminds you to cap your telescope and click “OK” before really starting
the sequence. Go have a cup of coffee…
When you come back, you’ll find that the “Dark Subtract” checkbox is now checked, and the program
is reminding you to uncap your telescope. You’re set up to take long exposure pictures with automatic
dark frame subtraction now:
Step 3: Frame M1 and set up a guidestar
Finding M1 will be the first real challenge for that “First Night Out”. Even with a 1 minute exposure,
M1 is essentially invisible. Plus, the field of view at this focal length is so small that star-hopping with
the camera will be very difficult. This is where GOTO becomes a real life-saver. Focusing on that
bright star also serves as a reference point for your GOTO telescope. I centered Capella as above,
then instructed my mount to swing to M1. I set an exposure of 1 minute and clicked on “Long exp”
and “Preview”. The Count Down window shows the time left in the exposure:
As you can see in the picture above, M1 is virtually invisible, but I’m trusting that it’s in there
somewhere. So, we’ll get ready to take a bunch of exposures and accumulate them.
Autostar Suite has a feature which they calling “Tracking” and which other manufacturers often call
“Track-and-accumulate”. In this mode a succession of pictures are taken, aligned in realtime, and
then summed in realtime. This feature allows you to take a set of pictures with the effective exposure
of one long exposure, but without autoguiding. If you telescope is accurately polar aligned, then short
exposures without autoguiding will be sharp enough to allow you to stack a bunch to increase the
overall effective exposure. In order to align the pictures, you have to identify a star that Autostar will
use to align successive pictures. Pick a bright star without any nearby neighbors to confuse the
program (easy in this case!) and draw a box around it with the mouse as shown above. Autostar Suite
will now use this star to align and accumulate frames.
Step 4: Start taking long exposures
OK, I’ll admit something, I cheated a little. I used an autoguider setup since I wanted to take long
exposures of M1 (5 min) and I had a feeling that my mount wouldn’t track accurately at 1100mm for 5
minutes. However, the test of Autostar Suite’s Track mode was still valid because there was still
some frame drift because I wasn’t autoguiding on a star very near M1. In other words, Autostar Suite
still had to align frames before accumulating them.
Here’s the setup I used:
1)
2)
3)
4)
5)
6)
7)
8)
Set up for 5 minute exposures
Make sure “Dark Subtract” is checked
Set up “Image Process” to “Deep Sky”
Set “Min Quality” to 0 to force Autostar Suite to select and stack every exposure
Set “Evaluation Count” to 1 for the same reason and make sure “Combine” is checked
Put in the object name
Click on “Save Proc…” to bring up the popup and
Set up to “Save Every Composite Image”
In normal operation, Autostar Suite will just save one composite image. However, since there are a lot
of airplanes in my area, I didn’t want to chance having an airplane fly through the frame and have its
trail saved into the image. When “Save Every Composite Image” mode is set, every successive
composite (e.g. 1, 1+2, 1+2+3, 1+2+3+4, etc) is saved, so at least I have the previous images if a
plane flies through the field-of-view.
Here’s the result after clicking on “Start”:
Note that M1 is now visible (GOTO rules!) and that Autostar Suite has drawn green crosshairs
marking the position of the guidestar.
Here’s the result of 13 exposures, 65 minutes of total exposure, straight out of Autostar Suite:
And here’s the result after some histogram tweaking, color adjust and slight color boost in Photoshop:
Pretty impressive for a $299 camera and with most of the work done by Autostar Suite!
Second Light – M42 with Megrez 80 and 0.63 focal reducer
At this time of the year, M42 is a great DSO object for initial imaging because it’s bright, large, easy to
find, and spectacular to image. Because the DSI has a small imaging chip, a short focal length would
be needed to get a decent field-of-view on M42. Doing the calculations indicated a desired focal
length of around 300mm. That’s a LOT shorter than an 8” SCT focal length of 2000mm, and shorter
than what can be achieved with an 8” SCT and 0.33x focal reducer (660mm). Fortunately, I had a
Megrez80 achro refractor and Celestron 0.63x reducer on hand, which gives almost exactly 300mm.
While the 0.63x isn’t designed for refractor applications, because it’s also designed to correct for the
field curvature of an SCT which is different than a short focal length refractor, the small image sensor
means it will work well.
I also had the “variable T to SCT adapter” from the Meade 0.33x focal reducer handy, which is
needed to get approximately the right spacing between the 0.63x reducer and the DSI. I Put it all
together, slap onto the Megrez80, focus on a bright star, then swing over to M42 and compose.
Again, a guidestar was selected and a series of pictures taken. Because the dynamic range of M42 is
so great , I took a series of photos of 30 seconds, 2 minutes, and 5 minutes exposure, planning on
compositing them together later.
Here are the results, straight out of Autostar Suite:
16 x 30seconds:
4 x 2 min:
4 x 5min:
As exposure time increases, more of the surrounding nebulosity is captured, but the central area is
burned out because it’s so much brighter than the surrounding areas that it saturates the CCD. In the
4 x 120s and 4 x 300s images, the burned out regions aren’t just white, they have an unpleasant
mottled green appearance.
To get a nice picture of M42, compressing the dynamic range of the image, I stacked the three
pictures. On top, was the 4 x 300s image, with the burned out regions masked out so that the
underlying image could show through. Underneath was the 4 x 120s image, again with the burned out
regions masked out, and finally on the bottom was the 16 x 30s image. Each image was black point
adjusted and curved to match the previous layer.
It’s easier to explain with a picture:
The resulting image was then composited together, slightly histogram adjusted further in Photoshop
and then “Despeckled” and “Dust and Scratch Filtered” and finally gently unsharp masked:
A very nice image.
Summary
At the beginning of the review I said you could get deep sky pictures “out of the box” in one evening. I
certainly did. However, there are some caveats:
-
my telescope was very well polar aligned
I had a bunch of telescopes and reducers available to get different fields-of-view
for some of the images I used a $3k autoguiding system (including scope)
I have a lot of experience with astrophotography
I think, realistically, if you have never done ANY imaging, you won’t be able to get comparable
images your first night out. It’s no fault of the DSI, it’s that there’s just too much stuff to learn, polar
aligning, just getting focus (which can take HOURS the first time you try it), and image post
processing.
However, for the price, this is the easiest “introductory” camera I have used, and is definitely capable
of imaging virtually all of the Messier catalog with an impressive effort-and-cost-to-results ratio.
DSI likes:
-
everything you need is “in the box”
one USB cable control is MUCH simpler than competing “parallel port assisted” designs
Autostar Suite track-and-accumulate works well
decent sensitivity and noise immunity
automatic dark frame mode
DSI dislikes:
-
odd IR filter easy to damage – standard 1.25” thread-on filter would have been better
CCD isn’t temperature controlled, but hey, it’s only $299
no obvious ways to automatically do flat frames
Bottom line – Meade has set an impressive, new, low price point for entry level color DSO imaging. If
you want to dabble, this is a great way to start.
Click to Discuss this article in the forums

Meade DSI: Deep Sky Astrophotos “Your First Night

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