Thursday, March 28, 2013

Astrophotography - time is priceless

In action
Typical astrophotography setup contains typical elements: camera, telescope and the mount. The importance of components is hovewer of different order (especially assuming some budget). The most important is the mount - we should spent not less than 50% of total money for this component. The telescope focal ratio (diameter to focal length ratio) is quite important - it determines the total exposure time required. The camera is also important, however as long as we are not in a scientific grade instrument class, all amateur CCD cameras will provide decent and comparable results.
This time we will say a little more about how different aspects of our setup will affect the total exposure time required to achieve some measurable results. Let's define our goal first: we need to picture Owl Nebula (M97) that has apparent magnitude 9.9mag and angular diameter 3.3 arcmin, that gives us surface brightness value of 21.2 mag/arcsec2. We want to have signal to noise ratio (SNR) equals 40 on our final picture. Now we can calculate how different parameters affect total exposure time.
Note - we don't exposure single 300 minutes photo. We do 60 five minutes exposures and stack them. And the result is very similar to one 300 minutes photo.

(Geek mode on)

Camera dark current
This parameter is often overrated. Modern thermoelectrically cooled astrocameras has dark current value very low. Even modern DSLR cameras has this parameter under control, unless we do our sessions in the ambient temperature that exceeds 20C. Let's take a look into the graph:
Horizontal axis reflects sky background flux - the 20mag/arcsec2 means quite decent environment we can see some of Milky Way. 21-21.5 means dark, rural sky. 19-19.5 means city suburbs. Vertical axis is calculated total exposure time. We can see that camera dark current does not affect total exposure time much. All other noise sources are more important in this scenario.

Camera read out noise
Read out noise is the constant value of noise added to each frame taken by camera. It is added always. Another graph:
Here we can see some more significant differences. Camera read out noise affects total exposure time, especially when our camera has this parameter higher than 5e. For decent sky conditions and two cameras with 5 and 10e read out noise for the latter one we need to make total exposure time 50% longer to achieve our goal.

Camera quantum efficiency
WTH is quantum efficiency? It is amount of photons that we caught with camera pixel that will be converted to the actual electrons released that will product measurable signal. So if 100 photons will come to the pixel, and they will release 50 electrons we have QE=50%. Take a look at another graph:

Yeah, that is not surprise - the more effective our detector is the more signal we got. It is linear. DSLR cameras have QE between 30 and 50% (however Bayer matrix makes it not fully usable). Amateur CCD cameras QE varies between 40 and 65%. And last but not least...

Telescope focal ratio
Focal ratio (f stop) is the ratio between telescope diameter and its focal length. Fast scope (or camera lens) is the one that has large diameter and short focal length. It also means it is fast - you can achieve our goal quicker. Last graph:
Yeah, pretty impressive. Popular astrophoto refractors with ED lens have often focal ratio f/6 - f/8, that means they are slow. With dedicated astrophoto newtonian scope with f/4 ratio you can get the same result 2-4 times faster. ED refractors are more compact, do not require collimation, but it comes for a price. 

Summary
Dark sky - priceless. Each graph shows that suburbian sky with 19mag/arcsec2 sky glow will require four times longer total exposure time than under rural, dark sky. Even after this four times longer exposure the image will not be the same - the most faint picture regions will suffer more from light pollution. Image dynamic range will be compromised.
Focal ratio - often underestimated. Fast scope gives good results fast. Slow scope gives them later, but more comfortable.
Quantum efficiency - important, but available within limited range. Scientific grade CCD cameras cooled with liquid nitrogen can have QE 80% and more. Amateur ones are in 40-60% range.
Readout noise - quite important, especially for small pixel size cameras when full well depth is low and we need to collect many short frames to avoid oversaturation. Also more important under dark skies or when imaging with narrowband filters. 
Dark noise - could be important, but its value is so low comparing to readout noise and sky flux noise that it can be really forgotten. Is visible when picturing with DSLR during hot nights. More important with narrowband filter exposures.

Clear (and dark) skies!

Monday, March 25, 2013

45GB in 45 minutes


The night after my birthday party the sky was again a little foggy and additionally Moon was shining like hell, so the only option was again to picture the Moon. This time I decided to create the mosaic of the total Moon visible surface. I succeeded in 95% as you can see above :) The mosaic contains 31 single pictures that were stacked from 31 short movies of 600 frames each. 45GB of uncompressed data was collected and registered to create this single picure. The one above is resized, the original one, about 4000x4000px large (4MB) can be downloaded from here: https://www.dropbox.com/s/0ov6r42ez5r2qbv/moon-23.03-fin.jpg . Seeing conditions (that affect picture resolution) was not top notch, but the resulting picture is pretty detailed anyway. As you can see the closer to the terminator line, the more interesting the landscape details are. But the areas where the Sun is very high above also exposes some details, although they appear quite flat.
Sample single piece of mosaic looks like this:

Clear skies!

Thursday, March 21, 2013

Moonwalk


Mare Imbrium 
Just another night, a little bit of haze, a little bit of high clouds and gibbous Moon. So no temptation for deep sky astrophotography, but just a few moments in the backyard for Moon gazing. Well, in fact Moon film making, because imaging of Moon and planets is not just taking pictures. Actually we make a short movie (like 300-3000 frames) and then use dedicated software (like Registax) to select best frames and stack them into one, detailed picture. We do it, because atmosphere turbulences making different parts of image blurred (it is called seeing). But if we take many single pictures, there is a chance of having different regions with high quality in the different frames. Then software start to work - it aligns frames, selects best regions and stacks it into one single picture. 
This is also the way these pictures were recorded. Although my setup is not quite suitable for planetary (high resolution) imaging, I do it from time to time. I use my QHY5 guiding camera, put it into main scope with barlow lens and effective focal length of this system is about 2400mm. I also use infrared filter that passes only infrared light, so this spectrum range is less affected by seeing conditions (Astronomik IR 742). All pictures in this blog entry has scale 1px ~ 750 metres (enlarged).


Rupes Recta in the center
Here are the results of this short session (terminated by solid cloud cover) - you can click every picture to enlarge. The first image is centered around famous Rupes Recta (Straight Wall) that is about 300m high and over 100km long. At the bottom starting from the right are craters: Alphonsus, Alpetragius, Arzachel, Purbach  and many, many others (I recommend current anniversary edition of Virtual Moon Atlas for this kind of studies)

Next image presents Plato crater (101km diameter) filled with lava, so it has relatively flat bottom. You can see crater walls shadow at its bottom. Under Plato there are Montes Alpes  up to 2400m high and famous Vallis Alpes across the mountains. Right part of the photo is covered with Mare Imbrium. At the bottom Cassini and Aristillus craters, and separated Mons Piton above them.

At the top center part of this photo we can see Tycho crater (86km diameter, 4800m depth) coming out from the shadows. 1500m high central mountain in this crater is not yet visible. Many other craters around - you can see different crater generations - younger craters are placed over the older ones. 

 Montes Apenninus (up to 5400m high). To the right Archimedes crater (86km diameter). 

Another Montes Apenninus  picture with Eratosthenes crater at the top left (60km diameter, 3570m height). You can see two of its summits going out of the shadows. 


Clear skies!

Pin point stars on sale



Although chip size in my Atik 314L+ camera is relatively small (12mm diagonal) coma from my f/5 newton is visible at the image corners. So I started to look for another coma corrector (the old one was sold together with my modded Canon 20D). 
Important things for me were:
 - price :) 
 - corrector should shorten my scope focal length if possible, so my camera field of view would increase
 - acceptable image quality



M1 Crab nebula pictured without coma corrector.
You can see coma in the picture corners.
I have made a small research and found out Sky Watcher coma corrector as a potential candidate. It is beeing sold under different brands and additionaly it was on sale at my local retailer.
So for not much more than 100$ it came to me :) Built quality is decent, corrector itself is relatively lightweight, and according to specifications he gives corrected field of diameter 44mm and works for telescopes with focal ratio f/4 - f/6. And also provides focal length reduction factor 0.9 - 0.95x (yeehaa!).

So, lets go with some pictures.
Optimal corrector to image plane distance for this model is 55mm. On the image taken with this distance set you can see no coma at the corners, overall quality is good (stars FWHM was kept). Effective focal length was shortened to 679mm so it means reduction factor of 0.9x.

Picture taken with 55mm corrector - CCD distance.

Next picture was shot with distance set to 50mm. We can still see no coma at the corners, good stars quality and focal length was calculated to 683mm.

Picture taken with 50mm corrector - CCD distance.
Last picture shows the same star field with corrector - CCD distance set to 45mm. Still no coma, good star quality and effective focal length of 692mm. 
Picture taken with 45mm corrector - CCD distance.


This flexible CCD - corrector distance with decent correction result is probably valid for my small chip camera that covers about 1/4 of corrected field diameter for this device. When using with some larger chips one probably need to be more strict with the optimum corrector - CCD distance. 

Last image is kind of comparison - it presents bottom right corner of the pictures imaged for different chip - corrector distances and without coma corrector in optical path. It is also enlarged to 150% of the original scale. You can see that within this 10mm range of corrector distance the field is well corrected and stars are properly shaped. 
Overall I am pretty happy for this single piece of astro gear. It meets my requirements and comes at a very reasonable price. I also hope it will cover some larger field of camera view in the future, when I maybe some day will be able to exchange my camera with bigger sensor model (like KAF8300). I would reccomend this coma corrector for amateur use, when you need decent image quality with a very little price and you agree to have a little focal length reduction. I am eager to test it with some 8" f/4 newtonian that will give effective f/3.6 focal ratio and that means pretty powerful astrograph.



Clear skies!

Monday, March 18, 2013

Black Eyed Piece

Messier 64 galaxy is also known as Black Eye or Sleeping Beauty galaxy. Both these names corresponds to the spectacular dark band of absorbing dust near the galaxy nucleus. Galaxy itself is a spiral galaxy that is about 24 mln ligt years away and can be found in Coma Berenices constellation (Berenice's Hair). 
Recent galaxy studies had led to discovery, that interstellar gas in the outer regions rotates with the opposite direction than inner regions (till about 3000 light years from galaxy center). This is believed to cause many new stars creation in the layer that splits these two regions. Astronomers says the reason for this unusual behaviour was a collision of two galaxes about one billion years ago. 
Picture above was taken with regular setup (6" newtonian, Atik 314L+, L filter) with total exposure time 120 minutes. Single exposure was limited to 3 minutes due to very bright galaxy core that was oversaturated during longer exposures. Picture scale is 1.9 arc sec / px.

Saturday, March 16, 2013

Space interactions

M51 Whirlpool Galaxy and its companion - NGC5195 (click to enlarge)
Whirlpool Galaxy (M51) was one of my birthday night targets. It it placed in Canes Venatici (Hunting Dogs) constellation about 23 mln light years from Earth (or Sun, whatever :). It is large interacting spiral galaxy discovered by Charles Messier in 1773. 
Recent simulations bear out, that its companion (NGC5195) caused M51's spiral structure due to passing through the main disk of M51 galaxy about 500 to 600 mln years ago. The picture has been made using standard setup with luminance filter only. Total exposure was 90 minutes while single exposure time was set to 5 minutes. 
When you enlarge the picture you will notice some other very faint galaxies in the field. One of them is above the 'bridge' that connects M51 and its companion. Its symbol is IC4278 and no red shift was yet determined for this galaxy, so no exact distance was yet calculated. However quite probable value can be around 650 mln light years away. So that long the light travelled from IC4278 galaxy to be finally captured in my device. Nice catch :)

Tuesday, March 12, 2013

Faint chasing - update


Yet another cloudy night, so I took yet another look into my NGC4565 picture and found out a bunch of faint stars near to the galaxy. These stars looked like they did not want to be anonymous anymore, so I decided to at least found out how faint they are. The task was not so trivial, but after installing Astrometrica software (http://www.astrometrica.at/default.html?/catalogs.html) I managed to find them (not all although) in the database:



It turned out that one of the faintest stars registered on my picture (marked with red arrow) has visual brightness of 20.7 mag. Magnitude scale is non linear - faintest stars visible in fairly good conditions with naked eye are 5 mag. In excellent conditions we can see stars up to 6-6.5 magnitude. Each one magnitude more means stars 2.5 times fainter, so we can calculate that 20.7 magnitude star is about one million times fainter than stars visible with naked eye under good conditions. It is quite amazing how telescope with only 6 inches diameter can chase astronomical targets under suburb sky with magnitude limited to 5 mag only.

UPDATE
As Pulsar from http://astropolis.pl found out there are even more faint objects in this picture. Here is another Astrometrica screen shot presenting star with apparent magnitude 21.3mag

It means this star is about two millions times fainter that the ones visible with naked eye under dark sky.

Wednesday, March 6, 2013

Mortal Combat - DSLR vs Atik

Just before weather break down I managed to gather some more photons that have been travelling about 8 milion years to be eventually captured in my astrophoto setup. The target - spiral galaxy NGC2403 has been chosen for a purpose. This galaxy was one of my first objects pictured with Canon 20D camera some time ago (probably has not changed much during this about one year period :) ). This time picture was shot using Atik 314L+ camera, but I was able to collect only 40 minutes of total exposure (comparing to 80 minutes of exposure made with Canon 20D about one year ago). However this comparison makes sense, because another 40 minutes of exposure should be made to collect colout for this black and white Atik image to reflect actual color image exposed using Canon DSLR.
Anyway, let us see some result:


Both pictures was exposed using the same setup - GSO 150 f/5 newtonian scope. It is quite fortunate that both Canon 20D and Atik 314L+ have similar pixel size, so the image scale is about the same. However please keep in mind, that Canon camera has RGGB Bayer matrix (Atik is monochrome camera) and also IR filter, so it is less sensitive to the red light (especially with regards to H alpha emission line). Bayer matrix also means a little bit worse spatial resolution. Additionally more thermal noise is present for DSLR camera (comparing to thermoelectrically cooled Atik camera), but also light pollution noise has some significant effect especially under suburb sky with limiting magnitude about 5mag.
Nevertheless the conclusion is fairly simple - DSLR astrophotography was real fun and gave me great opportunity to get into deep space astrophotography with decent bang for a buck ratio. "Real" CCD astro camera on the other hand even under suburb night sky gives us brand new picture quality of our precious sky targets. However it comes at a price of new problems to fight with - especially for monochrome camera with color filter set.

Tuesday, March 5, 2013

What is Leo looking at?


Mystery resolved! Leo is looking at NGC2903 barred spiral galaxy :) Galaxy is about 30 mln light years away and is quite nice object for astrophotography. Unfortunately weather this night was not quite good - seeing got worse and also some high clouds appeared, so the photo has not as many details as I would like it to have... Anyway another LRGB picture approach, 150:40:40:40 minutes of exposure, binned 1:2:2:2. 

Monday, March 4, 2013

Needle Galaxy

Needle Galaxy (aka NGC4565 or Caldwell 38) is an edge on spiral galaxy in Coma Berenices contellation. It is about 40 millions light years away and its apparent magnitude is 10 mag. It is huge galaxy - more luminous than famous Andromeda galaxy. It is prominent celestial masterpiece that Messier missed in his catalog.
The photography was taken during one night with fairly good seeing conditions, but a little bit haze present that limited naked eye magnitude to about 5. LRGB frames were exposed with total times 150:40:40:40 minutes and 1:2:2:2 binning. Single exposure times were 5 minutes for L and 2 minutes for color filters. Setup as usual - GSO newtonian scope 150/750mm, Atik 314L+ camera with color filter wheel placed on autoguided HEQ5 Pro mount.

Only luminance channel is presented below:



Clear skies!

Saturday, March 2, 2013

Crab hunting

B/W image of Crab nebula
First light of my new second hand CCD camera went for first Messier catalog object - Crab nebula. After over one month waiting for clear skies I was able to expose first piece of material. Through luminance filter 2 hours (24x5min) exposition was made, so first frame for luminance layering LRGB image was made.

Crab nebula (Messier 1, NGC1952) is a supernova remnant in constellation of Taurus. It corresponds to bright supernova recorded in 1054. It's diameter is now 11 light years, and it is about 6500 light years awau from Earth. In the center of Crab nebula there is pulsar, very dense neutron star, 28-30km across with a spin rate of 30.2ms.

Two days after luminance was exposed I pointed my setup again to Crab nebula and exposed R, G and B frames. Red channel was exposed for 90 minutes. Both G and B channels were exposed for 40 minutes and with binning set to 2. After many work with layers, channels, masking, and balancing here we have my first LRGB astrophoto:


Crab nebula LRGB photo (120:90:40:40min, bin 1:1:2:2)

It is not very colorful at all, but my intention is rather to obtain somehow "natural" look of the nebula (although noone can say how does it look actually :) )
And at the end the photo of my setup in action:)


Clear skies!

Friday, March 1, 2013

Arduino telescope focuser real life tests

In the first part (ASCOM telescope focuser with Arduino) short description of the project was presented. Last night I have noticed a few stars (yeah, maybe a few dozens) and it was enough to do some real life tests of the focuser project. 

FocusMax software was chosen to deal with homemade telescope focuser. After few moments of setup the V-curve of the system was determined. The process ran surprisingly fast and without any problems. Both side slope values were pretty similar, so I was quite happy with the result. 
After this I spent like half an hour on focusing for different star fields with different filters, and the success factor was 100%. Each time the system was able to reach focus point within 30 seconds. HFD value was in the range of 1.7-2.15 depending on automatically selected star and current filter in the optical path (currently my set contains only UV/IR cut, R, G, and B filters produced by Baader Planetarium).


So it seems everything works fine and I am even more prepared for (hopefully) incoming starry nights...