Dealing with UK weather, the dew challenge

Observing from the UK presents its fair set of challenges, between finding a clear sky, good seeing and dealing with dew.

In this post, I'll cover what I've ended up doing on my battle with dew.

The problem

The dew problem is very common on SCT scopes due to their large Schmidt plate in the front, facing the sky.

Understanding the physics of dewing or condensation helps fighting it. In short, condensation takes place when the humidity in the air is high and a surface colder than the air is present. In this case, the droplets of water in the air go rest on that surface, creating dew and waisting your observations!

Now why is the scope colder than the air? After all it is only sitting there and should be at the same temperature, isn't it? Well things can get colder or warmer through 3 mechanisms:

• conduction: this is when 2 parts are in contact and exchange heat
• convection: this is when a liquid/air helps transfer heat between 2 parts (your blow dryer is a good example of using convection to transfer heat from the blow dryer to your hair)
• radiation: this is due to electromagnetic waves. it happens in the vacuum of space. A good example of radiation is the heat you feel on your face standing a few meters from a very hot wood fire. 

Radiative cooling is the problem for us observing the sky. The deep space radiation is very cold (3 kelvin). The atmosphere (water vapor in the air) radiation is still pretty cold but warmer. The ground radiation is warm in comparison. All 3 contributes to the temperature of our optics. On a clear night, where the air has a low water vapor content (as compared to on a cloudy night), you get less "warm" water vapor radiation. And this is sufficient to cool anything facing the sky. This is why on a clear dry night it can feel very very cold whilst a cloudy night with the same air temperature will feel warmer. It's also why you get frost on your car after a clear night eventhough air temperature didn't drop below 0 (but exposed surface did). To learn more about this phenomenom, this is a good read.

Perventing dew

So to fight dew, all we have to do is to keep the optics a few degrees above dew point (the point at which water droplets condensate on surfaces). In practice, think you need to keep your optics above the ambiant air temperature by just a few degrees.

1-prevent cooling by reducing your exposure to radiative cooling. This is the effect of the dew shield. These long tubes in front of the scope simply create a new radiation source at air temperature (the shield itself). They also by their nature reduce the amount of water that can reach the optics as most would condensate on the top part of the shield rather than the bottom where the optics are. The material doesn't matter much but a sturdy plastic or yoga mattress certainly works like a charm.

2-warm your optics. this is what dew heaters are for. they are quite simply resistive bands that you wrap around the scope. put current and they get warm, warming the optics and therefore keeping them a few degrees above the dew point.

FAQ: do dew heaters create turbulence? Well since you are heating only a few degrees above air temperature, you're not going to create much turbulence at all. So you'll be fine.

What I've done

I started with a dew shield as shown below. 

It works wonders, but on those humid nights, observing objects high up in the sky, it will reach its limits and in my case, the scope can start to fog up after a few hours. I used to use a blow dryer and remove the dew which works fine, but I wanted a more permanent solution

So I bought from Modern Astronomy at the Astrofest a 10" dew heater strip and a Hitecastro single channel dew controller.

 

Now getting the dew shield on top of the strip on the scope was a bit of a pb as the strip is too thick for the dew shield to close, so I decided to combine them. I opened up the dew strip on one side, removed the foam from the plastified resistor band since the dew shield already has one and stiched everything back together. 

Here is the finished result. As you can see, I kept the velcro band from the dew strip to neatly store the wire.

All that remains is a clear night to put it to the test!

New LX200 10" ACF Scope!

After years of using a 5" Newton F/5 Perl Vixen running on a SuperPolaris mount, I recently upgraded and acquired a second hand Meade LX200 10" ACF OTA powered by an EQ6 SynScan mount. Here are the 2 scopes.

I'm really pleased with the new scope. It came with a dew shield, the 8x50 finder scope, a Meade #1209 Zero Image Shift microfocuser, a 0.63 foc

The EQ6 came with an extended dual saddle for increased stability, a pied for permanent observatory setup, the Synscan handheld controller and a GPS SynScan module

From there, I've acquired 2 eyepieces, a 10mm Delos and a 40mm SWA. I also upgraded the diagonal to a 2" Quartz Dieletric Start Diagonal from William Optics.

For DSLR imaging, I've also acquired a WO prime focus T2 camera adaptor.

I'm now in the process of discovering this new scope which offers plenty of possibilities for years to come. One of my goals is to have it set up on a pier in the garden in a permanent observatory, but first I want to get a handle on remotely controlling the mount and camera for deeksky imaging by leveraging the fantastic work of the EQ6 community: the EQMOD project. I'm also going to venture into software to remotely control my Canon EOS 500d for imaging runs

In future posts, I'll be covering in details this journey!

moon, jupiter and venus and ISS

Taking advantage of the moon, jupiter, venus show. The ISS came through at the right time for a nice picture taken from London. 30s exposure ISO 200 on Canon 500d with 10-22mm wide angle

This picture was featured in spaceweather.com on Feb 25 2012

ISS and Space shuttle Discovery

I was looking forward to the last flight of space shuttle Discovery and was hoping to catch it. I discovered that it would pass over London about 20 min before docking with the ISS, so the chances of having both in the field of view of my telescope were good. At 18:36GMT, the tandem rose over London as the cloud cover started to thin out. It was looking like a single brightening star. But through the finder of my scope I could distinguish 2 dots. Frantically taking as many pictures as my camera could, I was manually tracking the tandem, hoping that a few shots would be good enough. Well it was!!! scope: 130mm/720mm newton, 20mm eyepiece, canon EOS 550D, 1/250s at 1600 ISO

The picture was features twice in spaceweather.com (here and here) on Feb 27, 2011

Summer night

I managed to get a great night of astrophoto stationned on the parking lot of Kynance Cove near The Lizard. Skies were nicely dark on the south as all there is is the atlantic until Spain, which is about 1000km away!

So I spent time imaging objects in Saggitarius and also got a nice shot of NGC253 when it was about 5 degrees above the horizon. This gives you a good idea of how clear the sky was.

All pics here

Summer fun

We spent the last 2 1/2 weeks exploring the south west of England, namely Dorset, Devon and Cornwall, all the way to its most southern part, Lizard Point.

Lot of fun, great hospitality, black starry nights, fantastic seafood, wet camping, many friends. Well all is captured in pictures here

Cosmic perspectives

(move your mouse over the picture to see the Baidu M1 Rocket trail on a single 60s shot)
The other night, I took some pictures of the famous Leo triplet, Messier 65, Messier 66 and NGC 3628. The atmosphere was amazingly stable and so I went on to try to record 1 hour of 60sec exposures. It turned out that only 27 of the 60 pictures had good enough tracking to be used for the final picture, but result turned out pretty good (mouse over the picture to see the difference between one 60 sec shot and the stacked processed result).

One interesting thing I discovered while reviewing the raw shots is that on 2 of them, I had captured a trail. After quick measurement, it was something moving at about 26' per minute. Clearly too slow for a plane or a low altitude satellite. Calsky.com, the excellent satellite viewing prediction website, proved to be remarkebly useful in finding what it was. It turns out that this trail was the Baidu M1 rocket (orbital details), launched in 2007, and orbiting on a very elliptic orbit (523km x 21046km). At the time of passage, it was distant by 6707km.


Exposure Data

• Lens: Newtown Perl-Vixen 130/720
• F/stop: f/5.5 (focal)
• Mount: Perl Vixen Super Polaris mount

• Exposure: 27 min total exposure, composed of:
• 27 x 60 sec (120 min) unfiltered RGB @ ISO 1600 Canon 500D
• Mode: Raw
• White Balance: Auto
• Calibration: Flat, Darks and Offset in DeepSkyStacker
• Processing: Standard deep-sky adjustment in Photoline

• Date Start 2011-Apr-09 02h02 BST, 2011-Apr-09 03h13 BST
• Temp: 9C, 8C
• Location: West Witterings, UK

Image Scale

There are a few astro bloggers that I think do a fantastic job of sharing their expertise in the field of astrophoto. Jerry Lodriguss is certainly one of them. His blog post "Catching the Light" is a fantastic source of info to understand more about how to take great pictures.

In a recent post on Image Scale, he describes how to calculate the image scale on your camera, that is, what field of view each pixel is representing.

So an approximation of the formula (converted in metric system) goes like:

P = (206 * S) / (FL)
Where:

P is the image scale per pixel in arc seconds
S is the size of the pixel in microns
FL is the focal length in millimeters

(the real formula is P=3600*arctan(S/FL), but since S is very small compared to FL, the simplification will be good enough for astro use)

Example: Calculate the image scale per pixel for a Canon 500D camera with 4.7 micron pixels when used on a Perl Vixen 130/720 telescope (720 of focal length).

P = (206 * S) / (FL)
P = (206 * 4.7) / (720)
P = 1.3 arc seconds per pixel

So each pixel of the 500D on the scope will cover 1.3" of the sky.

Jerry goes on with saying that this is very useful to estimate the smallest details you hope to record. You typically would like your image scale per pixel to be 2x to 3x smaller than these smallest details (eg 0.3" to 0.5" to record 1" details on Jupiter).

How do you do this? Increase your focal length with either a barlow, eyepiece projection or lens on your camera held up to the eyepiece of the scope.

Something else to consider as Jerry points out is the seeing. Let's say the seeing is 1". If you are doing deep sky astrophoto, then with the additional tracking errors and other atmospheric variations over the time of the picture, you're unlikely to reach the seeing true limit, so you're probably correctly sampled at 1.3"

But in planetary, where exposure times are very short, it's worth considering the 2x or 3x oversampling as the seeing is less of a real limit.

About Canon 500D noise

DPreview.com is one of those sites that do an amazing job of dissecting cameras down to a level which can become useful for astrophoto.

For example their measure of noise levels for the 500D is something I think might be useful to estimate the optimal ISO / # pics required for deep sky imaging and stacking.

ISO Std Dev

100 3

200 4

400 5.8

800 8.3

1600 11.3

3200 15.8

6400 24

12800 35.7

However, if you have a clue on how to do it, drop me a line

Volcanic ash, sunset, moon, venus and mercury

The sunset on April 16th was clearly affected by volcanic ashes in the south of London, UK.
The sunset was unusually hazy and dusty, most likely due to the volcano that erupted in Iceland a few days before and which ash plume disrupted air-traffic over northern Europe. The second picture shows an early Moon in conjunction with Venus and a very dim mercury

I took the picture from the loft of my house, using a canon eos 500d and an 18-200mm tamron lens set at 200mm for the sunset and at 25mm for the conjunction.

global warming...

I'm generally interested in what's happening out there, in space. I became introduced to astronomy by my grand uncle Christian when I was about 10 year old, looking through a telescope at the moon... That was it.

Since then, I've spent countless nights looking at stars and photographing them and reading about space, cosmology, birth of the universe, astrophysics, etc etc. I'm also fascinated with space exploration.

I'm also very aware of my impact on the world I live in and global warming. But I somehow never connected the 2.

A few days ago, I came across this post on spaceweather.com that suggest that in the last 8 years, the global warming has been compensated by natural phenomenons such as the solar minimum, El nino and volcanism.

And tonight I was told that human influence on global warming is of the order of 1%. In fact, global warming is happening on others planets such as Mars, Jupiter.

Being curious in nature, and not taking any as the absolute truth, I want to start tracking all things I can find about the influence of natural phenomenons on Global warming. This with the goal of plotting all the data to see if any pattern comes out.

So here are a few interesting articles. The natural phenomenoms to be considered are:
-solar irradience
-planet orbit
-planet wobling (precession of equinoxes)
-planet magnetic field position and strength
-position of the sun in the galactic plane


http://www.newmediaexplorer.org/chris/2007/01/22/inconvenient_facts_about_climate_change_global_warming.htm

http://thecriticalthinker.wordpress.com/2008/11/01/climate-change-felt-on-other-planets/

http://news.nationalgeographic.com/news/2004/12/1206_041206_global_warming.html

http://www.timesonline.co.uk/tol/news/uk/article1720024.ece

http://www.spaceweather.com/

Turtle attitude

Aah! Mr T nous a bien fait rigoler ce weekend... Sans commentaire.

and 7 months!

The last 3 weeks have been amazing in terms of change.
first 2 teeth meant lots of tough nights
crawling is now the way to go
seating on my own is easy-weasy!

6 months spring time

Theo's discovering spring, grass, and cowboy boots. And he clearly likes it...

Theo's 5 month adventures

Image via Wikipedia

Fun with Daddy is pretty cool when you are 5 month old. Clearly Daddy enjoys it a lot as well.

So here comes this serie:
1-Theo thinks Daddy's big nose and mouth is quite fun to play with,
2-At St James park, Daddy does a bit of gardening/flower lesson with Theo... hum how does this tulip taste?
3-Theo likes his compotes and purees. There's not doubt about it.
4-And a bit of a rest on the sofa isn't bad either.

Enjoy...

Mamy P's in town... Yihaaaa

Theo is treated by his texan grand mother this week. It's so great to have Paula come over and know Theo. And Theo seems to like her a lot, given how the 2 play together!

Theo - 4 months

I've been bad... and good. Just ended up spending most of my time with Theo and therefore not publishing pics of him... Well here you go a little 4 months gallery

random photo meme

Just got tagged by my great friend and top artist Bethany in bethanyblog..

Fun one, here is how it goes:
1. Go to the 4th picture folder on your computer.
2. Post the 4th picture in that folder.
3. Explain the picture.
4. Tag 4 more bloggers.




So this is what I would call the best cappuccino in the world. A piece of art, a concentrate of kindness, an elixir of happiness. When I used to live in Roma, I had this little tiny cafe in Via Giulia, Cafe Peru. Its owner, Alfredo, is the most delicious person, a true Italian full of passion and wonderful generosity. Alfredo is a magician, and he distills his secret in his cappuccino.
When I was tired, or barely awake because I had to catch a plane at 6am to go to Milan, I would first stop in his cafe. He would open it for me at 4:30am. Reading my mood of the day, he would craft his cappuccino like a maestro transcending his art. The result was simple, I never left his place without a smile on my face and my heart as light as the froth of his magical potion! He is one in a billion. I just wish I knew someone with his passion and art for coffees in London. The day would be brighter, even in the middle of the winter.

I tag. (sorry only 3).. Astrobleme, mudbug, mbaudier

Theo through the lens

I always imagined that I would do some nice shots of Theo when time would allow. Well it was just a matter of taking the time. Sunday we setup the environment to do some cool close-up shots that he will certainly love on his wedding day 20 or 30 years down the road.

Full screen slideshow here

Another snow

Snow came in for the second time this year... And what a snow!!! About 20 cm fall overnight. Our street is transformed, dressed with the most beautiful coat of fluffy stuff.

Of course the children love it! Enjoy the day