Astronomy and DSLR Astrophotography Blog

Yesterday I have received e-mail from Mathew, asking me about usefulness of SCT telescopes for deep-sky astrophotography. He agreed to reply to his question in this post, since this is pretty general question and others may find it useful too. Here’s his message:

“Hi,

My name is Mathew Oehler, and over the past several months I have become more interested in astrophotography, after having general photography as a hobby for about 6 years.

Last year a family friend was gracious enough to give me a 100mm Orion refractor telescope, and I have gotten excellent use out of it in viewing Saturn, Jupiter, and the Orion nebula among other things.  More recently I purchased a T-ring that enabled me to connect my Canon Rebel to the telescope, and was disappointed in its capability.  I would ideally like something designed to reach deeper, so I can get some clearer shots of nebulas and galaxies. I have heard that Schmidt-Cassegrain telescopes are designed for this purpose, but I’m having trouble finding good information amongst the sea of astrophotography information on the internet.  Are SC telescopes good for this, and will a standard one have an equatorial mount that I can put a motor onto?

Thank you in advance for any help you can provide me!”

The answer is not very simple (as always). The main reason why SCTs are so popular telescopes is their portability and compact size. They have a short optical tube because the light goes three times through it before it reaches the focus. The consequence is that they have long focal length comparing to diameter of the primary mirror which means slow optics (high f-ratio) – typically between f/8 and f/12. And here’s the catch! F-ratio is a king at photography because it tells you how many light the optics gathers per pixel. The lower f-ratio means more light, which means lower ISO setting (less noise) and shorter exposition to go deeper. This is the main reason why SCTs are not so good and popular among astrophotographers. In spite the fact they are compact, they have long focal length, small field of view (a lot of nebulas on the sky are larger than the diameter of the Moon) and require long exposition. Usually they come with mounts which are not capable to guide accurate enough to achieve pin-point stars at longer expositions (5min and more). All their characteristics suits for planetary and moon imaging, where longer focal lengths are needed and guiding is not so critical. On good mounts and with company of high detective CCD cameras they are good optical instruments for small planetary nebulas and galaxies, but I would highly dissuade this type of imaging the deep sky from beginners.

Mathew, I suppose that you have an achromatic telescope, because I have heard just good things about 100mm APO Orion refractor and it is good imaging instrument. I suggest you to start imaging at wide angle, since this is the easiest and the cheapest option. DSLR in combination with kit lens is good start point and your first object are star trails (if you don’t have a mount with tracking) or Milky Way (if you have the mount). It’s good to have in mind that you need to “build” your equipment from the ground, that means that first you need a good tripod, then mount and at the end the optics and camera. Note that good mount is astrophotographer’s best friend and it’s worth to buy a good one in the beginning, because you can put on whatever you want (just camera with kit lens or 16 inch RC). Later on you can buy a telephoto lens or APO middle-sized refractor which has many many objects in range (practically all the M catalogue and long list of NGC objects). If you are interested in little smaller objects you may find 8-10 inch f/4-f/6 high-quality Newton good instrument as well. At the end there are more exotic telescopes like SCTs, RCs which cost a fortune and need very expensive and good mounts (in range of 6000+ USD).

I hope I have answered to your question. You are welcome to leave a comment if I haven’t.

And if anyone have a question related to astrophotography just contact me. I’ll try my best to solve your problem …

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Finally I have more time. The exams are almost finished so I have three months of holidays now and I hope that I will take some good astrophotos in this time and post it here. I will also continue with Astrophotography Tutorial but for today I have prepared something more fresh. The last week I have ordered new autoguiding camera for my setup. The previous one, Atik ATK-2HS, was not mine actually so I decided to buy the new one. I have ordered QHY5 from UK, the astronomy store named Modern Astronomy. The shipment to Slovenia costed only 10 pounds and it took only 4 days to arrive.

The first impression was very positive! Even though it is “made in China” it looks very well designed and made. The build quality is on very high level. It is not plastic like Atik, but metallic. The original package comes with camera, T-2 to 1,25″ nosepiece adapter, USB cable and setup instructions. I have also ordered RJ-11 cable for direct autoguiding via camera.

I got impressed when I unscrewed the nosepiece because of the size of the CCD. It seems really huge comparing to tiny CCD detectors in webcams (Atik ATK-2HS also uses ordinary 640×480 one). The resolution is 1280 x 1024, more specification are available at the QHY website.

QHY5 without nosepiece

The best thing is that QHY5 has T-2 thread which perfectly fits to my autoguiding telescope so I can just screw it on.

QHY5 on the autoguiding telescope

As you can notice it has two ports in the back side. One is USB 2.0 port for connection with the computer and another is ST-4 compatible autoguiding port for direct connection with the mount. I am using Vixen SkySensor hand-controller and I though that it is compatible but in fact it isn’t, because Vixen uses it’s own standard for autoguding. The connector is the same but wires are connected differently so today I am going to make a new cable. The camera has also build-in red LED diode which is too bright and could be disturbing in a completely dark so I pasted a black stripe over it.

Back side of QHY5

The camera comes with one A4 paper sheet of instructions how to setup. The drivers are not included, you have to download it from the Modern Astronomy site. The setup is really simple, you just install the generic driver and plug in the camera. Then you have to install additional driver for the autoguiding software you are using, in my case Guidemaster. All the main autoguiding software are supported, including ASCOM platform.

To sum up, I am very satisfied with the purchase. Two days ago I had a first light and everything worked fine. I was planning to capture M102 but ended with just 2 pictures because it clouded. This is 100% crop of one of them as a proof that camera guides OK:

100% crop of M102

I hope you enjoyed the review. If you have any questions don’t hesitate to ask under the comments

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So let’s continue with the tutorial. The last time we have chosen the object. Today we will take a look at some common camera settings for astrophotography. In this tutorial I will be using modified Canon Rebel XT (350D). Modified means that I have replaced original filter in front of CMOS sensor with replacement filter of Baader. I have listed some good guides how to do it here.

Let’s go back to the topic. The camera is important piece of equipment since it’s capturing the light. Not all the cameras preforms the same but there are some common setting that it’s good to setup before we go out under the stars.

The very first thing is the photography mode of the camera. Here we have just one choice – M (Manual) mode because of one simple reason: DSLRs are not meant to be astrophotography cameras but daylight cameras. So the longest exposition that automatic modes enables us it’s 30s. But that’s far not enough for deep-sky astrophotography. We need longer exposures and the M mode has a “bulb mode”. That means that the exposition is as long as the shutter is pressed on a camera. Of course, we cannot hold the button on a camera for 5 minutes so the wire trigger is necessary.

We select M mode

The second thing is mirror lockup. Almost all the DSLRs enables lockup function. Mirror lockup means that the mirror in the camera, which reflects light to the viewfinder, locks up some seconds before the start of exposition. And why is that so important? Because if you are using telescope with long focal length, then every single tiny movement is noticeable on a picture. And when the mirror “jumps up” it shakes the system and bright stars get tails and that’s what we obiously don’t want to have on the pictures. So the mirror lockup function triggers the mirror some seconds before the start of exposition so the system can settle down before the exposition is started.

Enable the mirror lockup function

Next important thing is ISO speed. This one is still a hot topic among astrophotographers since some claims that lower ISO is better and others who prefers higher ISO settings. Anyway, two years ago I have made a test on Youth Astronomy Camp which revealed that Rebel XT preforms the best at ISO 800.

Selecting the ISO 800

For additional processing it’s the best that we use uncompressed data so the RAW mode is the right decision. If you prefer having JPGs also, then you can select RAW + JPG.

RAW (uncompressed) format

Another not so important thing is LCD brightness. I strongly recommend to set this as low as possible because in the night our eyes are adapted to the dark and if you want to check for example the sharpness of the images on LCD with the full brightness then you’ll get blind for some minutes.

The last but not the least function is information LCD illumination. This function enables you to see the settings specified in the dark. Just press the button.

Info LCD illumination

Thank you for reading. So far we have chosen the object and specified all the camera settings.

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The very first thing we have to do when we start an astophotography is to choose the object. We have to know what are benefits of our equipment, what we are planning to capture, what is FOV (field of view) of our telescope and camera, how “deep” our system goes and so on. Let’s take a look at some of this questions.

When we are deciding which object we will be capturing we can help ourselves with different astronomy software. They display imaginary sky and we can take a walk into nigh sky, choosing the objects, checking the rising, setting, magnitude, distance, … All this parameters are important when we are selecting the target for astrophotography.

There is a lot of different programs available on the web for astronomy. Some are free and others are paid. There are some of them:

• Starry Night
• Hallo Northern Sky
• Cartes du Ciel
• SkyMap
• Stellarium

In this tutorial I will be using Starry Night. It is paid but it offers endless of useful options like FOV and has  really a huge database of objects.

First of all we have to know what type of object is appropriate for our astrophotography setup. The predispositions for deep-sky astrophotography are:

• Telescope of telephoto lens
• Mount for tracking that tracks very accurate (under 2″ of error) or has option of auto-guiding
• DSLR or CCS camera with remote control of exposition

In Starry Night we put the parameters in the the section Equipment:

Then we select FOV of our telescope and camera. The program displays us the rectangle that out astrophotography setup “sees”.

So, what’s next? We have to choose the target that we are going to capture. Here are some basic directives:

• Make sure that object is in the field of view at least 3 hours in the total darkness. It has to be at least 15-20° above the horizon all the time of the capturing. Make sure that you have specified your date/time and location accurately!
• Make sure that object roughly fits to the field of your telescope’s view. The rectangle that we have specified before helps us at this step. Make sure that it is not too small – it doesn’t make any sense if you are photographing a very small planetary nebula at 600mm of focal length.
• Make sure that it is not too dim. Almost all the objects from Messier catalogue are OK, and all the bright objects from NGC makes good target as well. If the object is dimmer than 10th magnitude then bigger telescope then 8″ is recommended.
• If you don’t have GoTo or it’s unreliable then make sure that you are able to find the object in the night sky. Help yourself with bright stars, print the carts out.
• If you are using german equatorial mount then make sure that object doesn’t pass the meridian during the planned time of capturing.

Some good objects to start with: M45 (Pleiades), M42 (Orion Nebula), M44 (Beehive Cluster), M27 (Dumbbell Nebula), NGC7000 (North American Nebula), M31 (Andromeda). They are all bright and easy to find even with binoculars.

And here we are. We have the object.

Next time we will take a look at the conditions in the athmosphere that affects to the astrophotography – the weather, seeing, transparency and more.

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I have decided to publish step-by-step how-to tutorial for astrophotography beginners. It’s International Year of Astronomy and I am pretty sure that there is a lot of people who are interested in astronomy and astrophotography, but they don’t know where to start.

Weekly I will publish articles guiding you trough the process of astrophotography – from equipment to the imaging tips and processing of images on the computer. Each article will cover specific part and every single of them is important for good results.

There are several different types of objects up there and they require different type of equipment, capturing, processing. In this tutorial I will focus on deep-sky astrophotography with DSLR camera and with amateur equipment in range up to 5.000$. If you are beginner – don’t be afraid! There is a possibility to start astrophotography with much lower budget and get stunning results. You just need to be patient and don’t give up.

Contents:

• Part I: Choosing an Object
• Part II: Camera Settings
• Part III: The Telescope

Don’t forget that we all learn from mistakes. With patience and enthusiasm everything is possible. Enjoy the Universe and Clear Skies!

NGC6888 - The Crescent Nebula @ Primož Cigler

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As we all know the optical system needs to be clean to provide very sharp and contrast image. All the small ‘dirties’ and dust affects to the image quality and reduces it.

If we use our telescope for night observations or astrophotography then the dust is collected on the optics in time. To get the best of our telescope we have to clean it sometimes. It is very important that we do that very carefully because we won’t be able to repair any of the scratches we may do during the cleaning.

Today I’ll give you some important information how to clean the primary and secondary mirror of the Newton type of the telescope.

Before we do anything we have to prepare some things we will need during the cleaning:

• Clean kitchenware that is larger at least 50% then diameter of our primary mirror fulfilled with warm water and a bit of detergent
• A clean towel
• Distilled water (approx. 1L)

Disassembly:

First of all we remove the primary mirror. We unscrew the screws which holds the cell of the primary mirror in the tube (Picture 1):

Picture 1

When we do that we carefully (!) remove the cell of primary mirror. There are different types of primary mirror holders so we have to figure out how to remove the mirror from the cell (Picture 2):

Picture 2

The disassembly of the secondary mirror is easier. We just unscrew the screws in the spider and that’s it. Just make sure that you are holding the secondary mirror all the time with one hand while with the other you are unscrewing. Otherwise it may happen that the secondary mirror falls to the floor and broke up.

Cleaning:

When we are sure that we removed all the parts which holds the glass we pick the mirror (Picture 3) and put it into the kitchenware with warm water. We just let it in it for some minutes. After that we grab it with both hands and starts moving it left and right in the water and all the dust will just flew away. We are doing that for some minutes. If we can still notice a dust on a mirror we can remove it by soft circle movements on the glass, but be careful here! Everything you should do in the water (Picture 4):

Picture 3

Picture 4

When we are sure that there is no longer dust and dirt we take the mirror out of the kitchenware and wash it with a distilled water over the bath. After that we don’t touch the mirror surface anymore. We just put it on a towel and let it to dry.

The same procedure we repeat with the secondary mirror (Picture 5). Anyway, it is not necessary that we remove the mirror from the holder because often it is glued to it.

Picture 5

After both the mirrors are dry we assemble the telescope back. We just need to be careful to make everything just in the back order we disassemble it. If we have done everything OK then our optics is prepared again to provide the best image quality!

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There are several different types of equatorial mounts but surely the German Equatorial mounts are the most popular among amateur astrophotographers and astronomers. Let’s see why?

At German’s type of the mount we have two axis: one is parallel to the Earth’s rotate axis and another is rectangular to it. This is because when the mount is tracking there is no need to move both axis as the same time but just one. This makes those mounts so popular and useful because in theory you need just one motor to track the moving of the night sky. You also don’t get any other errors like field rotation or similar.

The axis which moves from east towards west is R. A. (Right Ascension) axis and the another one that enables us to move telescope from South to North is named Declination axis.

At this version of mount we need counterweights to balance the whole system. This procedure is very important to do it as precisely as possible because the better balanced mount slews better!

When setting-up we have to do everything in the right order to assure that in the end our mount tracks as precisely as possible:

1. Put the tripod to the flat and solid ground.
2. If you are using tripod make sure that the leg labelled with “N” (North) is positioned right to the North. Help yourself with a compass.
3. Attach the equatorial head to the tripod or pier.
4. If your mount has a polar finder check the time and a date, rotate the two wheels on the R. A. axis to the right date and time and check the position of Polaris (Northern Hemisphere) or the Southern Cross (Southern Hemisphere) in the polar scope and align the stars with the marked positions.
5. If necessary do a drift-align (the guide how to do it follows in the next posts).
6. Put a telescope on a mount.
7. Balance the whole system with counterweights and moving of the telescope tube.
8. Enjoy the good tracking!

That’s short review of the German equatorial mount. More read:

• http://en.wikipedia.org/wiki/Equatorial_mount
• http://starizona.com/acb/basics/equip_mounts_gem.aspx
• http://science.howstuffworks.com/telescope5.htm

Image source: http://www.company7.com/vixen/mounts/counterweights.html

Thanks for reading!

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