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View from a Scope | November 20, 2017

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Getting Started with Deep Sky Astrophotography

Getting Started with Deep Sky Astrophotography

When I was deciding which telescope to purchase I had read a lot of articles that suggested for novices something on an azimuth mount would be a good place to start. This kind of mount would allow you to simply push your telescope in the direction you want to look. The argument put up for recommending these to beginners is that they are a no hassle, pick up and go way of stargazing with a scope.

However they are not suited for deep sky astrophotography, Although initially I felt I would be doing more observing than photography I still wanted the option to give it a go, so in the end I decided on a telescope on a equatorial mount.

Why Mount Why?

The choice of mount is the first thing you need to consider before getting into deep sky astrophotography. An equatorial mount will allow your telescope to counteract the earths rotation, this essentially means when set up correctly if you have something in the centre of your view it will stay there until you move onto something else.

When you are looking at deep sky objects you will only see a grey cloudy patch, this is because the light receptors in our eyes are just not sensitive enough to pick up the extremely faint light coming from these objects.

When we look at something imagine it like our eyes taking 24 photos a second of that object, are brain combines these images for us and this allows is to perceive things as moving, this is great when you are hunting a Lion in the Serengeti, but not so good when you want to pick up faint nebula colours. Our eyes have only got 1/24th a second to pick up all the colour that we see. What we need is an eye that can take less photos per second in order to pick up more colour.

This is where the DSLR comes in and used with an equatorial mount this will allow the camera to look at the same spot for a longer period of time, we can control how many photos it takes per second, thus letting more light in from the object which helps pick up the faint colours and details that our eyes miss.

Enough Theory, Lets Get Practical

Ok so now that we have gone through some the theory in how deep sky photography will work, I guess its time to start putting this into practice.

Items We Need (Red signifies the equipment I have)

Equatorial Mount (EQ3-2)

Telescope (Skywatcher 150p)

Mount Motor (EQ3-2 Dual Axis Motor)

DSLR Camera (Canon 400d)

T-Ring

T-Ring Adaptor

Bahtinov Mask

The Setup

For this section I will be using my own telescope setup as the demo unit.

First off I don’t have much experience with different types of mounts in relation to astrophotography but in my experience I would think that an EQ3-2 mount would be the minimum for deep sky photography.

Step 1.

When setting up the mount it is important to get the best polar alignment you can. A good polar alignment will allow your scope to arc across the sky following the lines of the stars. this will help ensure that your target stays central.

There are a number of ways to polar align, and regardless if you are imaging or observing you will do some form of polar alignment before use (with an EQ mount)polarscope

Initially I was doing my polar alignment very roughly, by just looking through the hole in the mount where a polar scope should be and trying to line this up as best I could with Polaris. You can get lucky using this method and could be able to get exposures of about 30 secs. I would however recommend investing in a polar scope. This will allow you to get a fairly accurate polar alignment pretty quickly. potentially pushing your exposures up to 45 secs (remember more exposure means more light).

The best way however is to do a polar drift. I have tried this a few times but it is a long process. Hopefully I will be able to cover it in some detail in a later post.

For now lets assume we are using a polar scope. The image on the left shows you Polaris aligned using a polar scope.

Step 2

Once the mount has being polar aligned we need to get all the equipment we need connected up. First thing is to turn on the mount motor, this will start tracking the sky at the same rate as the earths rotation.

Now attach the telescope to the mount. I would recommend at this point consulting a star chart and deciding on your target. What I have learned is that it is easy to get distracted and jump around from target to target and not getting anything good on any of them. My advice would be to decide on a target and focus on getting the best photos of that target you can. Leave some time at the end to enjoy the view.

Step 3

So now that you have decided on a target, you need to go and find it. Use a low mag eyepiece for this (I use a 25mm but 30mm would be ideal). Between using you finderscope and the primary eyepiece you should be able to find your target. this can be quite difficult and can take some time but be patient.

Now that the target is central in the eyepiece it is time to connect the DSLR camera. Be careful doing this, you don’t want to accidentally move the telescope.

The camera connects to the scope through a T-Ring and a T-Ring Adaptor. My own scope has a T-Ring Adaptor fitted to the focusing tube so I only needed a T-Ring. The camera with the adaptors connected should click in place. Describe the connection process with pictures

Step 4 – Stay Focused

The key to a good deep sky photograph is focus.

There are a few ways you can do this but I have found that the best way by far is to use a bahitov mask.

bahtinov_4mm

This mask produces diffraction spikes on any bright object such as a star. So point your telescope with the mask attached towards a bright star. If your camera supports live view you can start focusing straight away using the image display to help you focus.

My camera does not have live view so I take exposures lasting 5 / 8 secs. The diagram below demonstrates the different levels of focus.

Focus_b_masks

Thanks to http://www.madpc.co.uk for the image.

Once you have the diffraction spikes in the correct place your camera should now be in perfect focus. Remove the Mask and get snapping.

Conclusion

A final note, I am very new to deep sky photography, so this article is as much a reminder to me as it is a help to someone else. But the techniques outlined above are ones that I have found help me. Without doubt there are better ways of doing this and I am sure I will learn them with time.

The important thing to take away from this is that setup is the key to enjoying deep sky photography. Take your telescope out while it is still light and arrange all the equipment you need to be close at hand.

Spend time on your polar alignment and focus and try not to get distracted with to many targets. Some nights I find myself jumping from target to target and not getting any good shots of any of them. Decide on your target before you set up, Get an idea where it is in the sky and once you have focus spend time on framing the shot, maybe getting a nice bright star in the shot.

Take as many shots as you can but unless you are autoguiding don’t take exposures longer than 30 secs.

You can view some of my photographs on this page 

The next post will concentrate on processing these images in Deep Sky Stacker and Photoshop.

 

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