Astrophotography Journal


Copyright © 2004 Nick Glenister

The trouble with telescopes

Before we start I would like to tell you about a website I visited when I was choosing what to buy. Unfortunatly the exact page I looked at at the time no longer exists but Celetron do have knowledge base which still contains most if not all of the information I found several years ago Celestron Website

Looking back now my choice of telescope was not altogther the best however it was a great telescope. I chose a Newtonian Reflector because it was far cheaper than an equivalent aperture Cassigrain and is better at observing deep sky objects and astrophotography compared with a refractor. Were I able to afford a Cassigrain at the time I would have bought one and looking back I now wish I had waited and spent more (more on this later). I chose the C6-N / C150-HD which was a good compromise between aperture at 6 inches and cost but is much larger and heaver than a cassigrain and is not at all portable. A great deal of confusion surrounds the actual telescope I have got. The only difference between the C6-N and the C150-HD is as follows:

Based on this information I chose the C6-N as this had the lowest f-number (refer back to the Film SLR journal entry as this explains f-numbers for camera lenses - as I wanted to take photographs of very faint object I figured a lower f-number would allow more light onto the film in a shorter time). When I got the telescope it contained instructions for the C150-HD, this wans't much of a problem as they are both identical in all other respects but I did wonder why both existed and why there is a £70 price difference.

The telescope comes with a CG-4 equatorial mount and a 20mm eyepiece but I also needed motor drives to fit to the mount in order to automatically track the stars over long exposures. In addition to this I also purchased two T-rings for attaching my cameras, a 6mm lens which over significant magnification over the 20mm that came with the telescope and a Barlow lens which doubles the effective magnification of a lens effectively turning my 20mm into a 10mm and my 6mm into a 3mm (please note that the suggested eyepiece for the telescope I bought to achieve maximum magnification was 5mm, I will discuss this at a later date. Go here to read this).

When I received the telescope I was faced with the challenge of assembling it all. This is far more complicated than you might think and you are constantly hampered by an overwhelming desire to put it together as quickly as possible and go out and use it. Expect to spend several hours putting together your first telescope. I will quickly mention each of the problems I encountered with the C6-N which came about from rushing into it and not carefully reading the instructions first.

Attaching the Telescope tube to the mount.

The tube has two metal braces around it which were fitted when I received the telescope. These attach to the mounting platform. My first mistake was removing them in order to assess how they attached to the mounting platform. Once I figured this out I attached them to the mounting platform and then tried to fit the tube on. It worked but required far more effort than the correct way.

Attaching the finder scope

This was a bit silly really I put the holder the wrong way round and subsequently had no control over it. Had I read the instruction first I would’ve seen that the holder was back to front. A clear case of being over eager to get out and use the telescope and cockiness at thinking I new how to do it.

Attaching the T-Ring adaptors

The telescope I have (C6-N) comes with a built in T-Adapter. Do not confuse this with the T-Ring they are two separate pieces both of which are required to attach your camera to the focal point of the telescope. The instructions said I should remove the eyepiece from the eyepiece holder and unthread the eyepiece holder from the focuser assembly and thread the T-ring onto the exposed T-adapter threads. Unfortunately the thread on the T-ring was the same as the “exposed thread”. It turns out I was removing both the eyepiece holder and the T-adapter together and once I figured how to separate them it was easy but it did require sending an e-mail to Celestron technical support. It was at this time that I came to the crashing realisation that the eyepiece is not used when attaching the camera to the telescope. Prior to this point I had assumed that the camera attached to the eyepiece with the use of the T-Ring so that the photos you take with the telescope benefit from the extra magnification of the eyepiece giving you incredible photos of the planets and deep sky objects. This was a serious disappointment to me and essentially turned the telescope into an extra long telephoto lens.

Eyepieces

There are a wide range of eyepieces you can purchase but you are limited to those you can use by your telescopes aperture. It was explained to me that the smaller the eyepiece in mm the higher the magnification but at the cost of the brightness of the image. In my experience this does hold true but to me there is a far more obvious limitation in the focusing when using smaller eyepieces. For my telescope I have calculated (using the rules and information taken from celestrons website) that the smallest eyepiece I can use is 5mm. I instead chose a 6mm as this was more affordable but when I come to use it the focusing is hairline and very difficult to get right. This came as quite a surprise and not least a disappointment as I didn’t expect the focusing to be so tight and having found it to be so I was futher disapointed that the design of the telescope didn’t incorporate a finer/more fluid focusing dial. Still you get what you pay for!

Taking photos with your telescope

Taking photos with your telescope is where the fun really begins. This is essentially the same practice of taking photos of the stars without a telescope. All you need is an SLR camera (but you might have success with a compact camera, more on this later) capable of long exposures using the bulb mode and a cable release.

You have three choices on how to take photos with your telescope:

Attaching the camera to the Focal point
The camera is attached to the telescope using a T-adaptor and the correct T-ring for your camera. Its important to know that the eyepiece is removed prior to attaching the camera so you are limited to the telescopes focal length.
Attaching the camera to the back of the telescope (piggyback)
This is essentially no different to using the camera on its own but if you have a motor driven equatorial mount you can make much longer exposures which is essential for deep sky objects.
Afocal coupling
In this configuration the camera is not attached to the telescope but 'looks' through the eyepiece in the same way you do. The huge benefit to this is that you can gain the advantage of the magnification of the eyepiece but the down side is the object will quickly move out of view. In the future I will be attempting to make an afocal mount that will allow the camera to attach onto to the telescope but adjustable so that it is still afocal. This will allow long exposures while having the benefit of the extra eyepiece magnification so I’m hoping for superb photos of the planets once I’ve completed it. I hope to bring this too you as part of this journal.
Manually held (Afocal coupling)
You can also use non SLR cameras as I did with this photo of the moon taken with a compact digital camera hand held at the eyepiece. The magnification and amount of light given off buy the moon make this possible. If the object is any smaller the compact camera would not perform as well.