Capturing Images

"Prime focus" in SCTs is actually a misnomer because the point of primary focus in a SCT is at the secondary mirror, not at the visual back. Apart from a particular configuration that places a camera at the secondary mirror in certain Celestron scopes, the correct term for this type of focus is "cassegrain focus." Having said that, the term prime focus is most commonly used and I will also.

For prime focus astrophotography, I use canon's 300D [digital Single Lens Reflex (DSLR)] camera. It is a great camera with superb light sensitivity and low noise. Like all digital cameras, it is however relatively insensitive to the colour red - but then so are our eyes under dim light conditions.

I am no expert but one important thing I have learnt is that capturing deep sky images with the LX90 is not at all an easy undertaking. Before you can even open the shutter of my camera a number of things are required - polar alignment of my scope, periodic error training, and some method of guiding manually or autoguiding. Each of these initial steps are topics in their own right that I cover on other pages.

In this section, I am not presenting universal truths but a simple account of what I do when I am attempting prime focus imaging with my LX90. Other amateur astronomers will have other methods that work well for them and I encourage you to explore the ideas of others. If you decide to read on, my hope is that you can find something that interests you.

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Physical connections
Electrical Connections
Image acquisition

Physical connections

To attach my camera to my LX90, or any other SCT for that matter, you will need both a T-mount (T-adapter) and T-ring. The T-mount has at one end a female thread that screws to back of my scope and at the other end a male thread that screws into my T-ring. The T-ring connects to my DSLR camera in the same way a lens does and is specific to the camera you have. If you are using a focal reducer, this attaches between the back of the LX90 and the T-mount. The proper configuration is shown in the Image 1 below.

This image shows the method of attachment of Canon's EOS 300D to the LX90. In this instance I have also used an f/6.3 focal reducer. The Baader T-mount is of the short variety (16 mm) and reduces the risk of the camera hitting the base of the telescope during slewing or imaging.

Ensure that my camera is firmly locked (finger tight) at the correct orientation so that the field of view matches the field of view of my planetarium software. This is generally so that the long axis of the image is parallel to the ground. Of course, it is acceptable to lock the camera in any orientation. Indeed, you may have to do this to fit the object of interest into the field of view. The Baader T-mount used as per above is of exceptional design that can lock in any position. It was obtained from Alpine Astronomical in the USA.

Electrical connections

When you release the shutter of my camera to take an astrophotograph, you would not wish to do so with my finger. This is fine for daylight photography where very short exposures are typically used and the effect of camera shake is negligible. For longer exposures therefore you will need some type of cable release.

My camera's shutter is controlled by computer. To do this you need a special cable to connect the camera to the computer serial or parallel port. For more details on this check out my serial cable project. I also use the USB cable that came with the camera to adjust exposure settings via DSLR Focus (see below) and (occasionally) upload images to my computer. I say 'occasionally' because mostly I write images to a 512 Kb CompactFlash card simply because the write time is much faster.

So, there are two cables connected to the 300D for imaging - one is a standard USB cable, the other plugs into the remote 2.5 mm audio-type socket and connects to a serial port on my laptop or notebook PC (see Image 2 below).

This image shows the USB (top) and serial shutter control (bottom) cables exiting the 300D camera. For details on how to make your own serial cable, click here.

The port side of my Dell Inspiron 8500 notebook is extremely busy. I only have two USB ports and I really need three, so I use a 4-bay USB adapter that plugs into one of the USB ports and a USB to serial adapter that plugs into the other. The latter connects to the serial shutter control cable. As it turns out, this is assigned COM5. The true serial port controls LX90 goto slewing via the Meade #505 cable that connects to the Autostar 497 handbox. For autoguiding, the Shoestring Astronomy Guide Port Interface plugs into the parallel port and passes signals to the LX90 APM CCD port.

The connections at the back of my notebook PC are detailed in Image 3 below.

This image shows the connections at the back of my notebook PC utilised during a night of autoguiding and imaging with the 300D. From these connections and cables I can slew the scope to any object (serial #505 cable), autoguide (using a webcam attached to a spare USB port via the parallel port guide Port Interface), and control exposures in my 300D (using the 300D USB cable and serial shutter control cables).

Image acquisition

I'd like to break image acquisition down into four distinct steps - selecting a target to photograph, slewing to the target, focusing on the object, and firing the camera shutter.

1. Selecting a target
The first step in acquiring an image is to locate a suitable target object visible from my observing location. Three software applications that I use for this purpose are AstroPlanner by Paul Rodman, Deepsky Astronomy Software by Steven S. Tuma, and Starry Night Pro 4.2 by Space.Com. All three of these applications have planning components.

2. Slewing to target
Apart from when I undertake the iterative polar alignment routine, which I do with AstroPlanner, all slewing is done using Starry Night Pro. This is very easy software to use for slewing. All you need to do is locate the target, right click on it, then select 'Slew to...' The slewing commands are sent through to the Autostar via the #505 cable. Below is a screenshot of Starry night Pro showing the southern sky sue South-East of my front yard.

Screenshot of Starry Night Pro 4.5.2 showing the night sky from my location on November 20, 2004 at 2200 hrs. Equatorial coordinates are shown in red. The South Celestial Pole is upper right. Image copyright © 2003 Space.Com.

3. Focusing the target
The software that I use for focusing my 300D through the telescope and controlling the camera settings and exposures is DSLR Focus. DSLR Focus is a masterful application which is now very popular among, particularly, Canon DSLR users because it controls all features of the camera that are relevant to astrophotography (ie. the ISO setting, the image size, shutter speed or exposure duration, and aperture if you are not at prime focus).

DSLR Focus assists focusing by taking and displaying an image after each adjustment of the focuser. The latest version supports autofocus with ASCOM compliant electronic focusers. With each image a number of statistical methods that analyse the selected target star inform the user whether they are getting closer to focus or further away. DSLR Focus works very well and is the best method I know of achieving pin-point accuracy focus.

4. Firing the shutter
The software that I use to control the 300D is also DSLR Focus, although you can also use ImagesPlus by Mike Unsold or DSLRControl by Steve Barkes (parallel port control only). DSLR Focus has a number of additional features that support the easy acquisition of images such as Exposure Sequencing where you can set up a sequence of exposures at different ISO values and exposure durations. This is particularly useful if you are taking images of bright nebulae such as M42 (Orion Nebula) where short exposures are also required in order to properly expose the core region. it is also very useful for taking a series of dark frames at the conclusion of your night of imaging.

I always image in Canon RAW mode and mostly use ISO 800. Currently I am experimenting with autoguiding and the duration of each exposure ranges from 2 to 5 minutes. Depending on the imaging target, I may take 20 or more images. With DSLR Focus, I can sequence these images easily, push the Start button, and then go inside for a cup of tea while the images are taken for the specified duration at the specified ISO value and are automatically stored on the CompactFlash card. DSLR Focus even counts down the remaining time for each image by voice.

Below is a screenshot of DSLR Focus covering some of the many of its focusing and imaging functions.

Prime Focus Imaging with the LX90 and 300D