Basics - Astro

A telescope uses either lenses (refractors) or mirrors (reflectors) or both lenses and mirrors to collect and magnify light.

Each method has its advantages and disadvantages.

With lenses, the focal point of different wavelengths of light differs by several millimeters which causes the resulting image to have red, green or blue fringes, depending on which colour is actually in focus.

To rectify this multiple lenses and certain types of glass is used which results in either achromatic (red and blue having the same focal point) or apochromatic (all colours having the same focal point) telescopes.

When using mirrors, usually a parabolic primary mirror at the back of the telescope reflects light to a secondary mirror at the front of the telescope.  The secondary mirror can be flat and set at 45 degrees, reflecting the light to an eyepiece or camera at the side of the front end of the telescope (Newtonian) or can be hyperbolic or parabolic and set parallel to the primary mirror, reflecting the light through a hole in the primary mirror to an eyepiece or camera at the bach of the telescope (Cassegrain).  This "folded" light pass can significantly increase the focal length of the telescope

The focal length of the telescope determines the magnification of the resulting image.  I higher focal length will reduce the field of view and increase the magnification.

The amount of light a telescope (aperture) can gather depends on the diameter of the lens or primary mirror and the focal length.  Thus the aperture a 1000mm focal length telescope with a primary mirror diameter of 200mm is 1000 / 200 = 5 usually referred to as f5.  The lower the aperture ("f-number") the more light a telescope can collect.

It is possible to use a smartphone camera held to a telescope eyepiece to take astro photos, however a better option allowing much longer exposure times is to attach a camera to the telescope in place of the eyepiece.

Adapters are available for most DSLR cameras which attach them to the telescope, however the biggest disadvantage is that the CMOS sensors used in DSLR cameras heat up with the longer exposure times required for DSO photography and this heat leads to noisy images.

Dedicated astrophotography cameras are available, also with CMOS sensors, but with coolers which can maintain a sensor temperature of up to 45 degrees below ambient temperature.

These cameras are available as either colour or mono cameras.  Colour cameras capture the complete image at once, but the sensitivity is only one quarter that of a mono camera which uses filters (luminance, Red, Green, Blue) to capture each colour separately and usually requires four times as many images as a colour camera.  A mono camera can also be used with "narrowband" filters, but that is beyond the scope of this document.

The sensor size, pixel size and therefore the number of pixels determine the resulting image resolution and field af view.

When using a mono camera and especially so with narrowband filters, "false colour" images can be produced (i.e. when using the "Hubble Pallette").

Two types of mount are available, Alt-Az (altitude-azimuth) and EQ (German Equatorial).  Both types can be used for astrophotography, but the Alt-Az mount is best used for short exposure solar system photography.

The EQ mount aligns one axis (Dec) to the earth's axis so that when tracking an object, only the other axis (RA) needs to move.

With a motorised mount, the tracking is (almost) automatic and generally keeps the selected object in the centre of the field of view.

However, even the very best motorised mount has some errors and without any feedback as to its precise position, there will be some "drift", where the object moves away from the centre.

Guiding is a method used to provide feedback to the mount and counteract most of the drift.  A second camera is attached to the telescope, either via a smaller secondary guide telescope or with the use of an "off-axis" guide mirror.

This second camera then continuously images the area of sky and software is used to "lock on" to a star.  Any drift is then counteracted by the software sending additional small movement pulses to the mount.

Degrees (symbol: °) are broken down into (arc)Minutes (symbol: ") and (arc)Seconds (symbol: '), where there are 60 minutes in a degree and 60 seconds in a minute.  Thus one arc second is 1/3600 of a degree.

The field of view is determined by both the focal length of the telescope and the size of the camera sensor and is measured in degrees, minutes and seconds.

All objects in the sky are given a "magnitude" or brightness value.  The higher the magnitude number, the lower the brightness of the object.

Manufacturers assign a maximum magnitude value to telescopes, objects above this value cannot normally be seen with that telescope.

Some objects in the sky are very bright and are assigned an negative magnitude.  The sun, for example has a value of about -27, whereas the brightest visible star in the night sky (Sirius A) has a value of -1.46.  The International Space Station has a magnitude around -2

This is the really simple part.  Just capture as many photos as possible.

Take as many exposures as you can and set the exposure time to as long as possible, but within limits.  Ensure that the brighter elements of the object are not overblown - use the capture software histogram function to do this.  Also, get to know the capabilityes of your mount.  For example, you may start to get oval stars with very long exposures if your tracking or guiding is not ideal.

Exposure time will also depend on the sensitivity of your camera sensor (smaller pixels/colour camera = reduced sensitivity) and the telescope aperture.

The images of the object as know as Lights.

You will also need to take calibration frames to improve the final image:

Darks - with the lens cap on using the same settings (gain/ISO/temp/time) as the lights.

Ideally, two other sets of calibration images should be taken:

Flats - place a white t-shirt (or similar) over the telescope illuminate with a flat light and set the exposure to produce a half-white image (again use the histogram to achieve this).  Gain/ISO and temp must be the same as the lights, and most importantly, the camera orientation within the telescope must be the same as for the lights.

Dark Flats - same settings as the flats, but with the lens cap in place.

There are many free and paid-for software packages available for download designed to process your images.

The original images are all considered as mono; if you have used a colour camera, they will go through a process known as Debayering which converts the mono image to a colour one.

The images will then be calibrated using the calibration frames and then normalised to ensure they are all very close in terms of overall brighness and colour.

Registration is the next step, where all the images are lined up and in the correct orientation.

Finally, the images are integrated into a single, final image.

When the above is complete, the image will be denoised, stars corrected and other setting adjusted (saturation, hue, contrast etc).

Save the images and publish :)