Beginning Astroimaging Session 4 Written by Geoff Smith

Session Summary Objective: Understand the various cameras you will need for astroimaging Covers: • imaging camera • guide camera • why you want/don’t want a dedicated astroimaging camera • matching camera and telescope • field of view considerations • focal length considerations • colour or mono • software for image acquisition e.g. Sequence Generator Pro, MaximDL, Astroart.

Imaging Camera This is your primary camera. It is the one which will capture your intended image. The range in price is enormous:

The Choices Not cooled, one shot colour, (DSLR, some guide and planetary cameras) Not cooled, monochrome (Some guide and planetary cameras) Cooled, one shot colour Cooled, monochrome Sensor types CMOS (Complementary Metal-Oxide-Semiconductor) CCD (Charge Coupled Device) https://www.atik-cameras.com/news/difference-between-ccd-cmos- sensors/

Cooled cameras Photons from the object release electrons from the camera chip. These electrons are counted and converted to a brightness reading. Electrons are also released due to the inherent temperature of the chip, producing an unwanted signal. The number of thermal electrons is roughly halved for every 6°C drop in temperature.

Colour vs Monochrome ALL CAMERA CHIPS ARE MONOCHROME. They only record light intensity, not the colour of the light.

One shot colour So-called colour cameras use a Bayer Matrix over the chip: The camera software then uses the intensities through the red, green and blue filters to assemble a colour image. There is necessarily interpolation with a (slight) consequent loss in resolution

Monochrome cameras These use a filter wheel to rotate filters into the optical path—typically Red, Green, Blue, Luminance

Dispelling a myth Myth: My imaging time is limited so I have to have a one-shot colour camera. I can’t afford to spend the time taking images through multiple filters. Fact: For a given imaging time, we can always arrange for a monochrome camera to produce a colour image that has captured more photons than a one shot colour camera

Guide Camera Purpose is to correct tracking inaccuracies of the mount. A star—the guide star— is selected and takes repeated exposures, usually 2–10 seconds in length. The object is to keep the guide star in the same position on the guide camera chip by sending corrections to the mount if it strays.

Guide camera examples Two cameras in one! SBIG dual chip cameras have two chips — a main camera chip and a guide chip.

More conventional guiders

Using a guide camera Guide camera on a separate telescope—the guide scope Off-axis guider

Colour or Mono Colour is cheaper—no filter wheel, no filters Mono is more flexible —not restricted to R, G and B filters Mono gives better resolution Very little difference in difficulty of processing

Matching Equipment The following factors need to be considered: Focal length Aperture Sensor size Pixel size A spreadsheet can be very useful http://www.deepskywatch.com/telescope-calculator.html

Some considerations Long focal length: Tracking and seeing more critical, bigger image scale Short focal length: Easier tracking, seeing less of a problem, less resolution Large pixels: Less likely to saturate, loss of resolution with short focal lengths Small pixels: Better suited to short focal lengths, saturate more readily, no gain if used with long focal lengths.

Do you really need a dedicated astrocamera? YES if you are serious about taking a large number of quality images NO if you are content to take the odd image of easy objects—Trifid, Lagoon Nebula, wide angle shots, eclipses, transits of Venus. A DSLR is all you need

Software for Image Acquisition Many astro cameras come with their own acquisition software Atik, SBIG, FLI, Starlight Xpress Some third party software for DSLR ImagesPlus, BackyardEOS, MaximDSLR However, people usually use a software package that is not camera specific Nebulosity, MaximDL, Astroart, Sequence Generator Pro (SGP) Nebulosity is a good beginner choice—simple to use, versatile, cheap (US$95). SGP is also highly recommended (US$99).

Objective: Capture long exposure pictures with a guided and accurately polar aligned tracking mount Motivation:  Obtain the highest possible quality in our pictures prior to processing Covers (in overview): • equatorial mount • polar alignment • mount control with a computer • guiding Equipment: • telescope • mount • tripod • camera • guide camera • equipment to attach/use guide camera (guide scope/off-axis guiding) to telescope • guiding software, e.g. PHD2 Includes: • various polar-aligning techniques • guide scope vs off-axis guiding • guiding Next Time

Homework Refresh your understanding of polar alignment: https://www.assa.org.au/resources/equipment-reviews/polar-alignment/ Here is a very interesting daytime method that uses an iPhone. https://www.skyandtelescope.com/observing/daylight-polar-alignment/ Autoguiding is not as simple as sellers of software and guide cameras make out. Here is an in-depth article on autoguiding http://acp.dc3.com/McMillanAutoguiding11-2005.pdf Don’t spend too much time here. As you get more experience with the vagaries of autoguiding there will come a time when it will be useful.