Astrocameras

Introduction

Astronomy really deals with six categories of object

• Stars
• Galaxies
• Nebulae
• Planets
• The moon
• The sun

Each has different challenges and different equipment requirements and there’s no way a single setup can optimise everything. However, my purpose is not to compete with the serious amateurs who live under dark skies with excellent seeing and have the option of a permanent setup, but to be able to indulge a long-standing hobby and share it with others.

In addition to the categories of object, we need to consider what we want to do. There are, these days, basically three approaches

• Visual astronomy. You look through binoculars or a telescope. If you want some kind of permanent record, you sketch or annotate. This is the traditional kind of amateur astronomy which was the only game in town for a lot of people before digital cameras became ubiquitous.
• EAA. You use a computer-connected camera and software which processes one or more images in near real-time to give you a screen view
• Astrophotography proper. You use a digital camera to capture multiple images and then spend a lot of time post-processing them in order to obtain the best possible result.

Most of the stunning pictures you see published come from full Astrophotography by people with a lot of time to spend and the money to invest in a good quality mount as well as decent optics.

Visual astronomy has an immediacy which nothing else can come close to. But for the casual observer, what you actually see through any telescope is much less dramatic than those pictures. At first sight, planets appear to be bright blobs with possibly some just visible gradations of darkness and washed out colour if any. With a bit of experience, a steady mount, and good conditions, one can eventually learn to concentrate on the detail at fleeting moments of good seeing, but after the initial excitement of seeing the rings of Saturn a couple of bands on Jupiter or the phases of Venus, one quickly hits a plateau with planetary observation.

Deep sky objects can also pall quickly. Many of them are called ‘faint fuzzies’ for a reason. Even the brightest have no visible colour to the naked eye, and if you aren’t lucky enough to be in a dark location, only the brightest are visible at all.

Double stars, especially bright pairs with different colours, are amongst the few exceptions. What you seen when you first look is exactly what you expect/hope to see. Experience, good equipment and conditions etc can significantly affect which doubles you can separate but that’s all.

Variable stars can be a source of great interest to the amateur, but a single look at a variable star is hardly a thrilling visual experience.

EAA offers an intermediate option between the two above and is of increasing interest. For deep sky observations, a relatively modest scope can easily beat the naked eye and a much bigger scope in terms of what you can see and whether you get any colour. For planetary too, you will likely get rather more detail than you would see by looking directly.

EAA also offers a chance of observing otherwise extremely difficult objects, such as Phobos, where the problem is not that it’s too close to Mars to resolve, but that the difference in brightness is so great that it’s almost impossible to differentiate from the glare. when observing visually.

• Bigger focal length for same objective size makes the image bigger but fainter.
• Bigger objective at same focal length makes the image brighter and improves the diffraction limit if seeing permits
• Small CCD size is fine as planet occupies a fraction of a degree.
• Fast video is ideal, as brightness allows short exposures, and image selection can eliminate bad seeing to a great extent.
• As interesting features on a planet can be well under an arcsecond, a bigger objective will reveal more detail if the video can get frames of really good seeing.

Conversely,

Useful reference info

https://diffractionlimited.com/matching-camera-optics

https://astronomy.tools/calculators/ccd_suitability

Image scale: [“/pixel] = 206.265 * Pixel size [µm] / Focal length [mm] 

Brightness factor: (Diameter / focal_length) ^2

Size of point image in seconds due to diffraction: 138 / Diameter ( in mm )

In the atmosphere, even excellent seeing limits resolution to 0.5 seconds for normal exposures, so effectively, you won’t get better resolution once your objective exceeds about 250-300mm. However, if using lucky imaging for planets, your best video frames could be better, and so an even larger objective might work better.

The desirable image scale for deep sky photography is 0.5-1.0 for excellent seeing 0.5-1.25 for typical seeing, 1-2 for poor seeing.

For planetary observation, you want 0.1 – 0.25 arcsecs per pixel.

We assume we use 2.5 barlow for planetary and not for deep sky, so 0.2 arcsecs per pixel for planetary is 0.5 arcsecs per pixel for deep sky, so we should aim at camera with pixel size 0.5 arcsecs in ideal conditions. Twice that would probably be fine in practice given typical seeing.

The formula at the top says Pixel size = 0.5 * focal length / 206.265 = focal length / 400

However, the above formula assumes a large enough objective. If the diffraction limit of the objective is such that the limit is that, then these numbers need adjusting.

Telescopes

• LX90 – 200mm x 2000mm f10 Ideal pixel size 5
• ED120 120mm x 950mm f8 Ideal pixel size 2.4
• ETX70 70mm x 350mm f5 Ideal pixel size 0.9 but objective too small )
• Finder 50mm x 200mm f4 Ideal pixel size 0.5 but objective far too small

Altair GPCAM ARO130C colour camera.

• Aptina AR0130 CMOS sensor – 1.2 MPixels
• Sensor Size: 1/3″ diagonal (4.8 mm x 3.6 mm)
• 1280×960 pixels
• Pixel size: 3.75 µm x 3.75 µm
• Approx. 17 fps @ max. resolution
• 1.25″ connection with filter thread
• USB2.0 and ST-4 port
• Lens thread: C-Mount

With finder scope, image size per pixel is 4.2 arcseconds. 3 secs exposure at gain 1000 is minimum for DSO. FOV is 1.46*1.09 degrees

With ETX70 image size per pixel is 2.4 arcseconds. 5 secs exposure minimum.

With ED120 image size per pixel is 0.9 arcseconds. 11 secs exposure minimum

With LX90 image size per pixel is 0.4 arcseconds. 20 secs exposure minimum

Sony A6000

• Sensor size 23.5 x 15.6 mm
• Sensor output 6000 x 4000
• Pixel size 3.91µm
• Lens – E mount
• ISO 100-25600

Canon 1100D

• 5.2 micrometer pixel size
• 4272×2848 pixel sensor size
• max ISO 6400

Again with ETX70 we have image size per pixel of 3.06 seconds and field of view of 3.6 by 2.4 degrees.

ETX70

With the Altair, the image scale is 2.21 seconds per pixel which means the pixels are a bit large for the telescope. With the Canon it’s 3.06 which is even worse. The Altair field of view is 47×35 minutes, and the Canon 3.6×2.4 degrees

ED120

The ED120 has a 120mm objective and a 950mm focal length, so Altair 0.81 image scale which is pretty good for excellent to normal seeing. Canon is 1.12 which is OK unless the seeing is excellent.

ZWO camera choice

I want a new camera of higher quality than the Altair which can do planetary and deep sky.

• ASI585 £344 pixel 2.9 8.2M