I'm going to need some help here as my knowledge of Astrophotography is limited, but I will try and bluff my way through with the help of some of my customers.
First up, here is a photo by Graham Leaver of the Horsehead and Flame Nebula in Orion, taken with a Full Spectrum Modified Sony a5000 mounted on a Celestron Apochromatic Refractor, ED100mm f9.0. The finished photo is a stack of 21x 6minute exposures.
Graham has kindly provided a brief guide to how the above photo was taken;
The total of 2hr and 6min is made up of 21 x 6min separate exposures which when put into a stacking program, the exposure times add up.
The Full Spectrum Sony a5000 camera, set to iso 800, is at the focal plane of a Celestron semi apo-chromat refractor, a 100mm f9 and through a Baader uhc filter to cut down sky light pollution.
The telescope is mounted on a Vixen atlux equatorial mount which is computer controlled and precisely guided by a secondary smaller telescope which has a smaller chip camera at its focus.
A software program (phd2 guiding program - free!) takes the picture from this secondary camera, notes where the positions of star images are and in succesive pictures say every 2-3 sec, notes if the image has moved and sends signals back to the main telescope mount making corrections to its position so keeping the image steady in the main camera.
The length of individual exposures depends on the amount of skylight that gradually fogs up the image.
Some filters are available that help reduce various wavelengths in the spectrum, ie sodium, mercury and are transparent to light transmission from hydrogen alpha and beta wavelengths.
If you're in a good dark sky area then these filters may not be needed at all.
The area in the photo is the area just below the lower of the three stars (Alnitak) in the belt of the constellation of Orion which is visible in the sky in winter, as shown in the illustration below left.
The image above on the right shows one of the single 6 min exposures straight from the camera, so you can see what a difference the extra 2 hours of exposure make when all 21 images are stacked!!
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Thank you Graham for the amazing photo and the explanation, personally I can't believe the result obtained from relatively inexpensive equipment, I was under the impression this sort of photo was only possible with huge telescopes on mountain tops or the Hubble Telescope in space. I know you can use wide angle to take amazing photos of the Milky Way, but I didn't realise that these Nebulas and stuff were so big, I have looked through telescopes in the past and not been particularly impressed, but obviously over 2hrs of exposure brings out details and colour that you cannot possibly see directly through the scope!!
A Full Spectrum converted camera is many more times sensitive to the Red light emitted by Hydrogen called the H-Alpha emmission line at 656nm. The Cyan/Blueish filter that is removed as part of the conversion normally blocks about 80% of the H-Alpha light, so it is essential that this is removed as Hydrogen is the most abundent element in the universe and it emits this red light when the gas is excited in nebulas etc.
For Astro work the Full Spectrum conversion is the first step in getting the sensor to be as sensitive as possible to all wavelengths of light, however we don't always want to photograph all wavelengths for several reasons. Firstly, most refractor telescopes or camera lenses cannot focus all wavelengths to the same point, which results in blurred stars. Secondly, light pollution from street lights and general external lighting exacerbates sky glow. A lot of this light pollution can be filtered out using specialist filters as can other wavelengths that are not required for a particular shot. These specialist filters can be added to the telescope or lens or even clipped into the lens mount of the camera. These specialist filters are mostly interference filters made by depositing very thin layers of chemicals onto glass which either transmit or reflect certain wavelengths [ Hot Mirrors ], by building up sometimes dozens of layers the filter manufacturers can tailor what wavelengths are let through and which are rejected. So for instance the emission lines of of Sodium and Mercury vapour streetlights can be completely eliminated as can unwanted Infrared. These can then increase the contrast of the desired object, by reducing the unwanted background light pollution. Which individual filters are used is beyond the scope of this article [and my knowledge], there are some excellent specialist Astro websites such as cloudynights.com etc. The photo at the top of the page by Graham Leaver is taken with a Full Spectrum Sony a5000 camera mounted on his refractor telescope, but he added a Baader UHC filter to reduce light pollution and IR, this transmits two bands of light 450-550nm and 600-700nm which between them cover most of the wanted wavelengths and reject the polluting light allowing longer exposures to gather as much "Starlight" as possible and by blocking the IR above 700nm reducing Star Bloat.