Wednesday, February 13, 2013

Refractor or Reflector, for Deep Sky Photography

If you target for faint deep sky imaging, then you will need a very good quality optics, and these come in two main designs, refractors and reflectors. Refractors are the ones with the main objective lens at the front, reflectors counts on a big light-collecting mirror and come in several different configurations. So here's your first big decision, refractor or reflector, and what size?

Also these telescopes seem to come with a variety of different types of mounting and controls systems, Altazimuth, Equatorial and the newer "Goto" system. Which do you choose? Unfortunately these are very difficult questions from the start.!

Modern day refractors are beautiful and durable instruments, especially those with multi-element lenses that are designed to cut down chromatic aberration. These refractors are often referred to as "apochromatic", but the point is, by using different glasses in the multi-element lenses, the splitting up of light into its constituent colors (just as light does in passing through a glass prism) is minimized.  If the refractor is to be your main imaging telescope you will need the more expensive three-element apochromat in order to eradicate "blue halos" around bright stars and "star bloating". If your telescope only had one type of glass in its construction, then it would split the light up into its constituent colors as it passed through the optical system, and finally the different colors would be focused in slightly different positions at the eyepiece end. There are filters you can use to cut down these unwanted effects, and these are usually referred to as "fringe-killers". These filters cut down the blue and infrared wavelengths passing through your refractor minimizing the blue halos, and the bloated (oversize) stars caused mainly by the near infrared radiation your CCD camera will be sensitive to. Since your CCD camera is invariably based on Silicon technology, it will detect radiation up to near 1 micron (10-6 m) in wavelength. Very deep red extends to around 0.695 microns, so the CCD is sensitive well beyond the eye's visible wavelength limit and it extends your detection limits to the near infrared. The filter may sound like a good way of turning a “cheap” refractor into an effective apochromat for imaging purposes, but of course there is a catch.

By severely attenuating any blue transmission you will find that your CCD simply cannot detect certain reflection nebulae such as the Iris nebula in Cepheus, or the reflection nebulae near the Horse-head nebula in Orion. By effectively cutting out the blue transmission you will also find that you cannot detect the faint clouds of nebulae around the Pleiades (seven sisters, Subaru, or Messier 45) either. So I'm afraid you can't cheat here and it is not possible to turn a sow's ear into a silk purse by this approach. However, there really is one way you can win!. but it is worth introducing H-alpha filters at this point. If you are forced to image in a heavily light polluted area, you will be forced to go down this route anyway, and these filters also allow you to image when the Moon is creating a lot of sky-glow. A hydrogen alpha or H-alpha filter only allows through a very narrow range of light wavelengths in the red region of the visible spectrum. This narrow range of wavelengths corresponds to the emission of hydrogen atoms in most of the large emission nebulae we image.

Moonlight, being reflected sunlight, means there will be some light at the H-alpha wavelengths present, but the filter bandwidth is so narrow that the amount of H-alpha that gets through from Moonlight is very small indeed, and of course all the other wavelengths are very severely attenuated. An H-alpha filter therefore means we can image emission nebulae (and other objects, like stars) in poor visibility skies caused either by light pollution or Moon glow. There is an added bonus. Since only one very narrow band of wavelengths is getting through the H-alpha filter we can now use our “cheap” refractor for very effective imaging! All the other wavelengths that would have been focused at different distances from the eyepiece, forming an out-of-focus image, are gone. We can get nice, pin-sharp, H-alpha images from the night sky with relatively cheap equipment, and with light pollution or Moonlight present. Sounds too good to be true, so what's the catch? The catch is the same as for the fringe-killer filter, but far more extreme. Instead of not being able to image just the blue region of the spectrum, now you cannot image any part of the spectrum, with the exception of a tiny band of wavelengths in the red. You cannot therefore take those pretty full-color images of the night sky with just an H-alpha filter, but you can take very dramatic monochrome images, and it is easy to convert these into (red) color images with image processing software that is readily available including Noel Carboni's astronomy tools for Photoshop.

I deviated a little from the most direct course there, but I felt it was necessary to point out some of the options, and the compromises, at this very important initial decision-making stage. So if your main imaging system is to be a refractor, I am implying that you need to buy a good quality apochromat if your aim is to take those pretty full color images you see in the astronomy magazines. A refractor has other advantages too over the alternative types of telescope. Since a refractor has no obstructing objects in the optical path, a refractor will give you better image contrast than a reflector; this is more important for planetary observation and imaging rather than for deep-sky work. The way a refractor is constructed also means that it tends to hold its alignment (of its optical elements) better than the reflectors, which means that (unlike with reflectors) you do not need to worry about collimation. Refractors also have disadvantages of course compared to reflecting telescopes. Depending on the region of sky you are working in, the eyepiece may be very awkward to get to, this is important only if you wish to view through the system, and it is totally irrelevant if you only want to image.

However, if you want to both image, and occasionally view through your telescope, then this hindrance is worth bearing in mind. Light-grabbing power is all about aperture, which is down to the diameter of the objective lens in your refractor, or the diameter of the main primary mirror in your reflector. Per inch of aperture, the refractor is the most expensive telescope you can buy, especially so if we are considering apochromats. I have seen very good imaging work carried out with refractors of just 90mm (3.5 inch) aperture, but personally I would put this at the very minimum you should consider for serious imaging purposes. However, you also need to consider portability if you do not have the luxury of a permanent-base imaging site, so a large 6-inch refractor may not be feasible for your particular requirements, but as mentioned earlier, aperture is everything (provided the quality is there) so you would want the largest aperture apochromatic refractor you can afford, or carry. 

One last little detail before we move on to consider reflectors specifically, and that is auto-guiding  If we want to take individual images with an exposure time in the order of minutes, then not only do we need to be equatorial mounted and polar aligned, of which more later, but we will also need to guide our main imaging scope by optically locking on to a guide star somewhere in the image we are taking, and then moving the imaging scope so that the guide star remains fixed in the field of view. There are three main ways this can be achieved. You may either need a second CCD camera to act as the guide star detector, or there are some CCD cameras that allow you to both image, and have a separate area for the auto-guiding function. If we consider the separate CCD camera first, then this needs to also see the same region of sky as the main imaging CCD, and this can be achieved in one of two ways. Either, the auto-guiding CCD is connected to a separate guide scope, or you can fit what is called an "off axis guider" in the eyepiece position of the telescope which allows you to fit the main imaging CCD in-line with the telescope, and the auto-guiding CCD at 90 degrees to the imaging CCD. Light is "split off" from the main light path to the imaging CCD by a prism that re-directs a small portion of the light to the auto-guiding CCD.

If you are using a separate guide scope then it is clear that this must be rigidly mounted like the main imaging scope so that both track the sky in the same manner, this can turn out to be very much harder than it sounds. The third option is to use a CCD that combines both an imager chip and a guider chip in the one integrated package. I personally have no experience of using combined imaging/auto-guiding CCD cameras but mention them here that for completeness and note that they do seem to operate very well. The only negative aspect I can see in having a combined imaging/guiding CCD is when you are imaging in H-alpha, or using narrow-band filters. Under these conditions you severely limit the number of photons reaching the CCD, including the guider, and this means you may need very long integration times to see an appropriate guide star. Such long integration times will generally lead to poorer auto-guiding.

References: Making Beautiful Deep-Sky Images: Astrophotography with Affordable Equipment and Software (Patrick Moore's Practical Astronomy Series)

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