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The Basics

Alot of people may have questions about telescope basics and the theory behind it. In this section of my website, I will actually DEMONSTRATE using actual astrophotos.

First, let's get the basics down.

#1) A telescope's most important aspect is APERATURE. By now, you probably already know this from tons and tons of websites and articles. Alot of people new to astronomy think that MAGNIFICATION is the most important aspect. It is NOT. Any telescope can "theoretically" achieve any magnification just by changing out the eyepiece. A telescope's true strength is in it's aperature. Aperature is the actual size of the primary lens or mirror. The larger the size, the more light grasp. The more light grasp, the higher the "theoretical" magnification can be pushed. As a rule of thumb, the upper limits for magnification is primarily dictated by the aperature in inches x 50. Hence a 4 inch telescope should not be pushed above 200x.

4 inches aperature x 50 = 200 power

This also converts to metric using the aperature size in mm x 2.

100 mm aperature x 2 = 200 power.

Remember, 25.4 mm = 1 inch. So if you have an 8 inch telescope (approx 200mm) you would have a maximum theoretical power of 400x.

This is the general Rule of Thumb. On certain days when seeing is bad (i.e. turbulence in the atmosphere), you may only be able to achieve 20 - 30 times your aperature in inches instead of 50.

Your telescope's quality, eyepiece quality and eyesight will also affect the outcome of your theoretical magnification limit. Note that the higher you push your telescope in power, the less detail you will see. That's not to say that you shouldn't use high power when the need calls for it. High power is most often used for detailed Lunar or Planetary observing. You need to find a "sweet spot" that is a good mix of magnification and detail. You may get a crystal clear image of Mars at 50x but the image will be so small it will look like a dot, whereas, if you magnify Mars 600x (about the size of your thumbnail) it may be nothing more than a ruddy colored smudge. Again, with good optics you may be able to step outside of this boundary. I have pushed my 6" telescope to 406x with decent results. Even though I lost some of the detail on the planet I was viewing, there was enough left that I could discern surface features. Plus the view was large enough so I did not have to squint to see it.

#2) Focal Length and Magnification. Your scope's focal length and magnification go hand-in-hand. A scope's focal length determines it's focal ratio. Let's use my telescope as an example. I own a 6" newtonian reflector with a focal length of 750 mm. Since my scope's aperature is in inches we need to convert to metric.

Since there are approximately 25 mm per inch my scope would be:

25mm x 6 = 150 mm

Hence my aperature is 150mm. I have a focal ratio of f/5. This means that my focal length is 5x what my aperature is. My focal length is therefore 750mm.

Focal length = aperature x focal ratio

Focal length = 150 (aperature) x 5 (focal ratio) = 750 mm

A focal point is the length from the primary at which an image is perfectly focused. It's the same as using a magnifying glass on a sunny day to burn things. Too close or too far and there is no heat produced by the magnifying glass. Get it just right and you will burn a hole through newspaper, leaves or bugs. (CAUTION: I AM NOT CONDONING BURNING BUGS AND STARTING FIRES...I AM TRYING TO MAKE A POINT). If you have a magnifying glass that's 2 inches in diameter, and on a sunny day you have to hold it about 5 inches above the ground in order to start burning things then you have a focal ratio of 2.5 or f/2.5 (5 inch distance / 2 inches aperature = 2.5) and a focal length of 5 inches.

Now that we have a crash course in Focal Lengths and Focal Ratios let's move on to how this corresponds with Magnification. Magnification is determined by the Focal length divided by the size of the eyepiece. Again. let's use my telescope as an example. I have a 150mm telescope with a f/5 ratio. We already know that the equation is 150 x 5 = 750. So my focal length is 750mm. Now, say we are using a 6 mm eyepiece. My magnification would then be as follows:

750 mm focal length / 6 mm eyepiece = 125x.

Using this eyepiece would yield 125x magnification.

125x power may be great for studying the moon and nebulae. But let's say I wanted to look at a vast star cluster. At 125x, only a small portion of the stars would be able to be seen in my field of view (FOV). For this reason I would probably use an eyepiece with a much larger size...say 25 mm. This would yield a magnification of 30x. (750mm focal length / 25 mm eyepiece = 30x) This is much more appropriate for larger star clusters (M44, M45 ect...)

The smaller the eyepiece size = the larger the magnification.

For detailed planetary study I would probably want to be less than 4 mm for an eyepiece size. At 4mm my scope would give me 187.5x. (750mm / 4mm = 187.5x) I have the choice of either getting a smaller aperature eyepiece OR utilizing a Barlow Lens. A Barlow Lens increases the magnification by extending a telescope's Focal Length. For instance, if I use a 2x barlow lens with my 6mm eyepiece the magnification would double. The number on the side of a barlow lens tells you by how much. (2x = 2 times, 3x=3 times ect...) Hence:

750mm focal length would now become 1,500mm focal length using my 2x barlow. Using my same 6mm eyepiece we have the following:

1,500mm / 6mm = 250x.

NOW I have enough power to see clearly the Cassinni Division on Saturn, the Great Red Spot on Jupiter and surface markings on Mars during opposition. Because the image is twice as big (and still within the rule of thumb for MY scope of 300x = 50 x aperature in inches) I have a large enough image that has alot of detail. An easier way to do this formula is to simply multiply the power of the eyepiece by 2. Either equation comes out to be the same.

Astrophotographers call this "slowing down" a telescope's speed. My scope is considered "fast". Any scope of f/5.0 or less is considered fast. Any scope over f/5.0 is considered slow. Slower scopes at f/10 or f/12 are almost always refractors or catadioptics (combinations of Refractors AND Reflectors). These would be Maks and SCT's. This is because reflectors usually have much larger aperatures than them because they are much cheaper in cost. A 7" quality made refractor would be camparable in price to a 15" Dobsonian or a 12" Newtonian Reflector. Imagine having a 15" Dobsonian with a f/12 focal ratio. It would be 180 inches in length (15 feet long).

There are pros and cons to each. A scope with a slow speed (f/12 for example) would give wonderful views of closeups of planets as well as the lunar surface without having to further "slow down" the scope. Magnification on these type scopes (usually refractors) is most times excellent with any eyepiece. Though the FOV (field of view) is smaller than a typical reflector the views are very detailed and there are very minimal coma abberations. Coma abberations are caused by the edges on mirrors. They are not noticeable too much on refractor lenses but on a fast scope with a mirror like mine...the edges of the FOV seem to distort the shapes of stars into "V" like shapes. However again, the cost is a key factor as well as the fact that with Achromatic refractors the views of bright objects gets a light purple hue around it's fringes. In this case an Apochromatic (or a Fluorite) Refractor, which are even more expensive, would be needed to eliminate this problem.

Anyway, let's move on.

Obtaining the Image

There are several methods of obtaining pictures including CCD cameras, modified webcams, digital cameras, film cameras and even video cameras (just to name a few). Other "less conventional" methods include electronic eyepieces that normally would be run by RCA cables to a TV. By using a video card with built-in RCA jacks, these images could be transmitted directly to a computer monitor.

I have used The Meade LPI, The Meade DSI and a modified webcam. They are all somewhat simple inexpensive CCD cameras. There are a great many CCDs out there from under $100 to over $10,000. Some allow you to image only the planets, the sun and the moon while others allow you to image faint DSO's.

The following are personal settings that I find to be most effective on days that the seeing is "average" to "good" using the Meade LPI:

Exposure: either 0.063 or 0.088, Gain: 40, Offset: 45, Mono: OFF, Align & Combine: Min Quality : 70% (for starters...move up higher if conditions allow), Evaluation Count: 8 (for starters...move up higher if conditions allow), Kernel Filter: For Jupiter go with a hard edge enhancement, this will accent the boundaries between the belts and the bands, For Saturn go with Medium for good definition in the ring divisions, Apply after: 8 Frames.

If you are new to Astrophotography, these are the only settings that you need to adjust. Unless you are much more advanced, other settings such as Histogram, Hue, Saturation and RGB scales can be tweaked AFTER the image has been obtained through an Image Processing Program such as Registax

Note: these are settings that "I" have found to work best with my setup. Settings will vary from camera to camera, location, conditions and telescope aperature, quality and type.

Moving on, here are some photos taken of Jupiter on April 28, 2006. The first shot is at my scope's regular speed (F/5) using the Meade LPI which is equivalent to a 6mm eyepiece. This yields me 125x.

Notice how bright the planet is and also how tiny it appears. Because of my scope's "fast" speed, I retain alot of light in such a small image which seems to overexpose the image. At this point I have a few options:

1) Darken up the photo of the planet. This can be done by either lowering the exposure time, lowering the gain, or lowering the histogram as well as a combination of any or all of these may do the trick. However, I am still left with an extremely tiny image. You would have to squint in order to see any real detail at this magnification.

2) Use a Barlow lens. By using the Barlow lens, I can slow down my telescope's speed (F ratio) from F/5 to a more moderate F/10 ratio. This will double the size of the planet as well as make it less bright. It becomes less bright because the same amount of light energy is transferred through the telescope, however, the energy is now dispersed over an area twice as large.

Notice how the detail in the planet is much more apparent in this photo.

By using a more powerful Barlow lens (say 3x) I can slow my scope's speed to F/15 making the image 3 times larger than the original photo, and 3 times fainter. But at this point my theoretical magnification of 375x may be alittle too much on most nights for my scope to handle (see the rule of thumb above...) not to mention that my scope does not have enough focuser travel to handle it. The most my scope can handle is about (2.5x) it's current speed, which would give me a focal ratio of F/12.5.

We can see that by continuing to magnify the image, all detail eventually becomes lost. With quality optics you may be able to outperform the standard rule of thumb (50x per inch of aperature) but, the seeing that particular night must be "perfect" in order to do so. Remember...try to find that "sweet spot" that is a good mix of magnification and detail. A photo can always be further increased in size through any photo editor.

For my final shot of Jupiter, I decided to use the 2nd of the 3 shots shown above. This shot had the best amount of detail and was also large enough to easily see. This is Jupiter's photo at F/10 that I have further blown-up through a Photo Editor. You can use any program that will enlarge a .bmp or .jpg file.

Final Processing of the Image

After the image has been obtained, further "tweaking" can be done. Using the picture of Jupiter as an example again, I realize that the color has a blueish/green tinge to it. This happens during atmospheric turbulence. Many times a light sources loses some of it's color spectrum in Earth's atmosphere if it is turbulent. The image received may be discolored, hard to focus or may have other distroted attributes. The easiest to fix is a distortion in color. By using a processing program I can change the color, hue and brightness of an object to make it appear more natural. I can also opt to align the image in the center, by cutting and cropping, so it looks more professional instead of having it all the way at the fringe of the photo.

I also like to align any visible detail in a left to right fashion (horizontally). This makes the image appear more uniform and again, professional. For example, all the shots of Saturn and Jupiter on my Astronomy Page have the rings or cloud bands lined up horizontally as opposed to an angle. Very rarely do the rings line up perfectly horizontally or even perfectly centered for that matter. By "processing" the image I can change all this. A great program is Registax. All these variables can be changed.

Below is the final image of Jupiter that I will go with. I have boosted the red scale on my RGB control in order to make up the odd coloring while cutting back on the blue and green. I also dimmed the picture somewhat so that the bands are more readily apparent.

A Final Note : In order to end up with a good picture, you need to start with a good picture. The results shown here are alittle less than what I would like. Being that the seeing this night was poor (2 out of 5) there is only so much I can do to improve this image. This is common of astrophotography though. Alot of patience and some luck is needed to get that "perfect shot". Even though Image Processing Programs allow us to "tweak" certain aspects of a photo, the better the picture starts out...the better it will end up. Don't expect to take a poor picture with alot of wind and turbulence and expect to process it to make it textbook quality. It won't happen (at least not in THIS day and age). We have not excelled that far yet. Maybe one day soon the technology will get there, but for right now we need to start with a decent picture.

This is just a VERY basic tutorial on Astrophotography. There is a wealth of information on other sites. I am still new to this offshoot of astronomy myself, but, I have already captured some photos that I am very pleased with in the past few months. If you are new to this site, try checking out my Astronomy page.

Hopefully this helps anyone understand telescope theory and principles.

Feel free to email me if you have any questions.