Equipment
My main imaging scope is an 8", f/4.9 Orion reflector. It is mounted on an equatorial mount with motor drives in Right Ascension, and Declination. It has an 8 X 40 finderscope, and a Telrad. Mounted piggyback using adjustable scope rings, is my guidescope, a 70mm, f/10 Orion refractor.
The adjustable scope rings make finding a guidestar easier by allowing the scope to be moved slightly in R.A. and Dec. in order to center or find a suitable star to guide on. The rings have 3 thumbscrews that can reposition the scope while still mounted in the rings.
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Adjustable Ring |
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Guiding
Even with motorized drives, when taking images, guiding is essential. The drives are accurate enough for visual observing, meaning they can keep an object in your field of view (FOV) in an eyepiece. But photography requires pinpoint accuracy in order to eliminate any trailing or misforming of stars. Guiding, either manually, or with an autoguider, can compensate for slight variations in the drive gears themselves, or fluctuations in control voltages that may throw off the tracking of the axis drives enough to produce trailing in exposures longer than 30 seconds or so. I use an autoguider which connects in a Prime Focus fashion to the guidescope. The autoguider runs through the computer via a USB port and also hooks into an ST-4 port on the motor drive controller. My drive controller wasn't equipped with an ST-4 port, but I was able to add one by buying a kit from Shoestring Astronomy, which has a plug that is attached to the circuit board of the controller and turns it into an ST-4 capable port. This little device which cost all of $7, saved me hundreds in having to buy a new mount that was ST-4 equipped. What the autoguider does when focused on a star is take a photo every 2 seconds (or various other time settings) and compares them to the last shot, and when run through the computer, makes adjustments by sending a signal to the motor drive through its' controller to bump the drives a slight bit to compensate for any variances in the tracking to insure that the scope is staying on target. Manual guiding requires you to find a guidestar in the same fashion, but using a lighted reticle eyepiece, keep a star centered in the crosshairs of the EP and then moving the controller by hand. This is tedious at best, since you must keep checking on the guidestar every 20 seconds or so, and then make just enough correction to keep your target centered. While also being careful not to overcorrect or bump the scope, which is a challenge when it is freezing or below, and you can't feel your hands, or you are getting eaten up by insects in the summer.
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Autoguider |
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Guider on Scope |
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Drive Controller |
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Motordrive |
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Polar Alignment
Before you ever get the camera near the scope, a good polar alignment is essential. This will ensure accurate tracking and the lessen the chances of mis-shapen stars and trailing. Some mounts are equipped to accept a "polar alignment scope". This is basically a finder scope that mounts in the axis of the mount, and can be aimed at Polaris. The pics below show the business end of the scope as well as its' location on the mount. The scope has an etched reticle showing a constellation map of Cassiopeia and Ursa Major and a target circle for Polaris in the scope itself. When you look through the scope, you line up the constellations relative to their current positions, and then adjust your mount to center Polaris in its' target circle. This will get you very close to polar aligned. After this is done, a "drift alignment" is still recommended for exposures of anything over a minute or so. If visual observing is all you are interested in, then a drift alignment would be a bit of overkill and isn't necessary.
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Polar Alignment Scope |
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Polar Scope Location |
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Alignment Reticle |
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Camera Adapters
For prime focus photography, you are going to need an adapter to couple your camera to the focuser of your scope. Normally for most scopes a T-ring is what you'll need. This is simply a ring that will mount to your camera the same way the lens does, with either a 1.25, or 2" barrel on the other end that will slip into the focuser the same way as an eyepiece does. These are made for practically all SLR's, whether they are film or digital. Below are examples, (clockwise from top L to R) of 1.25" t-ring; 2" t-ring; and two lo-profile adapters. Some scopes, such as my 8" reflector, have fast f/#'s and short focal lengths, and getting the camera close enough to the focal plane to achieve focus can be difficult without a lo-profile adapter, which brings the camera closer than a standard T-ring can.
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T-Rings |
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Focusing
Now that your camera is set up and your mount is aligned, one of the next things to be concerned with is focusing. This is one area that usually gives alot of trouble to beginners in this hobby. You cannot simply look through the viewfinder and focus as you would shooting some vacation shots. Also the auto-focus (if equipped) on your camera is now useless since you don't have a lens attached anymore. This is one place where the laptop comes in handy. I have the camera run into the laptop so that after a shot is taken, I can see it in full screen mode if I want, and then adjust focus accordingly. Depending on what you are shooting, you will soon learn about where in the travel of the focuser you should be in order to be close to focus. Some astrophotographers will mark their focuser tubes to get closer to focus with different cameras when switching from one to another. This way you can set focus when using an autoguider to a different mark than when using, say, a webcam. They will both focus at different distances, and when marked on the tube, you can get very close to where each device will focus without wasting alot of time moving the focuser back and forth.
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Focus Marks |
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Elec. Focuser |
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Focus Controller |
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Another nice device to have for focusing is an electronic focus controller. This connects to one side of the focuser shaft and can control focus without having to touch the scope at all. It is adjustable to from a slow movement for fine adjustment, to a faster movement for coarser adjustments. This can make slight adjustments more accurately and smoother than manually turning the focus knob. Once you are guiding, even the slightest touch of the scope can throw off your autoguider, which means calibrating it again. The electronic focuser eliminates this risk and can also make minute changes in the focus that are difficult to make manually.
Eyepieces
One of the most debated pieces of gear are eyepieces. Everyone has their favorites and reasons for them. I'm not going to go into all the ins and outs of Ep's for visual observing, but suffice to say you'll want at least one high magnifaction and one low magnification eyepiece at the very least. For astrophotography, the most used one would be a lighted reticle eyepiece. The one below is shown mounted in a diagonal, on my guidescope. This is usually about a 12mm EP with a lighted X or crosshairs when viewed through. Most have an adjustable brightness control which is nice when staring through it for long periods of time, or when used to track a rather dim guidestar. They are invaluable when doing a drift alignment, or guiding manually. Also good to have would be a high magnification EP, such as a 9-6mm which I use for zeroing in on targets once I think they are in my FOV (field of view). A widefield EP such as a 25-40mm is handy as well for narrowing down your FOV when looking for a specific target. The finder scope will get you in the neighborhood of your target, then you go to higher magnification gradually until you zoom in on your target and know what is in your cameras' FOV.
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Eyepieces |
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Lighted Reticle EP |
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Telrad
Another nice little device to have is a Telrad. This is a zero magnification finder. It does not magnify the sky when viewed through, but does project 3 concentric circles as a bullseye in the sky, making finding targets easier than a normal finder scope sometimes since you don't have to deal with the hundreds of stars displayed in a finder that aren't visible when seen with the naked eye. There are charts available that project the Telrad circles in scale against the sky and they make star hopping and locating objects much easier than a normal finder alone.
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Telrad |
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Red Dot Finder
Along the same lines as the Telrad is a Red Dot Finder. Like the Telrad, it is a zero magnification finder, but instead of projecting 3 concentric circles, it projects a single red dot, like a laser pointer or a laser sight on a rifle. The same advantages of a Telrad come into play with these, except there are no charts tailored to them and you simply point it to where you want to go and then use your magnified finder to zoom in on your target. Shown below is a red dot finder along with a focus controller on my guide scope.
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Red Dot Finder |
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Finders
There are different kinds of finders besides a normal "scope" finder. Telrads and Red Dot finders are probably the most widely used. One thing to remember when setting up your scope is to align the finders with their respective scopes before it gets dark. That way you can be sure what you see in the finder is where the main scope is pointed also. Finders all have adjustment screws that can be turned to center on whatever the main scope is centered on.
