During those minutes, the Earth rotates, the stars move and for a visual observer that's not a big problem. We recognize this movement while our eyes sample images at ~10 frames per second. A camera in the same situation would capture the movement of the stars and record them as startrails and to avoid those, photographers use a motorized EQ-mount
Telescopes for visual observation may not have any motors at all and that isn't a bad thing -- it reduces costs & simplifies setup. Astro- photographers need much longer exposres and therefore require a motorized mount to compensate for the Earth's rotation. Big Alt/Az-models for visual observation also have motors to accurately move the mount, and occasionally track a position in the sky, but that still will not eliminate the "field rotation". Meaning, the telescope will keep pointing at the same patch of sky and the star in the center of your view will stay put. But all the other stars will run circles around that one : /stargazer95050/30333005
To see that effect, I suggest you use Stellarium and perform these steps
- launch Stellarium and set the time to after sunset
- pick any (bright) star you like and press "[space]" to center it
- zoom in (press "[Ctlr] & [Up]") until the FOV shows ~10° -- exact amount doesn't matter
- Press "L" 3 times to accelerate time
- see the other stars rotate around the selected star
- at the end of the simulated night, the star will set and the horizon will be leveled
You will see the stars all remain in place and at the end of the simulated night, the horizon will appear at an odd angle.
The Alt/Az-mounts are popular with medium to large reflector telescopes and are offered at attractive prices. Again, there's nothing specifically wrong but you should be aware of the problems you may encounter when you try to utilize a setup tailored to visual observation and attach a camera to it.
This setup does nothing to eliminate the field rotation I mentioned above and it adds some peril to your camera. The fork mount has some clearance to attach a mirror-diagonal and eye-pieces, meeting the needs of a visual observer. Often that space is too tight to accommodate the necessary adapters and a bulky camera.
Sometimes owners place such mounts on a wedge to accomplish nearly the same as a real EQ-mount ==> also getting rid of the field rotation. If you are aware of this mechanical limitation or have made sure you can fit your camera between scope & fork, you can use a mount like this (plus wedge) for astrophotography.
The optimization for visual use extends beyond the mount -- it also affects the telescope and the combination of the two. It is a lot more convenient to look through an eyepiece that's 4'....5' feet above ground, and that's one reason why many telescopes intended for visual observation, place the eye-piece at the front instead of the back. The tube is nearly as long as the focal length and you stand upright next to the scope's open aperture as it points at the stars.
Other telescope designs place the exit pupil in the rear and as a result the eye-piece may be close to ground and will require you to look up at a steep angle. An EQ mount + camera are in such inconvenient position. The benefit of this design is a more compact scope. The tube only needs to be 1/2 ... 1/3 the focal length but the eye-piece for observation may be a lot closer to the ground.
A astronomer-novice also will overlook another detail -- one that rarely is mentioned in the glamorous descriptions. Newtonian & Dobsonian telescopes (intentionally) offer very little room to adjust focus..For one, the focuser is on the side and the focus-tube can intrude into the optical path. And second, you can directly attach an eyepiece and there's no need for a mirror-diagonal. Engineers took that into consideration while designing the telescope tube but unfortunately it means, you will not be able to get a SLR into focus simply because of mechanical constraints. You can't get the sensor close enough. Recently, there have been some exceptions -- a few Dobsonians advertise their ability to get a SLR-camera into focus. Very carefully read the data and better have a good return policy.
In comparison, telescope type used by astrophotographers attach the camera to the rear, where focuser tubes don't interfere. They are opposite to the central mirror obstruction and leave plenty of room to adjust focus or add accessories.
Even if you can manage to get your SLR + telescope into focus, please take a careful look at the setup and ask yourself if it is mechanically stable. Many eye-pieces are light-weight while a SLR is 2lbs or more. The off-the-shelf solutions aren't designed to adjust for such changes and achieving balance isn't the only issue. How will such a setup stand up to wind or shutter vibration ...
If you start, I recommend you pick a refractor first. The (big) reflector telescopes are tempting with their larger magnification but it takes time to learn and reflectors add a couple of extra issues you have to deal with
- the larger magnification means, every mistake you make has a much bigger impact
- you may be pushing the limits of your mount with that big tube
- for best results with that large magnification, you better also have good polar alignment & auto-guider.
- reflectors are more mechanically sensitive ==> need to collimate the two mirrors (plus camera)
One final example why starter kits and (low-cost) visual telescopes are not suitable for astrophotography comes from comparing the size of your camera sensor to the diameter of the tube of the eye-piece. A typical cropped sensor has a diagonal of 28mm. The tube is 1.25" wide and that's 31.75mm and it can be very long ==> the vignetting can be very serious issue.
The telescopes I'm suggestion feature at least 2"-wide (50.8mm) focuser tubes -- that larger diameter reduces the vignetting but still may not be wide enough to avoid visible vignetting on FX-size sensors (43mm diagonal) and that's why you find 2.5" & 3.0" focusers on the expensive telescopes. (PS : the impact of vignetting also depends on the focal length)
IN CONCLUSION
If you are interested in astrophotography, there are a couple of important reasons, for skipping certain categories of telescopes and mounts. These issues aren't so critical for pure visual observation
- Mounts may not include any (motorized) tracking
- Fork-mounts may have insufficient clearance for cameras at the rear of a telescope
- BY DESIGN there always is field rotation in Alt/Az mount (if you put them on a wedge, they work just like a EQ-mount)
- Newtonian & Dobsonian telescopes rarely have sufficient backfocus distance for a SLR
- the widely used 1.25" focusers will increase vignetting, even on small DX-sensors
- Many of these setups are wobbly even before you add a camera
Trying to take a cheap shortcut by attaching a $1000 SLR to a $200 starter-kit should give you some food for thoughts. If it would work, why would anyone buy the more expensive equipment ? And how come even average-quality SLR lenses cost $400, a sturdy tripod costs $400 and you may even choose to add a $200...$400 tripod-head.
If you thought those investments were worth it to support your SLR, please adjust your expectations on what GOOD ASTRONOMY equipment will cost. There are telescope accessories with truly astronomical prices but you can find very good quality at reasonable & moderate prices and I recommend you put those prices in relation to your camera & other SLR equipment. $200 telescope (incl tripod) vs $1000 camera doesn't seem right when OTOH you pay $200 just for a sturdy-enough SLR-tripod.
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-) it is designed to eliminate field rotation and includes the necessary motor(s)
-) you can balance a wide range telescope + camera
-) typically a "good" EQ-mount (with motors & GOTO & good load capacity) is a much sturdier than starter kits -- it admittedly is also more expensive
-) unlike many off-the-shelf kits, an EQ-mount is very flexible when it comes to add telescopes and can grow as your needs / telescopes grow
Not surprisingly, this can deter people
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