Image shift and mirror flop are the common phenomena seen on mirror type telescope, especially SCT. When either of them happens, you can see the image through the eyepiece shifting. The reason for these phenomena is the not-so-perfect movement (slightly to left or right) of the mirror. What’s the difference between them?
While focusing, “Image Shift”.
While slewing, “Mirror Flop”.
Both are actually the same thing. When focusing, you are doing the shift of the mirror back and forth, thus you made the imperfect movement and the movement caused the image shift; when slewing, gravity is doing it and it causes the mirror flop.
Before anything starts, I want to explain AA. AA is my own abbreviation, it’s Amateur Astronomer in full.
What’s the common requirement of an AA’s scope? Unless you are a serious AA with deep pocket, you won’t have an observatory that allows you to mount a gigantic telescope inside. Some AA travels to the dark site with his scope or just move his scope from his house to the backyard. It can’t be too heavy. So, portability comes into one of the major considerations. We want to get the largest aperture within the portability.
8″ SCT has it all. For example, my fork mounted LX90 has an aperture of 203.2mm, 2000mm focal length and weights about 24kg. I can carry it around easily. (Not for a marathorn though)
Left: 8″ LX90 of Edwin Kats from Netherlands.
Continue reading ‘8″ SCT The Best For AA?’
This is an interesting question, isn’t it? I only know about this today and I want to share it immediately.
Larger Newtonian (reflector) has a faster focal ratio. As you know, focal ratio is the value of focal length divided by diameter of primary mirror (aperture). So, that means faster focal ratio telescope has a relatively shorter focal length. Why does a large Newtonian (eg. 8″ Newtonian) have a fast focal ratio? The focal length of the Newtonian and refractor is almost nearly the length of the telescope. If a 10″ (250mm) Newtonian has a focal ratio of 10 (f/10), the length of the OTA is most likely over 2.5m. How can a single person handle such a bulky telescope? That’s why large Newtonians are always built to be “fast”.
What about the small aperture Newtonian? Manufacturer always made them slower and the secondary mirror is directly involved in this matter. When a Newtonian has a large secondary mirror, its focal ratio will be smaller in value. If a Newtonian has a small primary mirror but relatively big secondary mirror, the center obstruction (CO) is too great which will results in lower contrast images as well as observing. However, a large Newtonian won’t have much lower images because its center obstruction is relatively small.
The same theory applies to refractor too.
It’s so interesting to disassemble a telescope especially those have a very precise and big gears. I’ve disassembled my computer since I was 8 and 8 years later I disassembled my LX90. I will tell you how to do so step-by-step. However, take it at your own risk, I will not responsible for any damage done to the telescope. I suggest that you read all the procedures before you do anything. If possible, read one more before you start and it’s advisable to print this article with you or leave your monitor on when you are doing your work.
Continue reading ‘Remove The Corrector Plate Of LX90′
SOHO No.1200 Discoverer, Bo Zhou. Image courtesy of Renjiang Xie
1200th comet discovery of SOHO (Solar and Heliospheric Observatory) spacecraft was found by a Chinese amateur astronomer, Bo Zhou. The SOHO No.1200 was a tiny, diffuse, and very faint object. It’s detected in images taken with the spacecraft’s LASCO C2 coronagraph which he downloaded.
From this, we know that we don’t have to own a telescope to do researches or findings. There are quite a number of “virtual telescope” among the internet. I will try to collect as much as possible to benefit those who can’t afford to buy a research grade telescope.
I will create a page of the collection and more virtual telescopes will be added.
Astronomers use a system of magnitudes to indicate how bright a stellar object is. An object is said to have a certain numerical magnitude. The larger the magnitude number, the fainter the object. Each object with an increased single number is approximately 2.5 times fainter. The faintest star you can see with your naked unaided eye is about 6th magnitude whereas the brightest stars are negative number magnitude.
The skies and any instrument mentioned are at the best condition.
The magnitude of faintest star you can see with the telescope is called Limiting Magnitude. The limiting magnitude is directly related to aperture, where larger apertures allow you to see fainter stars. The atmopheric condition often reduce the limiting magnitude.
Photographic limiting magnitude is approximately two or more magnitudes fainter than visual limiting magnitude.
M=7.5+5logD
M: Magnitude (constant)
D: Diameter of primary mirror (cm)
The resolution of a telescope is presented in arcsecond (”). It’s related to the aperture of your telescope. The larger the aperture, the closer the object it can resolve. A high resolution telescope is handy when it comes to double stars observation, Moon features observation and many more.
θ: Resolution (in arcsecond)
D: Diameter (in mm)
You aren’t going to waste the ability of a GoTo capable telescope in this event, are you? Sun doesn’t “move” at the speed of stars.
The movement of the stars from east to west across the sky is caused by the rotation of the Earth. The movement rate is called sidereal rate. The movement of the Sun is slower 35% than the sidereal rate. So, you have to slower the tracking rate of your computerized mount (either an EQ mount or a fork mount).
For a Meade GoTo telescope, you have to change the tracking rate from “Sidereal” to “Custom”. The path will be Setup>Tracking Rate>Custom. As suggested by dick who is always active in modifying Autostar II firmware, the custom rate has to be adjusted to -3 FOR MOST OF THE UNITS. If your autostar controller is either StarGPS patched or dick patch kit patched, you can enter -3.5 for a more precision tracking rate.
For others, the tracking rate is suggested to altered to -3 as well. If this doesn’t work, try to find out another figure around this.
Beware of using an unfiltered telescope to point to the Sun, it will cause severe damage to both of your telescope and your eyes.
I would like to know the field of view I can archieve with my scope if I am going to buy an eyepiece, especially a wide angle eyepiece. Apply the formula below, you can get a rough figure of the true field of view (TFOV). Why is it just a rough figure? There are still several factors to be taken account but I think this is good enough. There is a more precise formula to do the same work.
TFOV = AFOV of Eyepiece / Magnification
The apparent FOV (AFOV) of an eyepiece is provided by the manufacturer. To know the magnification, please refer to Calculate Eyepiece Magnification.
Apparent Field of View of Several Eyepiece Designs
Orthoscopic - 43º
Plossl - 50º
Radian - 60º
Panaoptic - 68º
Nagler - 82º
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