Archive for the 'Telescopes' Category

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Why Does Larger Newtonian Have Faster Focal Ratio?

Published on January 23, 2007 in Telescopes. View Comments

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.

Remove The Corrector Plate Of LX90

Published on January 8, 2007 in Noteworthy and Telescopes. View Comments
Going to put it back

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.

In the Part I, I’m going to tell you how to disassemble the corrector plate. The photos below are clickable for a larger version.

Remove the screws on the black housing

(1) First, unscrew every screw on the black housing in front of the OTA. There are 4 on the top and 3 at the bottom. The screws have to be unscrewed by Allen Wrench. I have 3 from Meade when I bought the scope.

Unscrew the screws on the specification ring

(2) Afterthat, you have to remove the screws on the ring which have the specification of the telescope. Please make sure that you don’t point your scope down at this moment just in case the corrector plate fall down and result in severe damage.

Unscrewing done!

(3) Caution! Although you have all the screws undone, don’t remove the corrector plate now or you will regret for life.

Make a marking

(4) Now, check whether the Meade engineer made a marking over the edge of the corrector plate and the black housing. If he did, continue to the next step. If not, you have to do your own marking. Make sure that it’s visible!

(5) Do you see 6 pieces of cork slices filled in the gap between the corrector plate and the housing? I suggest removing them before you proceed to remove the corrector plate. After removing them, hold your hand on the secondary mirror (the center obstruction) and turn the scope down. If the corrector plate doesn’t fall down at this moment, try the way below.

(6) You suppose to see 2 big gaps, don’t you? I used a knife which was small enough to fit into the gap and insert it to the maximum (the knife holder may touch the corrector plate now). Use the way that you open a tin with a spoon to remove the corrector plate. I heard a loud “boom” when I successfully remove it. Do you still have the secondary mirror mounted on the corrector plate? You better have it untouched because once it’s removed, you can’t put it back precisely. You have to call UPS and pickup the item back to Meade. Haha… Frankly speaking, I did remove it accidentally. Luckily, I made a marking on the screws that hold the secondary mirror. Luck really plays an important role. I was able to put it back almost the same as before.

LX90 without corrector plate

(7) Okay, the corrector plate is removed. What shall I do next? I did cleaning of the corrector plate and removing of a toothpick which was ACCIDENTALLY dropped into the OTA via the baffle tube.

The Secondary Mirror

(8) Let me show you the back of the corrector plate which faces the primary mirror.

(9) No more, I am going to put the corrector plate back now. I restore the cork slices first. Wish you a happy removing! :D

Who Says You Can’t Do Anything Without A Telescope?

Published on December 12, 2006 in Astronomy News and Telescopes. View Comments
SOHO No.1200 Discoverer, Bo Zhou
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.

Limiting Magnitude

Published on December 3, 2006 in Telescopes. View Comments

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)

How Well Can Your Telescope Resolve?

Published on November 26, 2006 in Telescopes. View Comments

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=116”÷D(mm)

θ: Resolution (in arcsecond)
D: Diameter (in mm)

Tracking Rate for Transit of Mercury

Published on November 7, 2006 in Stargazing Notes, Telescopes and Transit. View Comments

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.