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RickB
Yoda
Offline
I've had both hands / wrists operated on for Carpal Tunnel over the holidays. I have one more week of sick leave before I return to work.
One of the things I've been doing actually for the past year is rebuilding my 8" Dobsonian telescope.
This is a Meade Starfinder I bought very cheaply on Craigslist several years ago. I was never very happy with the viewing from this thing so I thought I'd just rebuild it.
First I sent the primary mirror out to a place called NOVA Optical and got the thing tested then recoated with their highest reflective coating.
It's way better than new.
The viewing still sucked.
Next I thought about the focuser, the one that came with the scope was a rack & pinion job made almost entirely out of plastic. The rack had spots where it would stick so fine focusing was impossible.
I ordered a dual speed Crayford focuser, when I received it I had to show my kids as it was a perfect example of cheap crap versus quality materials and workmanship.
The old was also for 1.25" eyepieces and the new is for 2".
I also had to replace the finder scope, the cheap one it came with was useless.
I got a GSO 2 inch finder, which works great!
So I had to have a 2" eyepiece. I got a Baader Planetarium Hyperion 8mm - 24mm zoom. It's a really nice optic.
Still things didn't look good. Next I looked at my secondary mirror - yech. As with everything else on this scope the secondary was made of cheap junk.
It had black spidery crazing, and was not very reflective.
I got a new one, it's a small mirror so it was fairly inexpensive.
My hands felt good enough this week to work on that so I replaced the secondary mirror.
After alignment (Collimation) we had a nice clear night so I tried it out. I found that I could see the Orion Nebula but when I cranked up the lens it would fuzz way out.
This meant something was out of alignment.
After discussing the issue with another amateur astronomer who is into the hardware more than I am I got an idea.
I think I'm the only one who has tried this because I haven't read about doing it this way anywhere.
It is vital to have the primary, secondary and focusing tube all in alignment.
I put a center spot on my primary mirror, so I know that point is accurate.
What I don't know are many things, like:
1. Is the primary mirror mount "cell" centered in the tube?
It's a cheap POC so I have to assume the answer is no.
If it isn't then aligning everything else as if it were centered would cause problems.
2. Is the spider centered in the tube?
See the answer to #1 above.
Since it's actually most important to align these things relative to each other it's vital that the center of the spider be centered over the center of the primary mirror.
There is a hole in the center of the spider through which the secondary mount bolt is placed, a nut on the top holds it in place.
If a person were to slip a string through that hole and that string had a weighted pointer on the other end and it were long enough to reach almost to the primary mirror then a person could use a small level and make certain the tube was standing straight up (plumb) and then if the spider were centered that pointer inside would be pointing directly at the center spot on the primary mirror.
Yes, I'm talking about a plumb bob.
So I aligned it with a nice little home made plumb bob.
I found that the spider could not be centered properly as it came out of the factory so I had to use extra washers on two sides and none on two sides and then I could get it centered.
Then once everything was centered I collimated it and what do you know - it actually works.
It's way to hazy and cloudy to see much outside but I can focus on a fir tree about 1000 feet away and crank it up and see individual needles on the branches. Hopefully this will work well next time we have a clear night.
One of the things I've been doing actually for the past year is rebuilding my 8" Dobsonian telescope.
This is a Meade Starfinder I bought very cheaply on Craigslist several years ago. I was never very happy with the viewing from this thing so I thought I'd just rebuild it.
First I sent the primary mirror out to a place called NOVA Optical and got the thing tested then recoated with their highest reflective coating.
It's way better than new.
The viewing still sucked.
Next I thought about the focuser, the one that came with the scope was a rack & pinion job made almost entirely out of plastic. The rack had spots where it would stick so fine focusing was impossible.
I ordered a dual speed Crayford focuser, when I received it I had to show my kids as it was a perfect example of cheap crap versus quality materials and workmanship.
The old was also for 1.25" eyepieces and the new is for 2".
I also had to replace the finder scope, the cheap one it came with was useless.
I got a GSO 2 inch finder, which works great!
So I had to have a 2" eyepiece. I got a Baader Planetarium Hyperion 8mm - 24mm zoom. It's a really nice optic.
Still things didn't look good. Next I looked at my secondary mirror - yech. As with everything else on this scope the secondary was made of cheap junk.
It had black spidery crazing, and was not very reflective.
I got a new one, it's a small mirror so it was fairly inexpensive.
My hands felt good enough this week to work on that so I replaced the secondary mirror.
After alignment (Collimation) we had a nice clear night so I tried it out. I found that I could see the Orion Nebula but when I cranked up the lens it would fuzz way out.
This meant something was out of alignment.
After discussing the issue with another amateur astronomer who is into the hardware more than I am I got an idea.
I think I'm the only one who has tried this because I haven't read about doing it this way anywhere.
It is vital to have the primary, secondary and focusing tube all in alignment.
I put a center spot on my primary mirror, so I know that point is accurate.
What I don't know are many things, like:
1. Is the primary mirror mount "cell" centered in the tube?
It's a cheap POC so I have to assume the answer is no.
If it isn't then aligning everything else as if it were centered would cause problems.
2. Is the spider centered in the tube?
See the answer to #1 above.
Since it's actually most important to align these things relative to each other it's vital that the center of the spider be centered over the center of the primary mirror.
There is a hole in the center of the spider through which the secondary mount bolt is placed, a nut on the top holds it in place.
If a person were to slip a string through that hole and that string had a weighted pointer on the other end and it were long enough to reach almost to the primary mirror then a person could use a small level and make certain the tube was standing straight up (plumb) and then if the spider were centered that pointer inside would be pointing directly at the center spot on the primary mirror.
Yes, I'm talking about a plumb bob.
So I aligned it with a nice little home made plumb bob.
I found that the spider could not be centered properly as it came out of the factory so I had to use extra washers on two sides and none on two sides and then I could get it centered.
Then once everything was centered I collimated it and what do you know - it actually works.
It's way to hazy and cloudy to see much outside but I can focus on a fir tree about 1000 feet away and crank it up and see individual needles on the branches. Hopefully this will work well next time we have a clear night.
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