Hey all,

Astronomy newbie here, and need a little help. I axed a question on another thread, but perhaps need to start my own...

 I got the bug to build a telescope ever since using an 8" Newton with a Dobson mount last weekend.

Since I work in a cabinet shop with CNC capabilities, it should be quite easy to design, cut, and find materials eh?

That's not the problem... I need a good mirror, a good secondary, and a focuser (although I may make the focuser myself as well). I just don't hanker after spending weeks and months grinding glass...

So I went searching on eBay and found the 8 and 6 inch parabolic mirrors for pretty cheap.

My question is this, are either of these worth the money? Have you had any experience with these mirrors, or can you tell me anything just by looking at the specs? I'd like to design a scope around one of these, but don't want to waste time on it if they're not worth it.

here are the specs as advertised:


8"Mirror
http://tinyurl.com/2h5mmd
$47.95 S.B.
$69.95 B.I.N.
f=750
power=6
surface accuracy=1/4 lambda
Full Coating: al+sio
scratch-dig=60/40
chamfer 1x45'
clear aperture 8inches = 203mm
focal length 29.39inches = 750mm
focal ratio f/4
mirror thickness 0.75inches = 19mm
thickness/diameter ratio 1:10.5

6" Mirror
$23 starting bid
$38 buy it now
http://tinyurl.com/2ddmeh
Coating = al+sio
Clear aperture 6inches = 152mm
Focal length 29.38inches = 746mm
Focal ratio f/5
Mirror thickness 0.625inches = 15.9mm
Thickness/diameter ratio 1:9.6
Surface Accuracy = 1/4

Hi Mayk_It, to the forums.

I held off answering this post for a bit to give other members with more mirror experience (than I have) an opportunity to reply. But since there hasn't been an answer for you, I'm going to offer my opinions.

There are a few red flags that pop up when I see deals such as these that offer prices that to me seem too good to be true. In my 2 years of experience with telescopes, and reading different pages on equipment and mirrors, the prices these guys post seem way below what I'm use to seeing for a good quality mirror. Another flag that waves harshly in my face is the fact that they are an internet based business that deals in a multitude of items. They do not specialize in telescope mirrors at all. Their specialty seems to be on anything that can make them a dollar.

I would only buy a telescope mirror from a reputable dealer if it were me. Any problems you encounter would give you someone to go to for a resolution. I just can't see an internet based store that doesn't specialize in astronomy equipment, making good on, or even undertanding the problem you may be having with the mirror you purchase. Another red flag would be the one word included in your search description - Ebay. I would be very cautious about buying astronomy equipment off of Ebay. Although there are reputable businesses that advertise on Ebay, Ive yet to see one that I trust enough to buy astronomy equipment from. I think you would get the same reaction from alot of members here regarding Ebay. While there has been those that purchased telescopes on Ebay and have gotten a decent scope at a good deal, there have been just as many that have been left with lower quality equipment, or even no equipment at all, for the money they spent. You'll get varying opinions on this I'm sure. But for peace of mind, and quality mirrors, do some more research on buying optical mirrors before jumping on the "internet based store" bandwagon. Good luck!

That's my

Clear skies!

Well I found out you were absolutely right. I ran into a coupla guys on another forum who bought a couple of these, and they were spherical and the focal point was even shorter than advertised. So I guess I'll look elsewhere. If you know of a good 8 in or 6 in for sale somewhere... let me know.

I bought an 8" mirror for about $50 (incl shipping) on ebay, and built a 'scope around it. Maybe I got lucky, but it is excellent. All measurements were as advertised, and images are tack sharp. Sometimes you win one (grin).

Alright, right off the bat, don't buy it. I did, and built a scope around the 8" f4 mirror I picked up off ebay. The mirrors are not parabolic, they are spherical. The red flags here are first of all the short focal length, which makes for a much larger secondary mirror. I had to put a 2.5 inch secondary in my scope to get 100 percent of the light from the primary and no vignetting at the focuser. I also had to use a 2 inch focuser which I recommend anyway. The obstruction of the diameter of the primary by my diagnal is 30%. That is pretty bad. Also, the mirrors have optical defects because of the short focal length, you get very bad coma around the edges of the mirror, which makes the stars "blur" towards the outer edges of the mirror. I've done a lot of web surfing and the best optics for the price I've been able to find are here.

Hands On Optics

I would recommend picking up "NEWT for windows" its a nifty little program that helps you build a Newtonian telescope.  

HERE is a screen shot of my telescope in NEWT. You can see the performance info VS

the one im going to build HERE. I extended the focal plane 2 inches out of the focuser for my camera. You can actually get away with a 1.5 inch diagonal with an 8"F6. 

 Usually a well known dealer, and a longer focal length, make for better optics. I'm not sure where all these 8 spherical mirrors on amazon and ebay are coming from, but I would avoid them. Luckily I built an open tube design, so I will be able to easily replace my mirror without starting all over again.

Its kinda sad, I spent a lot of time on my scope and am not happy with the results. I turned around and picked up a 5"F8 on ebay for 40$ that had much better optics.

 

The listed specs are not even correct on those mirrors, an 8" mirror with a 29.39" focal length is f/3.6 .  That makes for a very "fast" mirror, and fast mirrors are extremely difficult to make.

 Just to give you an example, my WHOLESALE cost to purchase an 8" mirror to those specs in that auction (although, with a better scratch/dig rating) would be $900. 

May I suggest that you keep an eye on AstroMart in the Telescope Making section:

 AM Telescope Making and look for a mirror 8 to 10" in diameter with a f/ratio of f/4.5 to f/7 .

 

Building your first scope is a lot of fun,  and a scope with good optics can keep you happy for a lifetime.

 

Good luck to you!