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How to make a telescope?

by jijopaul
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jijopaul
#1
Sep8-13, 03:52 AM
P: 5
what will be the configurations for a simple 80x telescope?
i need to optimize it to the maximum resolution.
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davenn
#2
Sep8-13, 05:48 AM
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first you need to decide if you want to build a refractor --- lenses only or a
reflector --- mirror and lens ( eyepiece) mix

in general, the larger the f ratio the greater the resolution eg f10 higher res than f5

Dave
Drakkith
#3
Sep8-13, 08:39 AM
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I don't think the f-ratio has anything to do with resolution of the image except in the fact that slower f-ratios tend to have less aberrations in the lenses and mirrors since they are curved less sharply than faster f-ratios. Perhaps I'm misunderstanding something, Dave?

glappkaeft
#4
Sep8-13, 01:21 PM
P: 82
How to make a telescope?

First there are no "simple telescopes" that can maximize resolution so it's not possible to answer your question. There is also no such thing as a 80x telescopes except in the promotional material of entirely useless garbage telescopes (although then you are more likely to see 600x). We could help better if you explain more thoroughly what you are trying to do. This includes among other things:

* What is the purpose of this telescope?
* Are you going to make it yourself?
* What is your experience with using telescopes?
glappkaeft
#5
Sep8-13, 01:24 PM
P: 82
Quote Quote by Drakkith View Post
I don't think the f-ratio has anything to do with resolution of the image except in the fact that slower f-ratios tend to have less aberrations in the lenses and mirrors since they are curved less sharply than faster f-ratios. Perhaps I'm misunderstanding something, Dave?
You are quite right. The only truly important thing is aperture, the rest is just details in comparison. Important details mind but details nevertheless.
Chronos
#6
Sep8-13, 03:19 PM
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Unless you are an expert at optical figuring and testing, a refractor is a near impossible challenge for a fledgling ATM. Nor is it cheap. High quality optical glass is difficult and expensive to obtain, and you have multiple surfaces that require coatings - also not inexpensive.
chemisttree
#7
Sep8-13, 08:44 PM
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Building your own scope to perform well at 80X is trivial. The easiest way to start is with a refractor, IMO. Newtonians might be a little cheaper but not much in the smaller size scopes and they are fairly complicated to build since you need to fold the lightpath at a right angle. Two mirrors, each attached to an adjustable base and aligning all this with a focuser mounted to a tube at just the right distance from the end. Not extremely difficult but certainly harder than mounting the focuser one end of the tube and the objective on the other!

I've built several refractor telescopes. I currently use an 80mm f15 that makes 80 power with a 10mm eyepiece. This gives an exit pupil of 1mm which is fairly comfortable and the large f-ratio helps minimize chromatic aberration. With a light yellow filter it's almost a planet killer for not much money. I bought the objective and adjustable cell used for about $150 and mounted it in 4" PVC from Home Depot. Painted the inside with flat black and fitted it with a spare crayford focuser I had left over from a donor scope. It's great at high powers and handles 80X easily.

I have another 60mm f16.7 telescope I built entirely from parts I purchased online. $20 for the objective and cell, $15 for the threaded tube and $99 for the Crawmach crayford focuser. It's my favorite grab and go scope which I use at every public star party I attend.

Finally, I've just finished a 127mm Surplus Shed f9.4 refractor made from an objective/cell I purchased for $150 (now it's $210) a tube from Hastings Irrigation ($75 delivered for the 6" tube and 7" dew shield) and an old Jaegers focuser I've had for some time. It performs great at 80X with a 15mm eyepiece and the 1.8 mm exit pupil at that mag is very comfortable to use.

It's some money, but not a lot. Of course, if you think that anything like $135 is a lot of money for a telescope, it's expensive! And the prices I've outlined are for the optical tube only... you still need a mount and some way to attach the OTA to it. It too can be done cheaply... build a pipe mount from some 1-2" metal threaded pipe and fittings and a little wood and hose clamps. Used gear can be employed if you prefer. It seems that someone is always wanting to sell off their old gear when they upgrade.

Here is a step by step build for a 127mm telescope. Here is one for a 60mm f16.7.
davenn
#8
Sep8-13, 08:59 PM
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Quote Quote by glappkaeft View Post
You are quite right. The only truly important thing is aperture, the rest is just details in comparison. Important details mind but details nevertheless.
I should have commented on aperture as being significant larger aperture higher resolution

but still in scopes I own and scopes I have used, I have always seen a sharper image/better resolution in a hi f ratio scope eg my 2 scopes 10" f4.5 and my 9.25 " f 10
for splitting double stars and planetary features the f10 blows the f4.5 out of the water

I can only go by years of observations

Dave
chemisttree
#9
Sep8-13, 10:47 PM
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I agree with you Dave. It's been my experience that longer focal lengths give better resolution but only when you compare radically different focal lengths like you have at 4.5 vs. 10. But f6 vs f8? That's more dependent on aperture and quality. Quality probably more than anything.
Chronos
#10
Sep8-13, 11:25 PM
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So, you are saying a home made refractor is an easy project? I disagree.
Drakkith
#11
Sep8-13, 11:47 PM
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Quote Quote by Chronos View Post
So, you are saying a home made refractor is an easy project? I disagree.
Well not if you're grinding the lenses yourself or something...
davenn
#12
Sep9-13, 06:56 PM
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Quote Quote by chemisttree View Post
I agree with you Dave. It's been my experience that longer focal lengths give better resolution but only when you compare radically different focal lengths like you have at 4.5 vs. 10. But f6 vs f8? That's more dependent on aperture and quality. Quality probably more than anything.
yup, close ratios, aperture reigns, I agree with you :)

DAve
jijopaul
#13
Sep24-13, 08:15 AM
P: 5
I'm going to make it myself. I haven't got any experience in making telescopes. Refracting is prefered
enorbet
#14
May6-14, 04:43 PM
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Quote Quote by chemisttree View Post
Here is a step by step build for a 127mm telescope. Here is one for a 60mm f16.7.
Just wanted to thank you for those links. Those are beautiful. The second one appears it may be using a modified Surveyors stand. Do you know if this is so? or that it is a workable solution?
Chronos
#15
May7-14, 12:41 AM
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There is nothing simple about making a refractor primary. You have two surfaces to grind and polish for each lens and you need no less than two lenses to construct a useful primary. That is a lot of grinding and polishing, and, I haven't even addressed test stand issues. You also end up with four surfaces that need coated. Its no accident large refractor primaries are expensive.
Borek
#16
May7-14, 02:03 AM
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Back in seventies there was a hobbyist club in Warsaw where they were grinding manually their own mirrors for Newtonian telescopes. I believe their estimates were around 2000 hours of work for the 150 mm telescope (but I admit I am not sure if it was just grinding, or everything combined). Eyepieces were bought separately, mirrors were sent for coating in some industrial settings.

Technically process is not that complicated, if memory serves me well number of scopes they made was counted in hundreds. And from what I see there are tutorials on the web. I am far from saying it is trivial, but it is definitely a possible DIY project.
PAllen
#17
May7-14, 07:59 AM
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Quote Quote by Chronos View Post
There is nothing simple about making a refractor primary. You have two surfaces to grind and polish for each lens and you need no less than two lenses to construct a useful primary. That is a lot of grinding and polishing, and, I haven't even addressed test stand issues. You also end up with four surfaces that need coated. Its no accident large refractor primaries are expensive.
Chemisttree is clearly talking about buying the lenses, not making them:

" I bought the objective and adjustable cell used for about $150 and mounted it in 4" PVC from Home Depot."
brianhurren
#18
May21-14, 07:35 AM
P: 35
Quote Quote by davenn View Post
first you need to decide if you want to build a refractor --- lenses only or a
reflector --- mirror and lens ( eyepiece) mix

in general, the larger the f ratio the greater the resolution eg f10 higher res than f5

Dave
Not quite.. the grater the apature the higher the resolution up to about 12" then atmospheric disterbances blur the image so much that hight apatures beyond 12" dosnt make much difference, image just gets brighter. increased f ration equals high magnification for the same eye piece. magnification = f ration devided by eye-piece focal length


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