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normal mirror as secondary mirror?

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23 replies to this topic

#1 vicolodo

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Posted 20 February 2020 - 12:51 PM

Hello,

i have a question:

 

some years ago, in 2015 i bought a new GSO parabolic primary mirror 6" F4

 

I thought to have time/will to build my telescope, but i did not have this time,

so the mirror is still packed and never used.

 

Now i would like to give it a try, building a telescope without having to buy expensive parts specific for astronomy use, but only normal  stuff.

 

I can buy a plastic or metal tube in the near diy stores  , and can 3d print the parts to build a focuser, this should not be a problem.

 

But the other main part that is missing, is the secondary mirror.

I searched on online stores, and i found that an elliptical plane secondary mirror , with dimension that fits with my primary mirror diameter and focal lenght, costs about $100.

 

This is too much to give a try to my old (new) mirror.

 

So i wonder if i could use a 'normal' (that is a small mirror for bath, personal use etc) new good plane mirror.

I own a glass cutter, so i think i can cut it in the dimension and shape that i need.

 

What do you think?

Is it possible?

 

If not, why?

 

Let me know, thank you!

Lodovico



#2 petert913

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Posted 20 February 2020 - 12:55 PM

It would make a reflection and could be used for test purposes only.  But the surface quality would

be much to uneven to produce a good visual mirror.


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#3 randcpoll

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Posted 20 February 2020 - 12:58 PM

Not only that, "normal" mirrors have the reflective coating on the back, not the front, which really gives a poor image and could even give a slight double image since the front of the glass is also slightly reflective. Look for an appropriately sized secondary here on CN, you may have to be patient though.


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#4 randcpoll

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Posted 20 February 2020 - 01:00 PM

Better yet, post a "wanted" ad for one.


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#5 JoeInMN

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Posted 20 February 2020 - 01:08 PM

Yeah, the surface will be a lot wavier than a mirror made for telescope use should be. Also it will be alumimized on the back surface rather than the front, so you'll get the waves from three surfaces (front, back, and front again), and you might see a faint  ghost image from the uncoated front surface. But don't let any of that stop you from experimenting, if you've got the glass and the time and the curiosity; just keep in mind that what you end up seeing won't, um, reflect the quality of your primary.



#6 grtyvr

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Posted 20 February 2020 - 01:13 PM

 

 

But the other main part that is missing, is the secondary mirror.

I searched on online stores, and i found that an elliptical plane secondary mirror , with dimension that fits with my primary mirror diameter and focal lenght, costs about $100.

 

This is too much to give a try to my old (new) mirror.

 

So i wonder if i could use a 'normal' (that is a small mirror for bath, personal use etc) new good plane mirror.

I own a glass cutter, so i think i can cut it in the dimension and shape that i need.

 

 

If you have access to some old hard disk drives, the platters in those are reflective and first surface.  Jewelers saw will cut an oval out of them if you don't need one too large.  I think I heard of someone doing that on this board.


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#7 bobsorenson

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Posted 20 February 2020 - 01:30 PM

Some unusual first surface optical mirrors pretty cheap at Electronic Surplus.

#8 Pinbout

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Posted 20 February 2020 - 01:36 PM

 

 

i found that an elliptical plane secondary mirror , with dimension that fits with my primary mirror diameter and focal lenght, costs about $100.

 

This is too much to give a try to my old (new) mirror.

buy a prism coated on the hyp... use that.

 

but at f4 you need a nice first surface mirror... anything other than optically flat will ruin the image.


Edited by Pinbout, 20 February 2020 - 01:36 PM.


#9 TOMDEY

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Posted 20 February 2020 - 01:38 PM

Good question... but the answer is no. Things that "look like optics" are only that, nothing more. We had interferometers all over the place at work, so naturally would pop common items in the cavity to see what the instrument said. Turns out that 99.9% of e.g. reflective flat stuff, makeup mirrors, shiny plastic bobbles and bling, hardware store glass... anything you can lay your hands on --- all of it is clownish terrible, in terms of optical utility. It would be far more efficient to work a few hours of overtime and just buy what you need.    Tom


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#10 John Fitzgerald

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Posted 20 February 2020 - 01:49 PM

The original proposal is no good.  I would get a proper secondary.  You could always sell it.  You usually don't get something for nothing.



#11 vicolodo

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Posted 20 February 2020 - 03:57 PM

thank you to all!!

yes, i should buy the secondary ...i will buy it, i think...

 

but in the meantime i found this video that explains how to get a first surface mirror from a regular mirror...

 

https://youtu.be/IaEaaeajCSI

 

 

what do you think?


Edited by vicolodo, 20 February 2020 - 03:57 PM.


#12 Garyth64

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Posted 20 February 2020 - 04:08 PM

"So i wonder if i could use a 'normal' (that is a small mirror for bath, personal use etc) new good plane mirror."

 

NO, you need to use a first surface mirror that is  optically flat.

 

"but in the meantime i found this video that explains how to get a first surface mirror from a regular mirror..."

 

No, that will not work either for a telescope, you need to use a first surface mirror that is optically flat  (or a prism).


Edited by Garyth64, 20 February 2020 - 04:09 PM.


#13 vicolodo

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Posted 20 February 2020 - 04:41 PM

thank you, so you mean that removing the paint from back of the mirror, we get a reflective surface that is not optically flat

is not possible to try to flatten it using some polish ?

 

by the way, i found a cheap secondary mirror ..about 70 eur including shipping...

 

https://www.teleskop...minor-axis.html

 

i wonder if 63mm is ok for my GSO 6" F4...

there is also the 50mm size that is also cheaper than this...



#14 Garyth64

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Posted 20 February 2020 - 04:52 PM

   "so you mean that removing the paint from back of the mirror, we get a reflective surface that is not optically flat

is not possible to try to flatten it using some polish ?"

 

A bathroom mirror is not optically on either side.  They are just too thin too.

 

No, don't even think about trying to make a bathroom mirror optically flat by polishing.  Put the bathroom mirror aside and forget it.

 

The primary mirror maybe be good, so just go buy the size secondary you need for it, and use the scope.  As an f/4, it basically designed for wide field views of stars and DSOs.

 

(sorry to be so blunt)

 

Also, what is the size of this mirror you found that you say will work?  ( are you ready for some math?)


Edited by Garyth64, 20 February 2020 - 04:57 PM.


#15 mcmaker

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Posted 20 February 2020 - 05:26 PM

100$ for secondary? That's some sick price. I paid 30$ for 63 mm minor Axis secondary.
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#16 vicolodo

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Posted 20 February 2020 - 11:48 PM

100$ for secondary? That's some sick price. I paid 30$ for 63 mm minor Axis secondary.

hello,

 

thank you for the information!

 

where did you buy it?

 

let me know, thank you!



#17 brebisson

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Posted 21 February 2020 - 12:59 AM

hello,

 

I found a bunch of 70mm on ali expres, but I know that they also have 63...

63 are often sold with 200mm primary, some of the vendor have them as bundle, or separately, so also look for that.

 

https://www.aliexpre...earchweb201603_

 

Else, GSO makes them, look for local retailers of GSO mirors.

My local (France) store has them for around 50€ (includes local taxes+waranty and the like, hence the higher cost, they probably come from the same factory as the ali express ones).

https://www.telescop...axe-c2x30313650

 

So, it should not cost you $100...

 

Cyrille


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#18 Garyth64

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Posted 21 February 2020 - 09:07 AM

I didn't see the diagonal sizes you had posted before.

 

Depending on your secondary placement, and how far outside the tube you want the focus to be, a 50mm secondary may work.

 

The 63mm could work too.    It all depends on how far inside the focus the secondary is placed.


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#19 Pinbout

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Posted 21 February 2020 - 09:43 AM

just remember this:

 

the less money you spend the less assured of flatness... which will affect image.

 

https://www.youtube....45PT43k&index=5


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#20 vicolodo

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Posted 21 February 2020 - 10:03 AM

just remember this:

 

the less money you spend the less assured of flatness... which will affect image.

 

https://www.youtube....45PT43k&index=5

thank you  .. so perhaps an auto built secondary performs better than the gso... consider that my primary is a GSO too... :(


Edited by vicolodo, 21 February 2020 - 10:04 AM.


#21 Pinbout

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Posted 21 February 2020 - 10:34 AM

thank you  .. so perhaps an auto built secondary performs better than the gso... consider that my primary is a GSO too... frown.gif

guess my point is... don't skimp on the 2ndry.

 

really the 2ndry should match the primary. 1/4~ to 1/4~  something like that.

 

your gso primary maybe a really nice mirror and you'll ruin it with a crappy 2ndry.


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#22 JoeVanGeaux

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Posted 21 February 2020 - 01:14 PM

I agree with those that recommend buying a "real" secondary... BUT... if you really wanted to have some fun while waiting for your new (and real) secondary, maybe you can salvage a first surface mirror from an old rear projection TV. 

The first surface mirrors on those things are HUGE (like, nearly 2x3 ft!). If you have a glass cutter and pattern you may get closer to being able to see something with one made from this.  They are very thin, however, so I don't know how they'd hold up in just any kind of cell or mounting system. 

In any case, don't waste another breath thinking an ordinary household mirror will do anything but frustrate you to no end!!

Best of luck!

 

Joe


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#23 vicolodo

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Posted 22 February 2020 - 07:56 AM

thank you to all!
yes, i am going to buy a real secondary.
in the meantime, i would i would like to ask you a question:
i bougth this steel stove pipe from a local diy store

 

IMG 20200222 120232

it is about 50 cm long, and 18 cm diameter

i thought to remove my old c8 tube from its mount

 

IMG 20200222 120438

and 3d print an adapter in ABS for this stove tube

The c8 tube is 23.2 cm diameter, so i have about 2.6 cm offset on the sides that will be filled by the 3d printed adapter

My primary mirror is a 153mm GSO 60 cm focal length (F4)

I have calculated that the center of the focuser should be put about at 10cm from the border of the pipe, so if i put the mirror at the other end of the pipe , i have only about 40 cm distance from the secondary.
So from the center of the secondary to the focal plane point i have 60-40  = 20cm:
this means 9cm in the pipe, and 11cm in the focuser.

I have no experience about focusers, do you think that they can support focal point 11 cm far from the base of the focuser?

Do you think, instead, that i need a longer pipe? (for example, 60cm long)

 

What do you think about my project?

 

Perhaps you wonder why i would substitute the c8 ...it is because is very old, and F10... it does not fit with my deep sky observation needs.

 

Furthermore, perhaps the c8 mirror needs to be re-aluminated.

Let me know, thank you!


Edited by vicolodo, 22 February 2020 - 08:04 AM.


#24 Garyth64

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Posted 22 February 2020 - 08:33 AM

What are you going to use to hold the primary in place?

 

I don't think that 500mm tube is long enough. Your mirror will be outside the tube by 10mm,

 

You should draw it out to scale.


Edited by Garyth64, 22 February 2020 - 08:49 AM.

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