ndelo
professor emeritus
Reged: 08/02/06
Posts: 529
Loc: Light-polluted New Jersey
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I am planning on picking up one of the kits from Newport Glass. I want to start with a 6" mirror. Are there any caveats to this size? Would 8" be easier to start with?
Edited by ndelo (10/14/09 08:02 PM)
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Don W
Postmaster
Reged: 05/19/03
Posts: 14629
Loc: Wisconsin, USA
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I don't think an 8" is any more difficult than a 6" and you end up with a bigger mirror.
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Don Wyman
Obsession 18" f/4.5 #1166
W/Argo Navis DSC and Torus Primary
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bgilb
member
Reged: 07/05/09
Posts: 79
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I started with a 12" mirror. Still working on it.
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NeoDinian
Experienced Postmaster
Reged: 10/05/05
Posts: 12732
Loc: Rockford Illinois
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I followed the recommendations of my other club members...
I have large hands. For me, it was easier to do an 8". Doing something smaller would actually be a bit uncomfortable for me.
Hope this helps. 
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Neo... (Jeff)
-=-=-=-=-=-=-
10" LX200-GPS/SMT UHTC "Draco"
 Rockford, Il.
NeoDinian's Eye on the Sky!
Coming soon:
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ndelo
professor emeritus
Reged: 08/02/06
Posts: 529
Loc: Light-polluted New Jersey
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I'm only choosing a 6 because I have a 10" Z dob and an 18" obsession clone I built last year. 8" seemed too close to 10".
I was thinking of making something small, grab-n-go-ish, for quick peaks at the moon and planets and just for fun and to be able to have the experience of grinding out a mirror. This is why 6".
But, for some reason, I remember hearing that smaller mirrors posed challenges.
On the other hand, I'd like to pick the easiest size for a newbie.
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rwiederrich
Goldfinger
Reged: 11/17/05
Posts: 8334
Loc: Bremerton Washington
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Quote:
I'm only choosing a 6 because I have a 10" Z dob and an 18" obsession clone I built last year. 8" seemed too close to 10".
I was thinking of making something small, grab-n-go-ish, for quick peaks at the moon and planets and just for fun and to be able to have the experience of grinding out a mirror. This is why 6".
But, for some reason, I remember hearing that smaller mirrors posed challenges.
On the other hand, I'd like to pick the easiest size for a newbie.
A Nice 6"f/9 spherical mirror is a good start. Super little planitary scope..and you don't have to porabolize it.
Keep the mount low and the EP will be at eye level.
One of the students that is making a scope here at my shop..made an 8"f/11 spherical mirror.
Being his first he was happy with the longer focal ratio.
Good luck.
Rob
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www.goldmtobservingcenter.com
A great place for amateur astronomers, and ATM's to come and enjoy their hobby.
http://tech.groups.yahoo.com/group/HomemadeRefractorTelescopes/ My homemade refractor group.
www.vimeo.com/6014031
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Feidb
super member
Reged: 10/09/09
Posts: 128
Loc: Nevada
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I'm really happy to see you all making mirrors. It seems to be a dying art. I had not run across another mirror maker for almost a decade. Now I know of several who have at least tried one.
A 6" used to be the "standard" first mirror, but I wanted more so I got an 8" kit from a company called Telescopics. I can still smell that pitch lap 43 years later!
Congrats to all of you!
--------------------
Present gear:
16" Meade LightBridge
Meade 50mm straight through-finder
Lumicon green laser pointer
Orion Q-70 26mm, 32mm, and 38mm
Parks 2X 2" Barlow
Hyperion 17mm, 8mm
1 1/4" 18mm Russell Optics Bertele
1 1/4" 12.5mm and 6mm Coulter Optical Orthoscopics
1 1/4" X 2" 32mm Edmund Scientific war surplus Erfle
Tirion star atlas (white stars, black background) hand-laminated
Megastar
And a partridge in a pear tree
To nudge or not to nudge, that is the question
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alvin58
super member
Reged: 04/15/06
Posts: 101
Loc: North Shore Lake Superior
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ndelo,
I too have a finished 10" f/4 Dob, 6" Criterion and a 76mm Tasco. Yet.... I have a 4.25" Pyrex blank and tool and a 6" Pyrex blank that I am in the process of making a tile tool for. Still haven't decided which one to do first - but I also want to craft my own mirror. I think it will be a great experience. Allen
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HiggsBoson
Pooh-Bah
Reged: 02/21/07
Posts: 1118
Loc: Kal-li-fornia
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Ndelo
Making one’s own mirror can be a very satisfying experience. For me it was also my first and only telescope. Now I explore skies with a optic to which I have a deep emotional connection. My first good views of Saturn, Jupiter, M57, IIS and the Moon are all entangled with that mirror.
I made a 6” F/9 because I wanted to decide if I could do a 12”. Embolden by success I am working on a 12.5” F/4.5 for my deep sky telescope.
I took the 6” to an event related to the LCROSS impact last week and dozens of people were able to see the shadow of one of the moons cross the face of Jupiter in that scope.
Consider not just the size but the focal ratio. I understand that F/6 is near the sweet spot for a 6”. Longer gets hard to measure because the shadows are so faint and shorter will be easer to read and harder to figure. I would not suggest leaving the mirror spherical. A 6” F/9 spherical mirror just barely makes the Rayleigh diffraction limit.
At any shorter focal ratio a spherical mirror will not focus at medium to high powers. If you are at all concerned about being unable to create a good parabola I would suggest that you choose F/9 and attempt a parabola. If unsuccessful you can polish it back to a sphere in only a few minutes. You would be no worse off than if you had not made the attempt.
Getting the parabola was for me the most satisfying part of the project. This was enhanced for me last Thursday when an experienced astronomer comes over to my scope and says “ I understand that you made this mirror. Let’s see what it can do”. He viewed Jupiter at 175x. He looked into the eyepiece and his mouth came open… He looked at me and then back into the eyepiece… He paid me a complement before he left. I do not remember what he said, it was the look on his face that made the hours of grinding and polishing worth it.
Good luck and good seeing…
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Michael
ATM: 6" F/9 Newtonian Travel Scope
ATM: 12.5" F/4.5 Real Soon Now...
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ndelo
professor emeritus
Reged: 08/02/06
Posts: 529
Loc: Light-polluted New Jersey
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Quote:
A Nice 6"f/9 spherical mirror is a good start. Super little planitary scope..and you don't have to porabolize it.
Keep the mount low and the EP will be at eye level.
One of the students that is making a scope here at my shop..made an 8"f/11 spherical mirror.
Being his first he was happy with the longer focal ratio.
Good luck.
Rob
So is there a certain f/ratio above which one does not meed to parabolize a mirror? Does anyone have any good links they can provide to making a spherical mirror?
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Owen
professor emeritus
Reged: 06/21/07
Posts: 514
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Yup - normally F11 and above (to meet the rayleigh diffraction limit), but they do improve with parabolisation - and that, on a small mirror can be through flexing the mirror, rather than figuring..
Owen
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GaryS
super member
Reged: 10/30/06
Posts: 191
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Quote:
Quote:
A Nice 6"f/9 spherical mirror is a good start. Super little planitary scope..and you don't have to porabolize it.
Keep the mount low and the EP will be at eye level.
One of the students that is making a scope here at my shop..made an 8"f/11 spherical mirror.
Being his first he was happy with the longer focal ratio.
Good luck.
Rob
So is there a certain f/ratio above which one does not meed to parabolize a mirror? Does anyone have any good links they can provide to making a spherical mirror?
I'm going to jump in here to contribute the voice of dissent about the "leaving it spherical" idea. I think it's a bit of a myth that going long and spherical is meaningfully quicker or easier than going for a shorter focal length mirror that you have to parabolize. The thing is, making a *good* sphere at f/9 or longer isn't as easy as some people think. It doesn't happen automatically. The fact is, at that f/ratio, the mirror doesn't really want to be a sphere. At f/4, yeah, it *really* wants to go spherical -- that's one reason why figuring a short-focus mirror is challenging. What you want to do is pick a f/ratio between those extremes.
So, in my experience, f/6 is a good f/ratio to shoot for. You get a very useful telescope and in a 6-inch size, parabolizing isn't overwhelmingly difficult for a first-timer. Besides, what's the point in making your own mirror if you're going to bail out on the most interesting and meaningful part of the process?
So, damn the torpedoes -- full parabolization ahead!
Gary
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Sky & Telescope Contributing Editor
SkyNews Columnist & Blogger
www.GarySeronik.com
A place for stargazing enthusiasts.
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rwiederrich
Goldfinger
Reged: 11/17/05
Posts: 8334
Loc: Bremerton Washington
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Quote:
Ndelo
Making one’s own mirror can be a very satisfying experience. For me it was also my first and only telescope. Now I explore skies with a optic to which I have a deep emotional connection. My first good views of Saturn, Jupiter, M57, IIS and the Moon are all entangled with that mirror.
I made a 6” F/9 because I wanted to decide if I could do a 12”. Embolden by success I am working on a 12.5” F/4.5 for my deep sky telescope.
I took the 6” to an event related to the LCROSS impact last week and dozens of people were able to see the shadow of one of the moons cross the face of Jupiter in that scope.
Consider not just the size but the focal ratio. I understand that F/6 is near the sweet spot for a 6”. Longer gets hard to measure because the shadows are so faint and shorter will be easer to read and harder to figure. I would not suggest leaving the mirror spherical. A 6” F/9 spherical mirror just barely makes the Rayleigh diffraction limit.
At any shorter focal ratio a spherical mirror will not focus at medium to high powers. If you are at all concerned about being unable to create a good parabola I would suggest that you choose F/9 and attempt a parabola. If unsuccessful you can polish it back to a sphere in only a few minutes. You would be no worse off than if you had not made the attempt.
Getting the parabola was for me the most satisfying part of the project. This was enhanced for me last Thursday when an experienced astronomer comes over to my scope and says “ I understand that you made this mirror. Let’s see what it can do”. He viewed Jupiter at 175x. He looked into the eyepiece and his mouth came open… He looked at me and then back into the eyepiece… He paid me a complement before he left. I do not remember what he said, it was the look on his face that made the hours of grinding and polishing worth it.
Good luck and good seeing…
Yes I agree..f/9 is barely defraction limited..that is why I suggested it..because if he decided to porablize it the curve is more forgiving.
My original suggestion was going to be an f/11. But the focal length might put the EP still to high for some folks.
Plus as you say a long focus porabola is harder to see through a shadowgram then a faster mirror.
It's all preferences I think.
Good call though.
Rob
--------------------
www.goldmtobservingcenter.com
A great place for amateur astronomers, and ATM's to come and enjoy their hobby.
http://tech.groups.yahoo.com/group/HomemadeRefractorTelescopes/ My homemade refractor group.
www.vimeo.com/6014031
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rwiederrich
Goldfinger
Reged: 11/17/05
Posts: 8334
Loc: Bremerton Washington
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Quote:
Quote:
Quote:
A Nice 6"f/9 spherical mirror is a good start. Super little planitary scope..and you don't have to porabolize it.
Keep the mount low and the EP will be at eye level.
One of the students that is making a scope here at my shop..made an 8"f/11 spherical mirror.
Being his first he was happy with the longer focal ratio.
Good luck.
Rob
So is there a certain f/ratio above which one does not meed to parabolize a mirror? Does anyone have any good links they can provide to making a spherical mirror?
I'm going to jump in here to contribute the voice of dissent about the "leaving it spherical" idea. I think it's a bit of a myth that going long and spherical is meaningfully quicker or easier than going for a shorter focal length mirror that you have to parabolize. The thing is, making a *good* sphere at f/9 or longer isn't as easy as some people think. It doesn't happen automatically. The fact is, at that f/ratio, the mirror doesn't really want to be a sphere. At f/4, yeah, it *really* wants to go spherical -- that's one reason why figuring a short-focus mirror is challenging. What you want to do is pick a f/ratio between those extremes.
So, in my experience, f/6 is a good f/ratio to shoot for. You get a very useful telescope and in a 6-inch size, parabolizing isn't overwhelmingly difficult for a first-timer. Besides, what's the point in making your own mirror if you're going to bail out on the most interesting and meaningful part of the process?
So, damn the torpedoes -- full parabolization ahead!
Gary
I completely agree...however a first mirror can be intimidating and for some to avoid the parabolization step can still bring great satisfaction.
I agree a 6"f/6 is the sweet spot mirror...8"f/6 is too.
I used a mirror making student here as an example....he didn't want to tackle a parabola with his first...so he made an f/11. He cut the blank out as well...so grinding and polishing was as much as he wanted to tackle.
A sphere is undercorrected but at such focal lengths it is unnoticable.
I could be talking out of my ear..but that is how I see it anyway.
Rob
--------------------
www.goldmtobservingcenter.com
A great place for amateur astronomers, and ATM's to come and enjoy their hobby.
http://tech.groups.yahoo.com/group/HomemadeRefractorTelescopes/ My homemade refractor group.
www.vimeo.com/6014031
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Feidb
super member
Reged: 10/09/09
Posts: 128
Loc: Nevada
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MY 8" f/9.44 definitely still needed parabolizing and the shadows were really faint. The only reason it ended up that way was because I was 16 and still learning patience and got sick of grinding! Still, that thing is a great planetary scope and I used it for over 15 years... no actually 17 years. It was not bad for deep sky either, especially with a 32mm Erfle.
--------------------
Present gear:
16" Meade LightBridge
Meade 50mm straight through-finder
Lumicon green laser pointer
Orion Q-70 26mm, 32mm, and 38mm
Parks 2X 2" Barlow
Hyperion 17mm, 8mm
1 1/4" 18mm Russell Optics Bertele
1 1/4" 12.5mm and 6mm Coulter Optical Orthoscopics
1 1/4" X 2" 32mm Edmund Scientific war surplus Erfle
Tirion star atlas (white stars, black background) hand-laminated
Megastar
And a partridge in a pear tree
To nudge or not to nudge, that is the question
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rwiederrich
Goldfinger
Reged: 11/17/05
Posts: 8334
Loc: Bremerton Washington
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Quote:
MY 8" f/9.44 definitely still needed parabolizing and the shadows were really faint. The only reason it ended up that way was because I was 16 and still learning patience and got sick of grinding! Still, that thing is a great planetary scope and I used it for over 15 years... no actually 17 years. It was not bad for deep sky either, especially with a 32mm Erfle.
See..that is exactly the point. My student just wants a nice scope he can use as a stepping stone for something better in the future. Currently he is done and needs to reevaluate his position.....
He is going to enjoy this scope...and it will provide decent images for him...and that is exactly what he want's now.
Rob
--------------------
www.goldmtobservingcenter.com
A great place for amateur astronomers, and ATM's to come and enjoy their hobby.
http://tech.groups.yahoo.com/group/HomemadeRefractorTelescopes/ My homemade refractor group.
www.vimeo.com/6014031
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Owen
professor emeritus
Reged: 06/21/07
Posts: 514
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And it can easily be flexed to a parabola later, to sharpen things up
Owen
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3128
Loc: Northeast
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My take on it, 6" of aperture is easy to afford, and easy to handle, also works a bit faster than an 8. It's a nice size to learn on.
If you want to parabolise, I'd suggest F/5-6 area. It's not too squirrelly; especially at F/5-5.5. If you don't want to parabolise, I'd suggest F/11-12 and leave it a sphere. If you want to do F/9-10, I would recommend putting in at least some correction. Doesn't have to be fully corrected, just enough to tighten things up a bit.(say, 50%~ should be plenty) These basic guidelines should get a really fine mirror that would do very well.
One thing about long focus mirrors and edge issues- they sorta go hand in hand. The shorter mirrors are easier to get without this defect.
8"s on the other hand, I wouldn't imagine would reasonably go over F/8 on account of their overall length, so you're stuck with correcting them. It sorta cuts your options down. They do have a significant aperture increase over a 6"er, and will show more and brighter images. If this is an interesting aspect, go for an 8. For planetary, the conditions have to be fine for getting the most out of a good 8. The 6"s seem to do well most anytime except the really sour nights.
Additionally my philosophy is a bit different than what's been posted. If making your own optics is appealing, there's nothing wrong with setting your sights a little high. If you pull it off and get a good mirror, it'll last a lifetime.
Mark
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So many projects, so little time!
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Ed Jones
Pooh-Bah
Reged: 04/06/04
Posts: 1415
Loc: Sin-sin-atti
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My first mirror as a teenager was a 6 inch f/8.3 which according to Texereau was OK to leave as a sphere (with a 1/4 wavefront). I did a lot of observing with this scope and even though I knew it wasn't perfect I thought it was great and frankly it was better than many scopes I've look through today. So for a beginner I'd say if you are making your first mirror and are not too sure about parabolizing you can always test a 6 inch mirror f/8 or f/9 at conjugates. If you have room to place the light source about 30 feet away your mirror will be elliptical but will be 1/10 wave accuracy and would be an excellent mirror. This is perhaps wat Mark was saying.
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Ed Jones
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gregj888
sage
Reged: 03/26/06
Posts: 303
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Have you thought about a Groski Schiefspiegler in 4" or 6"?
Easy to make, teaches a lot, and you end up with a show stopping star party scope (if done at all well).
Just a thought.
Greg
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