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InkDark
Carpal Tunnel
Reged: 10/29/07
Posts: 1507
Loc: Montreal, Canada
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Hi all,
This week-end I visited my father and we took a look at his telescope. He bought it a couple of years ago in a department store (yes, I know, but at the time we didn’t...) It’s a Bushnell Newt on alt./az. Mount. It has a 114mm (4.5”) primary and a 900 mm FL. The EPs that came with it are beyond cheap and I won’t even mention the finder. OTOH, the 1.25” focuser is pretty smooth. Overall it is a cute little scope. I remember when he first bought it we’ve tried it a couple of times and were very disapointed. Now with what I currently know I think that the scope could serve some purposes. Maybe looking at the Moon, Jupiter, Saturn, M31 (from dark skies)...
I think that the primary is flat (non parabolic). 1) How can this affect the image quality and collimation?
I currently own a 6 inch Sky-watcher Dob. 2) How much of a difference could we expect between my scope and my Dad’s on the targets mentioned above?
3) Is this a worthless scope in your opinion?
BTW, I borrowed a 25mm Plossl to my dad so that he can try it with a better EP.
Thanks for your inputs.
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Jimmy
"Rarely Have So Many Understood So Little About So Much" - Palle Yourgrau
"...since that time, I have not complained about the weather one single time. I’m glad there is weather." – Alan Bean, Apollo 12
Edited by InkDark (08/18/08 08:34 PM)
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werewolf6977
Lord High Smasher
Reged: 12/15/03
Posts: 7444
Loc: Hanover, Ohio
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Don't really know about the optics, nor the mount. Take some of your plossl's from your skywatcher (oops, see you did), and see what that does for the views. As may be gleaned from my post about "Time With A Small Newt" here, and someone else's post about a 4.5" Meade Newt, you can see there is a demand for a smaller scope, and one can be made to be quite a nice, if small, scope...
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Pete
6" Apogee/LXD55 - "The Beast"
Starhopper 6" Dob - "Shiva"
Spaceprobe 130 EQ - "Spacey"
Bushnell Fatboy
The Abomination
Sun Pak Pro 7500 Platinum Edition
10X25 Bushnell Camo Roofies
7X35 Tasco Classic Plastic (good views though)
7X42 Tasco Rare Bird
10X50 Nikon Actions (Type 7)
15X70 Skymasters - "DroolMeisters"
One ratty old IBM 600E LapTop
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Protheus
Vaguely offended
Reged: 09/01/07
Posts: 4661
Loc: Illinois, US
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Quote:
I think that the primary is flat (non parabolic). 1) How can this affect the image quality and collimation?
Probably a flat primary would make collimation a moot point, since a flat mirror would not focus the light at all for you.  A spherical mirror, though, would collimate pretty much like a parabolic mirror, but would show spherical aberration.
Those dimensions make it almost an f/8, and I'm told that a spherical mirror is diffraction limited at f/9. So it won't work quite as well as a parabolic mirror at this focal length, but it may be close...
Quote:
3) Is this a worthless scope in your opinion?
Probably not. The optics are seldom a total loss. The mount is more often...
Chris
Edit: Stop this thing down to 100mm, and it would be diffraction limited (in theory...)
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"To tread the sharp edge of a sword;
to run on smooth-frozen ice,
one needs no footsteps to follow..."
"Well, people sometimes ask me 'how did you get involved in astronomy?' I said 'I got born, what's your problem?'" -- John Dobson
"In discussing the large-scale structure of the cosmos, astronomers sometimes say that space is curved, or that the universe is finite but unbounded. Whatever are they talking about?" -- Carl Sagan
Edited by Protheus (08/18/08 09:11 PM)
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InkDark
Carpal Tunnel
Reged: 10/29/07
Posts: 1507
Loc: Montreal, Canada
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Thanks guys for the replies.
Quote:
Probably a flat primary would make collimation a moot point, since a flat mirror would not focus the light at all for you. A spherical mirror, though, would collimate pretty much like a parabolic mirror, but would show spherical aberration.
Chris, I may be having some problems with the terms here. What does "spherical mirror" mean exacly and how is it different from a "parabolic mirror"?
Quote:
Edit: Stop this thing down to 100mm, and it would be diffraction limited (in theory...)
That's a good advice. We will try it. Thanks.
--------------------
Jimmy
"Rarely Have So Many Understood So Little About So Much" - Palle Yourgrau
"...since that time, I have not complained about the weather one single time. I’m glad there is weather." – Alan Bean, Apollo 12
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dmgriff
sage
Reged: 09/20/06
Posts: 258
Loc: 30 degrees latitude, USA
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If the focuser is good, that is a major start!
The mirror is spherical. You may wish to check the collimation. With an inexpensive Chinese mirror, you may expect things to deteriate at 150x +, depending on conditions. However, the alt/az mount may be a hindrance to tracking at high power anyway.
I am not selling the scope short, however. With decent plossl eps and nice collimation , I think you should get very nice views and use from this scope. If things work out with it, think about a Rigel type finder, light wt.
I have a 4.5in f8 Tasco Luminova ota with improved optics, 8x50 finder, on a eq3. Don't think I'll get rid of it any time soon, as it gets used. Competes with a 6in and 10in dob.
Good luck and Good viewing,
Dave
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werewolf6977
Lord High Smasher
Reged: 12/15/03
Posts: 7444
Loc: Hanover, Ohio
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A spherical primary is figured as a section of a sphere, IIRC,(forgive me if I over-simplify). However to perfectly focus aan objects image, the primary has to be figured to a parabola.
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Pete
6" Apogee/LXD55 - "The Beast"
Starhopper 6" Dob - "Shiva"
Spaceprobe 130 EQ - "Spacey"
Bushnell Fatboy
The Abomination
Sun Pak Pro 7500 Platinum Edition
10X25 Bushnell Camo Roofies
7X35 Tasco Classic Plastic (good views though)
7X42 Tasco Rare Bird
10X50 Nikon Actions (Type 7)
15X70 Skymasters - "DroolMeisters"
One ratty old IBM 600E LapTop
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dusty mirrors
member
Reged: 01/27/08
Posts: 20
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i have a very similar telescope. i have a meade 114eq-a. i have great fun with mine. i have been asking if there are others with cheap scopes like mine to compare notes with. i have done much work on improving mine since i got it. if you are inclined there are several things you can do to the mount. no its not worthless, its a lot better than a paper towel roll and its worth all the fun you can have with it!
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Protheus
Vaguely offended
Reged: 09/01/07
Posts: 4661
Loc: Illinois, US
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Quote:
Chris, I may be having some problems with the terms here. What does "spherical mirror" mean exacly and how is it different from a "parabolic mirror"?
What Pete said. Take a really large sphere, so large that the distance between one end and the other is twice the focal length of the scope, then cut of a section of this hypothetical perfect sphere which is the size of your objective, coat it with aluminum, and you have your telescope mirror. That's not _really_ how they make them, of course.
A parabolic mirror is figured somewhat differently to correct something known as "spherical aberration."
Quote:
Quote:
Edit: Stop this thing down to 100mm, and it would be diffraction limited (in theory...)
That's a good advice. We will try it. Thanks.
I meant that for illustration only. There's a good chance you don't need to do so.
In simple terms, spherical aberration happens when some of the light reaches focus at a slightly different point than the rest. This is likely to manifest as a kind of "soft" image.
Take a look at these:
Spherical Paraxial Rays
Parabolic Paraxial Rays
You see the overlapping of some of the returning light-rays in the spherical version, the ones that pass through the focal point, but don't come to focus there. That's the problem with a spherical mirror.
Now notice that this happens much more drastically towards the edge of the mirror. The more of the outer edge of the mirror you chop off (or, put another way, the "slower" the optic), the less spherical aberration you see. As I was saying, by f/9, it's supposed to no longer degrade the image compared to what you would see in a "perfect" mirror; you're limited only by the resolution limit of the aperture at that point. At f/8, you aren't quite there, you will get some spherical aberration, but I suspect it will not be bad. In other words, use the full aperture of this scope, and if spherical aberration drives you nuts, consider stopping 14mm off of the edge, and it will probably go away.
Chris
Edit: _twice_ the focal length.
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"To tread the sharp edge of a sword;
to run on smooth-frozen ice,
one needs no footsteps to follow..."
"Well, people sometimes ask me 'how did you get involved in astronomy?' I said 'I got born, what's your problem?'" -- John Dobson
"In discussing the large-scale structure of the cosmos, astronomers sometimes say that space is curved, or that the universe is finite but unbounded. Whatever are they talking about?" -- Carl Sagan
Edited by Protheus (08/19/08 03:00 PM)
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microbes
Pooh-Bah
Reged: 12/12/04
Posts: 1192
Loc: Romulus, Sector 12
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Quote:
It’s a Bushnell Newt on alt./az. Mount. It has a 114mm (4.5”) primary and a 900 mm FL.
Mine is on a EQ2 but I have the same scope. With some decent plossls It serves my purposes.
Quote:
With an inexpensive Chinese mirror, you may expect things to deteriate at 150x +,
I think that Bushnell's quality control may not be the best so your mileage may vary but with mine as long as the colimation is right and if the seeing is very good (here in Florida it sometimes is really good) I can go to 200-225X before the image degrades. On so-so nights 100-150X is a more reasonable maximum.
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Dirt Cheap Astronomy
Voyager 114X900 Newt EQ2 * Sky Chief 60X700 EQ1 * Cometron 62X300 EQ1
Sears Ultra Wide 7X50 Binos * Vintage 16X50 Binos EQ1
Books, Barlows, Eyepieces, Camera Adaptors & Other Esoteric Junk.
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boyd
sage
Reged: 11/30/06
Posts: 307
Loc: Florida
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I used to own a similar scope, probably from the same factory, but by another name. Being a corrected spherical mirror, you will never get really great images; but if you've never seen great images through a quality scope, then you won't know this to be the case. Here are the top three issues this scope will have even after you upgrade to a decent plossl ep.
1. The mount generates its own wobble. The scope simply cannot be touched during viewing, so you have to guestimate where the object is going in the ep, then move the scope slightly ahead of it and let go. Let the object drift through the ep. This is only a real issue during focus and Ep changes.
2. Because most correctors are plastic, and the spider arm is thick, you will get a large spike on star images. Just accept that they won't be pinpoints and move on.
3. Use of a barlow to ramp up the mag is very hit or miss depending upon what you are looking at. Essentially the barlow places another lense across the light path which increases the folds in the rays to four before it ever gets to the ep. This causes problems with dim images. Not a problem when viewing the moon, but you loose bands on jupiter, divisions in saturn's rings, and DS objects like the ring nebula are barely detectable as a smudge. This is far less than what a 4.5" should be able to do. Thus doubling your magnification isn't really an option on this type of scope.
Having said that, it does serve for casual viewing. For lunar it is acceptable. For planetary it will serve as long as you don't push the image magnification; accept the tiny clear image and be happy. For DS you will need clear steady nights and nothing smaller than a 25mm ep.
Hope this helps
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XT8
ETX70AT
C80EQ
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Protheus
Vaguely offended
Reged: 09/01/07
Posts: 4661
Loc: Illinois, US
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Quote:
I used to own a similar scope, probably from the same factory, but by another name. Being a corrected spherical mirror, you will never get really great images; but if you've never seen great images through a quality scope,
Hey Boyd, I'm thinking that this one is an un-corrected spherical mirror. If it were jones-bird, the focal length is likely to be 1000mm. Somewhat of a different set of problems to be expected, if he's lucky...
Chris
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"To tread the sharp edge of a sword;
to run on smooth-frozen ice,
one needs no footsteps to follow..."
"Well, people sometimes ask me 'how did you get involved in astronomy?' I said 'I got born, what's your problem?'" -- John Dobson
"In discussing the large-scale structure of the cosmos, astronomers sometimes say that space is curved, or that the universe is finite but unbounded. Whatever are they talking about?" -- Carl Sagan
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boyd
sage
Reged: 11/30/06
Posts: 307
Loc: Florida
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Like this:
http://www.bhphotovideo.com/c/product/421519-REG/Bushnell_789945_Voyager_Sky_Tour_4_5_114mm.html
It is a typical J-bird design, and it has a corrector.
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XT8
ETX70AT
C80EQ
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microbes
Pooh-Bah
Reged: 12/12/04
Posts: 1192
Loc: Romulus, Sector 12
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Quote:
Hey Boyd, I'm thinking that this one is an un-corrected spherical mirror. If it were jones-bird, the focal length is likely to be 1000mm.
If it's a jones/bird it would be a "Short tube". They do have a problem with higher magnifications. But every 114 X 900 (F/8) I've seen are long tubes, and they preform better than the short tube models.
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Dirt Cheap Astronomy
Voyager 114X900 Newt EQ2 * Sky Chief 60X700 EQ1 * Cometron 62X300 EQ1
Sears Ultra Wide 7X50 Binos * Vintage 16X50 Binos EQ1
Books, Barlows, Eyepieces, Camera Adaptors & Other Esoteric Junk.
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Protheus
Vaguely offended
Reged: 09/01/07
Posts: 4661
Loc: Illinois, US
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Well, I don't want to call you a liar, but that _really_ looks like a scope that should have an uncorrected mirror. The focal length (the actual focal length; the tube is nearly if not fully 900mm long as I see it) is too long for the corrector to do anything but make the image muddy. 
Additionally, the negative corrector of a jones-bird setup would extend the focal length beyond the length of the tube, and I'd expect it to do this by a significant amount, like 1.5x at least...
To give a counter-example, this is what I've come to expect in a jones-bird design. It's 127mm at -- looks like -- f/4 or so, but reports a focal length of 1000mm. That shows the negative corrector at work very plainly.
Maybe Jimmy can tell us for sure.
Chris
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"To tread the sharp edge of a sword;
to run on smooth-frozen ice,
one needs no footsteps to follow..."
"Well, people sometimes ask me 'how did you get involved in astronomy?' I said 'I got born, what's your problem?'" -- John Dobson
"In discussing the large-scale structure of the cosmos, astronomers sometimes say that space is curved, or that the universe is finite but unbounded. Whatever are they talking about?" -- Carl Sagan
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boyd
sage
Reged: 11/30/06
Posts: 307
Loc: Florida
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I honestly don't know what that scope is, but it isn't a full length scope nor is it a ST. I own a ST114, so yes I understand it looks wrong.
I was giving information based on the "spherical mirror" comments knowing that they have to be corrected by either a corrector plate (Schmidt design) or lense (Bird design).
Now that I'm looking harder, it may just be a parab mirrored newt. My main newt (xt8) seems much longer comparatively for a shorter f/, so looking at a few:
standard 114mm:
http://starnevada.info/ScopeReviews/M114.JPG
The probable culpret:
http://opticsbybushnell.com/library/bushnelldeepspace525x3telescope.jpg
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XT8
ETX70AT
C80EQ
Edited by boyd (08/19/08 01:32 PM)
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Protheus
Vaguely offended
Reged: 09/01/07
Posts: 4661
Loc: Illinois, US
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Ahh, ok, there's our problem. Spherical optics don't need corrected, and aren't always corrected, except in the case of faster optics. The jones-bird or schmidt-newtonian scopes you see are usually around f/4, and the primaries on schmidt-cassegrain or mak-cass scopes tend to be faster still.
Once you get to about f/7 you're in a grey area where you'll see spherical aberration, but it's not too bad. At f/9, the distortion of the aberration should be smaller than the resolution limit of the scope, and so, practically speaking, a spherical mirror at f/9 is fully corrected. (It's not, but you'll outstrip the ability of the scope to gather more detail before you start seeing aberrations.)
Anyway, I'm aware of a few commercial newtonians with spherical mirrors. I have a 76mm f/9, myself, which I've been trying to make a project scope out of, but haven't gotten around to re-mounting on something sturdy yet. Pete has an Orion spaceprobe (his 130eq) with an uncorrected, but somewhat shorter focal length spherical mirror. The current spaceprobe 130 has a parabolic mirror, but it apparently was not always so.
There is a corollary here. If all we're concerned about for a defect free view is the focal ratio, we can force a fast scope to become a slow scope by stopping it down. What I mean is, you can cut some of the light around the edges off, and force the scope to operate as a smaller scope of the same focal length. As I was saying, this scope is nearly f/9 as it is; you'd only lose 14mm from the diameter of the mirror to get corrected down to the resolution limit of the (now slightly smaller) aperture. One of our members, Nytecam, has taken a jones-bird, stripped the corrector off, and stopped it down, making something which -- somewhat oddly -- is called a "lensless schmidt." Here are his results. They really aren't bad. Remember, this is now a completely uncorrected spherical mirror, which used to be a department store jones-bird.
Chris
--------------------
"To tread the sharp edge of a sword;
to run on smooth-frozen ice,
one needs no footsteps to follow..."
"Well, people sometimes ask me 'how did you get involved in astronomy?' I said 'I got born, what's your problem?'" -- John Dobson
"In discussing the large-scale structure of the cosmos, astronomers sometimes say that space is curved, or that the universe is finite but unbounded. Whatever are they talking about?" -- Carl Sagan
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boyd
sage
Reged: 11/30/06
Posts: 307
Loc: Florida
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wow they'll sell anything nowdays...
I don't actually think it is possible to get a spherical mirror to focus on a single point, but I guess at long ratios the loss of light is marginal enough to go unnoticed.
I have a lenseless schmidt I've made from my old ST114, but it sux  HOwever, this is for another time.
Still, my advice holds other than the barlow issue
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XT8
ETX70AT
C80EQ
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Protheus
Vaguely offended
Reged: 09/01/07
Posts: 4661
Loc: Illinois, US
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Quote:
wow they'll sell anything nowdays...
Actually, I might be mistaken, but more often than not, these uncorrected spherical scopes seem to be older. This is unfortunate, since the performance might actually end up being better than the jones-bird corrected ones that you see so often (at the expense of having twice the tube length...)
Quote:
I don't actually think it is possible to get a spherical mirror to focus on a single point, but I guess at long ratios the loss of light is marginal enough to go unnoticed.
Exactly the case. As I said, it's supposed to be f/9 or so, where the image deterioration from spherical aberration becomes smaller than the scope's resolution limit.
Quote:
Still, my advice holds other than the barlow issue
I wouldn't argue with the rest of it.
Chris
--------------------
"To tread the sharp edge of a sword;
to run on smooth-frozen ice,
one needs no footsteps to follow..."
"Well, people sometimes ask me 'how did you get involved in astronomy?' I said 'I got born, what's your problem?'" -- John Dobson
"In discussing the large-scale structure of the cosmos, astronomers sometimes say that space is curved, or that the universe is finite but unbounded. Whatever are they talking about?" -- Carl Sagan
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proud uncle
Pooh-Bah
Reged: 01/22/07
Posts: 1336
Loc: Central Texas
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Quote:
Take a really large sphere, so large that the distance between one end and the other is the focal length of the scope, then cut of a section of this hypothetical perfect sphere which is the size of your objective, coat it with aluminum, and you have your telescope mirror.
The "distance between one end and the other" of a sphere is it's diameter. I think what Chris means to describe is a sphere so large it's radius equals the focal length.
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Kenneth
Zhumell 10" Dobsonian (f/4.9)
2" 32mm WA eyepiece
9mm, 12.5mm, and 20mm Plossls
6mm TMB/BO Planetary
2" 2x ED Barlow
Nikon 10x50 binocular (6.5 deg FOV)
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Protheus
Vaguely offended
Reged: 09/01/07
Posts: 4661
Loc: Illinois, US
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Quote:
The "distance between one end and the other" of a sphere is it's diameter. I think what Chris means to describe is a sphere so large it's radius equals the focal length.
That's what I should have said. I was thinking the focal point should be the center of the sphere, and it didn't come out that way.
Chris
--------------------
"To tread the sharp edge of a sword;
to run on smooth-frozen ice,
one needs no footsteps to follow..."
"Well, people sometimes ask me 'how did you get involved in astronomy?' I said 'I got born, what's your problem?'" -- John Dobson
"In discussing the large-scale structure of the cosmos, astronomers sometimes say that space is curved, or that the universe is finite but unbounded. Whatever are they talking about?" -- Carl Sagan
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