Opinion Please for Next Scope

On Saturday, January 13, 2001 at 10:30:44 AM UTC-5, wrote:

Based on comments by Bill Burnett, (an Intes Dealer), I was leaning
toward the MN66. He claims to do an optical workup, and the Sital
Mirror is rated at 1/6 wave, 93% reflectivity. The Sky & Telescope
article of last year raved about the Orion version of the MN61, and the
MN66 is an upgrade, (and an INTES Micro vs INTES product. I guess they
are the better considered of the two companies. Also Bill says the
Photographic limitation of the MN61 are lessened in the MN66.

Bill also says that the Meade mirrors are notoriously of bad quality,
and I could expect to return several before getting a quality product.
I had intended to get a used G-8, GP, or GP/DX for the MN66, keeping
the cost around $2000.

I am semi interested in getting a telescope myself. The most confusing area to me is the apparent lack of quality of the optics. 1/6 wave flatness (I assume peak to valley) for a mirror is awful and by itself will result in poor imagery. Is this figure typical? It makes me consider designing my own optical system and having it built privately. (Or spending the time to polish a blank myself.)

http://www.richardfisher.com

On 21/01/2016 13:54, Helpful person wrote:
On Saturday, January 13, 2001 at 10:30:44 AM UTC-5, wrote:

Based on comments by Bill Burnett, (an Intes Dealer), I was leaning
toward the MN66. He claims to do an optical workup, and the Sital
Mirror is rated at 1/6 wave, 93% reflectivity. The Sky & Telescope
article of last year raved about the Orion version of the MN61, and the
MN66 is an upgrade, (and an INTES Micro vs INTES product. I guess they
are the better considered of the two companies. Also Bill says the
Photographic limitation of the MN61 are lessened in the MN66.

Bill also says that the Meade mirrors are notoriously of bad quality,
and I could expect to return several before getting a quality product.
I had intended to get a used G-8, GP, or GP/DX for the MN66, keeping
the cost around $2000.

I am semi interested in getting a telescope myself.
The most confusing area to me is the apparent lack of quality of the
optics.
1/6 wave flatness (I assume peak to valley) for a mirror is awful and
by itself
will result in poor imagery. Is this figure typical? It makes me
consider
designing my own optical system and having it built privately.
(Or spending the time to polish a blank myself.)

Unless you are an experienced planetary observer the in focus image
quality from a lambda/6 error mirror will not be at all obtrusive.

The out of focus star test image will show any defects.

Except on a handful of very calm nights with excellent seeing the scope
will be limited by fluctuating phase errors in the atmosphere so don't
obsess on the relatively minor systematic errors of the mirror too much
unless you intend to put adaptive optics on it.

Time was when a lot of mirrors were lambda/4 and called diffraction
limited and the bigger light buckets were much much worse. That is a bit
borderline - certainly they are limited by their diffraction.

Nights where a 10" can be operated at its true diffraction limit with a
clean Airy disk are quite rare where I live. The jet stream sees to
that. Other locations like Florida and Hawaii are blessed with laminar
airflow and no nasty upper atmospheric features to mess things up.

At 4" aperture or less you will quite often get seeing good enough for
an Airy disk to be clearly visible.

--
Regards,
Martin Brown

On Thursday, January 21, 2016 at 9:26:02 AM UTC-5, Martin Brown wrote:
On 21/01/2016 13:54, Helpful person wrote:
On Saturday, January 13, 2001 at 10:30:44 AM UTC-5, wrote:

Based on comments by Bill Burnett, (an Intes Dealer), I was leaning
toward the MN66. He claims to do an optical workup, and the Sital
Mirror is rated at 1/6 wave, 93% reflectivity. The Sky & Telescope
article of last year raved about the Orion version of the MN61, and the
MN66 is an upgrade, (and an INTES Micro vs INTES product. I guess they
are the better considered of the two companies. Also Bill says the
Photographic limitation of the MN61 are lessened in the MN66.

Bill also says that the Meade mirrors are notoriously of bad quality,
and I could expect to return several before getting a quality product.
I had intended to get a used G-8, GP, or GP/DX for the MN66, keeping
the cost around $2000.

I am semi interested in getting a telescope myself.
The most confusing area to me is the apparent lack of quality of the
optics.
1/6 wave flatness (I assume peak to valley) for a mirror is awful and
by itself
will result in poor imagery. Is this figure typical? It makes me
consider
designing my own optical system and having it built privately.
(Or spending the time to polish a blank myself.)

Unless you are an experienced planetary observer the in focus image
quality from a lambda/6 error mirror will not be at all obtrusive.

The out of focus star test image will show any defects.

Except on a handful of very calm nights with excellent seeing the scope
will be limited by fluctuating phase errors in the atmosphere so don't
obsess on the relatively minor systematic errors of the mirror too much
unless you intend to put adaptive optics on it.

Time was when a lot of mirrors were lambda/4 and called diffraction
limited and the bigger light buckets were much much worse. That is a bit
borderline - certainly they are limited by their diffraction.

Nights where a 10" can be operated at its true diffraction limit with a
clean Airy disk are quite rare where I live. The jet stream sees to
that. Other locations like Florida and Hawaii are blessed with laminar
airflow and no nasty upper atmospheric features to mess things up.

At 4" aperture or less you will quite often get seeing good enough for
an Airy disk to be clearly visible.

--
Regards,
Martin Brown

Lambda/6 surface errors will give lambda/3 wavefront error on reflection. This is certainly noticeable in an optical instrument, even for my aging eyes. Your point about "seeing" being the usual limitation is well taken. However, on the occasions when seeing is exceptional it would be nice not to be scope limited.

Another factor to consider is that the resolution of an optical system falls off in a non linear manner with wavefront aberration for near diffraction limited systems. Hence I would expect a better scope to maintain good imagery in worsening conditions much longer than one that already has lambda/3 errors.

http://www.richardfisher.com

On Thursday, January 21, 2016 at 7:54:45 AM UTC-6, Helpful person wrote:
On Saturday, January 13, 2001 at 10:30:44 AM UTC-5, wrote:

Based on comments by Bill Burnett, (an Intes Dealer), I was leaning
toward the MN66. He claims to do an optical workup, and the Sital
Mirror is rated at 1/6 wave, 93% reflectivity. The Sky & Telescope
article of last year raved about the Orion version of the MN61, and the
MN66 is an upgrade, (and an INTES Micro vs INTES product. I guess they
are the better considered of the two companies. Also Bill says the
Photographic limitation of the MN61 are lessened in the MN66.

Bill also says that the Meade mirrors are notoriously of bad quality,
and I could expect to return several before getting a quality product.
I had intended to get a used G-8, GP, or GP/DX for the MN66, keeping
the cost around $2000.

I am semi interested in getting a telescope myself. The most confusing area to me is the apparent lack of quality of the optics. 1/6 wave flatness (I assume peak to valley) for a mirror is awful and by itself will result in poor imagery. Is this figure typical? It makes me consider designing my own optical system and having it built privately. (Or spending the time to polish a blank myself.)

http://www.richardfisher.com

the 1/6 wave measurement is not a flatness measurement. Flat mirrors don't form an image, and there are no flat surfaces in a MN66 scope. A 1/6 wave P-V optical system will be as near perfect as anything you are likely to encounter in a commercial scope. If it is a smooth 1/6 wave error, you can expect a very sharp star image at focus, and even if you star tested it, you might not be able to discern that tiny error.

Uncaoptic

On Thursday, January 21, 2016 at 8:54:45 AM UTC-5, Helpful person wrote:
On Saturday, January 13, 2001 at 10:30:44 AM UTC-5, wrote:

Based on comments by Bill Burnett, (an Intes Dealer), I was leaning
toward the MN66. He claims to do an optical workup, and the Sital
Mirror is rated at 1/6 wave, 93% reflectivity. The Sky & Telescope
article of last year raved about the Orion version of the MN61, and the
MN66 is an upgrade, (and an INTES Micro vs INTES product. I guess they
are the better considered of the two companies. Also Bill says the
Photographic limitation of the MN61 are lessened in the MN66.

Bill also says that the Meade mirrors are notoriously of bad quality,
and I could expect to return several before getting a quality product.
I had intended to get a used G-8, GP, or GP/DX for the MN66, keeping
the cost around $2000.

I am semi interested in getting a telescope myself. The most confusing area
to me is the apparent lack of quality of the optics. 1/6 wave flatness (I
assume peak to valley) for a mirror is awful and by itself will result in poor
imagery. Is this figure typical? It makes me consider designing my own
optical system and having it built privately. (Or spending the time to polish
a blank myself.)

Any telescope is better than no telescope at all.

(Now watch the idiots on this forum try to disagree with that.)

On Thursday, January 21, 2016 at 11:31:16 AM UTC-5,

the 1/6 wave measurement is not a flatness measurement. Flat mirrors don't form an image, and there are no flat surfaces in a MN66 scope. A 1/6 wave P-V optical system will be as near perfect as anything you are likely to encounter in a commercial scope. If it is a smooth 1/6 wave error, you can expect a very sharp star image at focus, and even if you star tested it, you might not be able to discern that tiny error.

Uncaoptic

The previous poster stated that 1/6 wave was the spec on the mirror, not on the complete optical system. I don't know how this can be for the whole optical system as their are different options for eyepieces.

http://www.richardfisher.com

On Thursday, January 21, 2016 at 10:05:09 AM UTC-5, Helpful person wrote:

Lambda/6 surface errors will give lambda/3 wavefront error on reflection. This is certainly noticeable in an optical instrument, even for my aging eyes. Your point about "seeing" being the usual limitation is well taken.. However, on the occasions when seeing is exceptional it would be nice not to be scope limited.

It is also good to not be limited by not having a scope.

On Thu, 21 Jan 2016 08:33:24 -0800 (PST), wrote:

Any telescope is better than no telescope at all.

I know people who abandoned amateur astronomy because their first
telescopes were cheap crap. They would have benefited from no
telescope at all. And I know people who greatly appreciate astronomy,
but have no interest in using telescopes.

On Thursday, January 21, 2016 at 1:01:59 PM UTC-5, Chris L Peterson wrote:
On Thu, 21 Jan 2016 08:33:24 -0800 (PST), wsnell01 wrote:

Any telescope is better than no telescope at all.

I know people who abandoned amateur astronomy because their first
telescopes were cheap ****.

I know people whose first telescopes were cheap **** and they did just fine in amateur astronomy.

They would have benefited from no
telescope at all.

Assertion made without evidence or proof.

And I know people who greatly appreciate astronomy,
but have no interest in using telescopes.

Irrelevant to this discussion.

On Thu, 21 Jan 2016 10:15:45 -0800 (PST), wrote:

On Thursday, January 21, 2016 at 1:01:59 PM UTC-5, Chris L Peterson wrote:
On Thu, 21 Jan 2016 08:33:24 -0800 (PST), wsnell01 wrote:

Any telescope is better than no telescope at all.

I know people who abandoned amateur astronomy because their first
telescopes were cheap ****.

I know people whose first telescopes were cheap **** and they did just fine in amateur astronomy.

I haven't suggested otherwise.

They would have benefited from no
telescope at all.

Assertion made without evidence or proof.

Pretty stupid response. People have told me, in no uncertain way, that
they gave up on astronomy because of bad experiences with poor
equipment. If you don't know such people yourself, with old telescopes
in their closets or garage, you have almost no connection with the
amateur astronomy community.

And I know people who greatly appreciate astronomy,
but have no interest in using telescopes.

Irrelevant to this discussion.

No. Like those who gave up on astronomy because of bad telescopes,
they represent another class of people for whom telescopes have no
value, and who therefore are better off without one.

I don't disagree that much of the time, for many people, a poor
telescope is better than none. But it's not an absolute truth as you
have presented it.