Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

OPT 8" f/9 Planet Pro Dobsonian

This topic has been archived. This means that you cannot reply to this topic.
105 replies to this topic

#51 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 81,883
  • Joined: 16 Jun 2004

Posted 22 April 2013 - 02:59 PM

Still, I can't argue against the soundness of a long-focus 6" or 8" newtonian as a planetary scope, and a lot of the arguments I've seen in favor of shorter focal ratios or larger apertures don't take all of the factors into account. With planetary performance, aperture only rules up to a point of diminishing returns.



This is the way I look at it:

- It's not shorter focal ratios or larger apertures, it's shorter focal ratios and larger apertures. All the points you make are important and represent a challenge to the builder.

- It's worth thinking in the fixed focal length, variable focal ratio/aperture paradigm rather than the fixed aperture paradigm. Typically the thinking is "what is the best 8 inch planetary scope?" I am more inclined to ask the question: "Which is the best planetary scope with a 1300mm focal length?"

From an ergonomic standpoint, comfort at the eyepiece, seated rather than standing, focal length is the most important factor.

So, all those disadvantages to a faster scope you mention, they are there and everyone who views the planets through a fast, larger aperture scope is well aware of them. But one should not be choosing between a 8 inch F/8 and an 8 inch F/4 as a planetary scope, I think we all know that the 8 inch F/8 will better.

On the other hand, someone looking at an F/4 is combining that with a larger aperture, in the fixed focal length paradigm, it's an 8 inch F/8, a 10 inch F/6.4, a 12.5 inch F/5 or a 16 inch F/4. In each case, if the seeing supports the decision, the larger aperture will be the better performer.

I have the series of ~50 inch focal length scopes from 6 inch to 12.5 inch. Around here where the seeing is typically quite good, the weather mild, each increase in aperture results in a more detailed planetary views. In other places, other combinations may be optimal.

Jon Isaacs

#52 Herr Ointment

Herr Ointment

    Cosmos

  • *****
  • Posts: 9,769
  • Joined: 12 Mar 2011

Posted 22 April 2013 - 03:02 PM

Well, on the very, very, very rare night that conditions are perfect for views of the planets or the moon the 8" F8.5 newtonian pictured here kicks the other three scopes butts.

Attached Thumbnails

  • 5815598-CN.jpg


#53 Jarad

Jarad

    Fly Me to the Moon

  • *****
  • Posts: 6,405
  • Joined: 28 Apr 2003

Posted 22 April 2013 - 03:12 PM

First of all, let's just jump right out and say that not everybody can afford a Zambuto mirror.


Certainly. And if we were asking the question "What is the most affordable scope that will produce outstanding planetary views", I would be inclined to point toward a 6"-8" long f-ratio scope. Cheaper mirror, and can use cheaper eyepieces. But that wasn't the original question. My answers below ignore the cost issue, and only focus on performance.

1. Coma. You can't get away from it at f/3.6 without introducing more optics into the lightpath.

Yes, but there are now excellent optical correctors for coma. It does add cost, but you can get still get excellent performance.

2. Central obstruction. At f/3.6 you need a BIG secondary mirror in order to have full field illumination. At that f/ratio, your central obstruction is approaching cassegrain territory and loss of contrast on planets will be DEFINITELY noticeable.



Depends on the size of the scope. You can build a 25" f3.6 scope and keep the CO at 20%. In larger sizes, you can keep the CO under 20%. Fast f-ratio isn't a goal in itself, you do it to keep the height down. In a smaller scope, I wouldn't go that fast. At 8", I see little reason to go faster than f6 for visual use (imaging is a different ballgame). I wouldn't recommend going to f3.6 in a scope much under 20". At those sizes, you will still be looking at a CO of not much over 20%.

Issues 3, 4 and 5 and 6 are all essentially cost issues. I agree that good fast optics cost more, and a good structure to hold collimation well costs more, and they require higher end eyepieces to avoid eyepiece astigmatism. But if cost isn't the driver, then those can all be dealt with.

As for the thermal issues, they are important. But I have seen a number of large scopes with thin mirrors and fans that produce excellent planetary images. Of course, this does also depend on your seeing. I have seen nights where 8" scopes are struggling with bad seeing, and are limited to ~100x. I have also seen a handful of nights where 18"+ scopes are pushing their diffractions limits (one night at the WSP in the Florida Keys was just incredible...).

If I lived in the Keys, where the seeing is good and the temperature doesn't drop much over night, I would be getting a larger scope. If I lived in the north with the jetstream overhead and nights that drop 30+ degrees compared to the day, I would be focused on something small that equilibrates quickly so I could make the best of what seeing there is. As for f-ratio, I prefer to keep my feet on the ground, or as close to it as possible, so my choice of f-ratio would depend on the aperture. I'd probably aim for a focal length of ~62" until the aperture got so large that I get down to f4 (say around 16"). Then once I break down and have to get the ladder, I'd probably jump up into the 24"+ range and get an f3.6.

Jarad

#54 Jeff Morgan

Jeff Morgan

    Voyager 1

  • *****
  • Posts: 11,537
  • Joined: 28 Sep 2003

Posted 22 April 2013 - 03:43 PM

Uncle Rod Mollise once said, "When it comes to planetary viewing, seeing is not the most important thing, it's the only thing."

In a thread like this, the first thing I do is look over and see what people live. We are captives of our own experiences so it's no surprise that someone who lives up near the great lakes or in the North East might have quite a different view than someone who lives along the coast in Florida or along the coast here in southern California.

When I respond to someone's post, I try to imagine something about their situation and hope to communicate something about my situation..

Jon


I would tend to agree. It's tempting when you see a nice off-the-shelf solution. But that long focal ratio by itself is not going to overcome prevailing bad seeing conditions. OTOH, it will do no worse than a faster mirror of the same aperture. And for the average person it would not require a ladder - perhaps only a step for the limited amount of times you work within 10 degrees of the zenith. So I really see no penalty in a 8" f/9 vs. something shorter. If sitting is your thing, get a Catsperch.

The long focal ratio also won't guarantee a mass-market mirror is going to be excellent (although it does stack the deck to an extent). It might indeed be very good - or it could have any number of systemic flaws resulting from high-speed production.

And then you get to the structure. A bending load is about the worst thing you can do structurally. It takes some engineering homework to get it right. That generally means large diameter tubes (2"+), short lengths (flexure increase with the cube of length), and ultra-light upper ends. Of course if you know the materials, weights, and dimensions this can all be easily calculated with great accuracy.

Unfortunately, all we have is the photo to judge and what I see is not encouraging (and the quality of the connectors is also not clear). If those thin-looking tubes flex too much, mirror quality is moot. Other than visual appeal of the clean lines of a single strut scope, there really is no reason to do this - in fact I would steer well clear of it. Triangle trusses are always better structurally.

I suppose if limited budget or workshop space were considerations it may be worth a chance. It may indeed be a wonderful performer. But I would definitely want a strong return guarantee from the seller.

#55 dscarpa

dscarpa

    Skylab

  • *****
  • Posts: 4,132
  • Joined: 15 Mar 2008

Posted 22 April 2013 - 04:06 PM

If someone who works at the store warns me about a major issue with a scope I steer clear. Discovery mirrors aren't low end, I looked through Jon's 12.5" and was very impressed. David

#56 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 81,883
  • Joined: 16 Jun 2004

Posted 22 April 2013 - 04:13 PM

And for the average person it would not require a ladder - perhaps only a step for the limited amount of times you work within 10 degrees of the zenith. So I really see no penalty in a 8" f/9 vs. something shorter. If sitting is your thing, get a Catsperch.



Jeff:

If you work out the geometry, it turns out that the eyepiece height of a dob like this one only changes a few inches (~4-5) between about 60 degrees and the zenith. In this case, it might not be so bad but judging from the 76 inch OTA length and the base design, it appears to me that the eyepiece height will probably be somewhere over 6 feet.

This means it will require a stool or short ladder for someone who is 6 feet tall down to about 60 degrees. I do think for viewing the planets, standing on a stool is a poor way to observe the planets, a tall ladder is better but the Catsperch is the right solution. I have one for my 16 inch and I can view seated at the zenith. But it's not that fun getting up there and I use my Starbound most of the time.

Jon

#57 careysub

careysub

    Mercury-Atlas

  • *****
  • Posts: 2,565
  • Joined: 18 Feb 2011

Posted 22 April 2013 - 04:47 PM

...
And then you get to the structure. A bending load is about the worst thing you can do structurally. It takes some engineering homework to get it right. That generally means large diameter tubes (2"+), short lengths (flexure increase with the cube of length), and ultra-light upper ends. Of course if you know the materials, weights, and dimensions this can all be easily calculated with great accuracy.

Unfortunately, all we have is the photo to judge and what I see is not encouraging (and the quality of the connectors is also not clear). If those thin-looking tubes flex too much, mirror quality is moot. Other than visual appeal of the clean lines of a single strut scope, there really is no reason to do this - in fact I would steer well clear of it. Triangle trusses are always better structurally...


It looks plausible that the struts are fine.

It appears that they are only 36" long (that is a really long bottom cage). If we assume a 1.5 lb load per strut (6 lb for the loaded UTA), a 1" tube (0.063" wall), and that the connection system is adequate to create a clamped cantilever (thus forcing S-bends in the struts) then the maximum deflection is 0.78 mm, vs the PAE collimation tolerance for planetary use (cf. Carlin) of 2.67 mm at F/9. A 1.25" tube cuts the deflection to 0.38 mm and would be within tolerance even without the clamped cantilever effect (1.52 mm).

#58 Sarkikos

Sarkikos

    ISS

  • *****
  • Posts: 31,144
  • Joined: 18 Dec 2007

Posted 22 April 2013 - 05:58 PM

Jon,

Uncle Rod Mollise once said, "When it comes to planetary viewing, seeing is not the most important thing, it's the only thing."

In a thread like this, the first thing I do is look over and see what people live. We are captives of our own experiences so it's no surprise that someone who lives up near the great lakes or in the North East might have quite a different view than someone who lives along the coast in Florida or along the coast here in southern California.

When I respond to someone's post, I try to imagine something about their situation and hope to communicate something about my situation.


You are correct, and I agree. But I think Uncle Rod uses hyperbole to make a point. Of course, if the seeing is poor, it is the only thing. But on a night of good seeing ... then what?

I live in Maryland in the Mid-Atlantic area. This is at the southern end of the North East. We don't live permanently under the jet stream, but it pretty much gives us a hard time from about Thanksgiving to Memorial Day. Most nights during that time will be mediocre to poor, although we get a few nights of good seeing. But during Summer and Fall, we can enjoy many nights of good to excellent seeing. Once in a great while, we'll have a series of excellent seeing nights, across a week or more.

So that's my experience with seeing here in Maryland. I know enough not to expect to see fine surface detail unless the seeing is good to excellent. But when the seeing is good to excellent, in general, I can expect more from larger aperture than from larger f number - of course, up to the point where the larger aperture begins to fail from things like lack of adequate thermal control.

Mike

#59 Sarkikos

Sarkikos

    ISS

  • *****
  • Posts: 31,144
  • Joined: 18 Dec 2007

Posted 22 April 2013 - 06:13 PM

Jon,

I have the series of ~50 inch focal length scopes from 6 inch to 12.5 inch. Around here where the seeing is typically quite good, the weather mild, each increase in aperture results in a more detailed planetary views. In other places, other combinations may be optimal.


That's what I've seen, too, when the seeing allows. For apertures larger than about 10" or 12", though, nearly all planet images I've seen have been poor, many downright terrible, and getting progressively worse the larger the aperture. Probably the telescopes needed better thermal control and better collimation. I'm skeptical about how often there is actually close collimation in telescopes that must be constructed again every time they are used, particularly when the observer never uses an autocollimator. Good enough for deep sky, but for planets, not so much.

Mike

#60 Sarkikos

Sarkikos

    ISS

  • *****
  • Posts: 31,144
  • Joined: 18 Dec 2007

Posted 22 April 2013 - 06:27 PM

Well, on the very, very, very rare night that conditions are perfect for views of the planets or the moon the 8" F8.5 newtonian pictured here kicks the other three scopes butts.


Well, you're comparing the long Newt against a couple SCTs and what looks like an achromat. Why am I not surprised that the Newt eats them all for breakfast?

:grin:
Mike

#61 Sarkikos

Sarkikos

    ISS

  • *****
  • Posts: 31,144
  • Joined: 18 Dec 2007

Posted 22 April 2013 - 06:31 PM

If you work out the geometry, it turns out that the eyepiece height of a dob like this one only changes a few inches (~4-5) between about 60 degrees and the zenith. In this case, it might not be so bad but judging from the 76 inch OTA length and the base design, it appears to me that the eyepiece height will probably be somewhere over 6 feet.


Not good for me! :noway:

:grin:
Mike

#62 azure1961p

azure1961p

    Voyager 1

  • *****
  • Posts: 13,816
  • Joined: 17 Jan 2009

Posted 22 April 2013 - 11:00 PM

Jon,

I understand your point, maxing out the aperture for a given focal length, but I found my own choice by considering what I wanted to see, what the resolution needed to be and how big an aperture and every bit as rock solid: how big a scope do i want to manage?

My Parks 10" f/5 was a full fifty pounds of OTA, while the albeit, cumbersome , 72" long 8" was only 35 lbs. Pondering a 10" f/7 with even more weight was beyond my envelope. A hefty 12" f/6 was even more so - despite the advantages.

Northeast seeing- Im convinced hot balmy humid rainforest heatwaves that plague CT summers can see.35" resolution on the better nights allowing big aperture to get some profitable views. The real hobbling bummer are the cold months. December thru late March are the WORST !!! The jetstream tethers a line to the state and lives there with the snow. Summer is a revelation with lazy slow seeing that seems to lethargic to move but barely on those nice to great nights.

Pete

#63 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 81,883
  • Joined: 16 Jun 2004

Posted 23 April 2013 - 03:20 AM

Jon,

I understand your point, maxing out the aperture for a given focal length, but I found my own choice by considering what I wanted to see, what the resolution needed to be and how big an aperture and every bit as rock solid: how big a scope do i want to manage?


Pete:

Mostly I am just trying to provide a different way to look at things, if you look at say F/3.6 versus F/8 without substantially increasing the aperture, it makes little sense as a planetary scope. If you think out of the box and allow the aperture to float and the size to remain fixed, it changes the whole equation. I remember the first time I saw a Discovery 10 inch F/6 PDHQ at OPT. Beautiful scope but it was as big as 12.5 inch...

I think the bottom line is that there is no optimal planetary scope, just the ones that works best for you, for me and for the other guy.

Jon

#64 Sarkikos

Sarkikos

    ISS

  • *****
  • Posts: 31,144
  • Joined: 18 Dec 2007

Posted 23 April 2013 - 06:01 AM

Pete,

My Parks 10" f/5 was a full fifty pounds of OTA, while the albeit, cumbersome , 72" long 8" was only 35 lbs. Pondering a 10" f/7 with even more weight was beyond my envelope. A hefty 12" f/6 was even more so - despite the advantages.


Weight and ergonomics count for something ... they count for quite a lot in my view. Too heavy and too high will automatically exclude some telescopes for me. Do I need someone else to help me mount the scope? Too heavy. Do I need a ladder to look in the eyepiece? Too high. IME & IMO, even if I need to stand to look in the eyepiece it's too high! :grin:

So far I've found my best mix of ergonomics, weight, portability and capability for planet observation with a 10" f/4.8 solid-tube Dob.

Mike

#65 Sarkikos

Sarkikos

    ISS

  • *****
  • Posts: 31,144
  • Joined: 18 Dec 2007

Posted 23 April 2013 - 07:45 AM

Pete,

Northeast seeing- Im convinced hot balmy humid rainforest heatwaves that plague CT summers can see.35" resolution on the better nights allowing big aperture to get some profitable views. The real hobbling bummer are the cold months. December thru late March are the WORST !!! The jetstream tethers a line to the state and lives there with the snow. Summer is a revelation with lazy slow seeing that seems to lethargic to move but barely on those nice to great nights.


This agrees with my experience in Maryland. Maybe it's a little better for us since we're a little farther south.

But winter is definitely the worst time for planet observation in our area. When I hear other observers talk about how winter is best for astronomy, I know they are not talking about planets, they do not live where I live, or they do not yet have enough experience to give an informed opinion. The ecliptic dips lower during the summer months, but I'd take a planet a little lower in the sky any steady summer night over poor seeing during winter. Also, I'd rather deal with mosquitoes than the cold!

:grin:
Mike

#66 Thomas Karpf

Thomas Karpf

    Vanguard

  • *****
  • Posts: 2,248
  • Joined: 09 Feb 2009

Posted 23 April 2013 - 08:44 AM

I've heard it said that the best 6" f/8 is an 8" f/6, and the best 8" f/6 is a 10" f/5. In general, and up to a point, larger aperture will show you more surface detail than will a larger f number.

Mike


Well then, the best 8" f/9 would be a 10" f/7.
http://www.teeterste...catalog_pk.html

#67 *skyguy*

*skyguy*

    Soyuz

  • *****
  • Posts: 3,636
  • Joined: 31 Dec 2008

Posted 23 April 2013 - 08:52 AM

Give two guys a $1000 to buy a complete 8" dobsonian scope. One gets an 8" f/4 scope and the other an 8" f/8. Who do you think is going to get the better high power views? The guy with 8" f/4 sure isn't going to end up with a Zambuto mirror in his scope! I'm also sure a 10" f/5 with a "perfect" mirror is going to beat out (probably not by very much) that 8" f/8 in high power planetary views ... but, at what ... 2X-3X the cost?

Unfortunately, in the real world most of us have to temper our purchases with what we have in the piggy bank. If cost were no object, we'd all own a Hubble Space Telescope for our observing pleasure! ;)

#68 Thomas Karpf

Thomas Karpf

    Vanguard

  • *****
  • Posts: 2,248
  • Joined: 09 Feb 2009

Posted 23 April 2013 - 09:18 AM

Give two guys a $1000 to buy a complete 8" dobsonian scope. One gets an 8" f/4 scope and the other an 8" f/8. Who do you think is going to get the better high power views? The guy with 8" f/4 sure isn't going to end up with a Zambuto mirror in his scope! I'm also sure a 10" f/5 with a "perfect" mirror is going to beat out (probably not by very much) that 8" f/8 in high power planetary views ... but, at what ... 2X-3X the cost?


Ahhh... But that's what makes scope comparisons so much 'fun'; the impossibility of comparing apples to apples.

Compare based on cost?
Compare based on aperture?
Compare based on length of tube?
Compare based on contrast?
Compare based on set up setup speed (and does the temp matter)?
Compare based on magnification/field of view (and which wins, high magnification or big field of view)?
Compare based on availability?

#69 Jeff Morgan

Jeff Morgan

    Voyager 1

  • *****
  • Posts: 11,537
  • Joined: 28 Sep 2003

Posted 23 April 2013 - 09:53 AM

And for the average person it would not require a ladder - perhaps only a step for the limited amount of times you work within 10 degrees of the zenith. So I really see no penalty in a 8" f/9 vs. something shorter. If sitting is your thing, get a Catsperch.



Jeff:

If you work out the geometry, it turns out that the eyepiece height of a dob like this one only changes a few inches (~4-5) between about 60 degrees and the zenith. In this case, it might not be so bad but judging from the 76 inch OTA length and the base design, it appears to me that the eyepiece height will probably be somewhere over 6 feet.

This means it will require a stool or short ladder for someone who is 6 feet tall down to about 60 degrees. I do think for viewing the planets, standing on a stool is a poor way to observe the planets, a tall ladder is better but the Catsperch is the right solution. I have one for my 16 inch and I can view seated at the zenith. But it's not that fun getting up there and I use my Starbound most of the time.

Jon


You're correct in that there is little height change towards the top of the arc. I did a rough estimate and came up with different numbers though. The listed figure of 76" is the length of the tube, not the focuser position, which does make quite a difference.

When doing a quick estimate like this I generally assume that the amount of height a mount adds to a Dob is cancelled by the height "removed" by the secondary to eyepiece (intercept) distance. This is actually pessimistic (especially with larger scopes), but let's run with it for now. So, the focuser position at zenith is about the equal to the stated focal length of 72".

The "average" American stands about 6' tall - 72". Figure your eyeballs are mounted about 4" below the top of your head. So, the "average" person has a maximum standing eyeball height of 68".

On the other side, one can quickly figure a normal and comfortable seated position of body height minus the distance from knees to hips. (Think about it, or just just look at how you sit now - your thighs are horizontal and contribute nothing to height when seated). For most people this is around 40".

Thus we have a normal comfort range of 40" to 68". Any shorter and you need to "scrunch" while you sit. And on the other end, you need a 4" tall step. For a 68" radius arc corresponding elevation angles are roughly 34 degrees and 71 degrees. Attached is pdf which illustrates this. Of course YMMV by body size and how efficiently the scope is designed and built.

Attached Files



#70 Jeff Morgan

Jeff Morgan

    Voyager 1

  • *****
  • Posts: 11,537
  • Joined: 28 Sep 2003

Posted 23 April 2013 - 09:57 AM

It looks plausible that the struts are fine.

It appears that they are only 36" long (that is a really long bottom cage). If we assume a 1.5 lb load per strut (6 lb for the loaded UTA), a 1" tube (0.063" wall), and that the connection system is adequate to create a clamped cantilever (thus forcing S-bends in the struts) then the maximum deflection is 0.78 mm, vs the PAE collimation tolerance for planetary use (cf. Carlin) of 2.67 mm at F/9. A 1.25" tube cuts the deflection to 0.38 mm and would be within tolerance even without the clamped cantilever effect (1.52 mm).



Plausible yes, but one must guess at some key numbers. I would want these before spending $1000 of my own money. Plus shipping, and return shipping if it does not pan out.

The photo appears to show foam wrapped tube for example. Perhaps they are 1". Perhaps they are less. Wall thickness is also unknown. Makes a big difference. Upper end weight is also a guess.

And the connector build quality is still a mystery.

#71 dscarpa

dscarpa

    Skylab

  • *****
  • Posts: 4,132
  • Joined: 15 Mar 2008

Posted 23 April 2013 - 09:58 AM

I would think a 8" Teeter STS would make a excellent lunar-planetary scope provided the optics are of high quality. The solid tube helps keep body heat out of the light path and should hold collimation if moved about with care. The longest ratio so far is F/6 but Rob might be able to make it closer to F/7. The 11" Teeter STSs are usually F/4.5 but the one I've got on order is F/5. Going with no cutouts in the base will make for a heavy but hopefully not nose heavy scope. Weight would be a lot less of an issue with a 8" STS that has no cutouts. David

#72 Sarkikos

Sarkikos

    ISS

  • *****
  • Posts: 31,144
  • Joined: 18 Dec 2007

Posted 23 April 2013 - 10:06 AM

I've heard it said that the best 6" f/8 is an 8" f/6, and the best 8" f/6 is a 10" f/5. In general, and up to a point, larger aperture will show you more surface detail than will a larger f number.


Well then, the best 8" f/9 would be a 10" f/7.


No, I've never heard that said ... until now!

:grin:
Mike

#73 Sarkikos

Sarkikos

    ISS

  • *****
  • Posts: 31,144
  • Joined: 18 Dec 2007

Posted 23 April 2013 - 10:16 AM

Lest we forget:

Central obstructions below 20% of the aperture are indistinguishable in practice from an unobstructed aperture, and for obstruction under 25%, performance can be very good.


Suiter, Star Testing Astronomical Telescopes, 2nd ed., p. 163.

Get down close to 20% CO and then don't worry about it. Is it worth further increasing the focal length of a given aperture if you wind up standing on a ladder or a nose-bleed chair to look in the eyepiece? A precarious perch is not good for extended observation of fine surface detail. There are other ways to enhance contrast besides increasing f number and decreasing CO.

Mike

#74 Sarkikos

Sarkikos

    ISS

  • *****
  • Posts: 31,144
  • Joined: 18 Dec 2007

Posted 23 April 2013 - 10:31 AM

Give two guys a $1000 to buy a complete 8" dobsonian scope. One gets an 8" f/4 scope and the other an 8" f/8. Who do you think is going to get the better high power views? The guy with 8" f/4 sure isn't going to end up with a Zambuto mirror in his scope! I'm also sure a 10" f/5 with a "perfect" mirror is going to beat out (probably not by very much) that 8" f/8 in high power planetary views ... but, at what ... 2X-3X the cost?


You pays your money, you takes your choice. For my own pay scale and ergonomic comfort, I've ended up with an off-the-shelf 10" f/4.8 OTA which I've put on a Dob mount, then optimized as much as I can. Works for me.

Unfortunately, in the real world most of us have to temper our purchases with what we have in the piggy bank. If cost were no object, we'd all own a Hubble Space Telescope for our observing pleasure! ;)


No sir, I would not want an HST. It's too cold out there in space, and I don't want to see a remote picture. I want to be right there in the field with whatever scope I have, my eye to the eyepiece and myself securely seated at all positions of the OTA.

:ubetcha:
Mike

#75 Sarkikos

Sarkikos

    ISS

  • *****
  • Posts: 31,144
  • Joined: 18 Dec 2007

Posted 23 April 2013 - 10:39 AM

I would think a 8" Teeter STS would make a excellent lunar-planetary scope provided the optics are of high quality. The solid tube helps keep body heat out of the light path and should hold collimation if moved about with care. They're F/6 but Rob might be able to make it F/7. The 11" Teeter STSs are usually F/4.5 but the one I've got on order is F/5. Going with no cutouts in the base will make for a heavy but hopefully not nose heavy scope. Weight would be a lot less of an issue with a 8" STS that has no cutouts. David


I'd rather have the 11" f/5 for lunar-planet. Is a somewhat smaller CO% better than a nearly 40% increase in light grasp?

Mike


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics