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Long slow Newtonians

equipment mirror making observing planet reflector
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#101 25585

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Posted 17 October 2018 - 02:57 PM

An 8" F8 Dob-mounted would be good. Long but not too heavy. Less stooping and if needed, a shorter platform. Should perform better than a F10 SCT as smaller CO. 


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#102 25585

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Posted 16 January 2019 - 06:28 PM

Last week I bought a Russian TAL 2 6 inch F8 OTA. Not lightweight through design but only size, like my other CCCP or Siberian stuff, built to last! Excellent finder, I think its eye lens is a TAL Plossl, gems in their own right.

 

Its almost an insult to make it a Dob mount, maybe if I used metal plate... but agricised plank will do with galvanised screws and fixings. 

 

Its focuser is only 1.25 fit but part of a classic, so I will keep it on but serviced. 

 

 

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Edited by 25585, 16 January 2019 - 06:28 PM.

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#103 25585

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Posted 16 January 2019 - 06:33 PM

I looked at Edmunds site but they did not have ordinary scope mirrors. Flats yes, but no apart from off-axis.

 

My TAL has a spherical mirror like SW F8 6" Newts.



#104 Deep13

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Posted 17 January 2019 - 01:56 AM

8" (20cm) f/8

8 in f8 1.jpg


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#105 25585

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Posted 17 January 2019 - 06:28 PM

What seems strange to me is that longer, smaller aperture Newtonians would be good in Dobsonian mounts but shorter FL ones are instead. Sure 6 inch F8 are commonplace, but 8 inch F8 would be fine at 64 inches. 10 inch F7 is about it for that size, feet on ground, and not too heavy. Glad to have a 10 F6 (as well as F5). 

 

For 12 inch F6 would be OK if transport and storage were simple. Still possible with a solid tube.

 

Photo of a 6" F11 Newtonian on Dob mount

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Edited by 25585, 17 January 2019 - 06:35 PM.

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#106 careysub

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Posted 19 January 2019 - 10:55 PM

I am putting money down this week on an 11" Zambuto FL 63" (F 5.73) quartz mirror , which I should get next year some time.

 

My plan is to mount it in a 12" ID insulated tube as a split tube Dobsonian, with the lower connecting ring acting as a rotating ring, resting against PTFE pads on the cradle.

 

When putting it together for viewing it will have the option of inserting a 9" aperture diaphragm over the mirror to turn it into a 9" F/7 (of course I can also do a 6.3" F/10 if I like).

 

The purpose is to build the perfect "apo killer" planetary scope, which will be comfortable to use: no climbing up on things, rotating for best EP position. I am 70" tall and figure that 63" is about the maximum for comfortable viewing close to the zenith (since I am going to use it on other objects besides planets).

 

I determined the maximum convenient FL and then looked at how wide a tube I was willing to work with, to get the most Zambuto glass consistent with the other objectives.

 

Another factor was that I already have a 13.1" FL 57.4" Swayze mirror truss Dob (very nice), so it did not make sense to crowd that in aperture.

 

PS: An additional advantage of the split tube is that I can have two upper tubes with different secondaries, a 2.14" for F/5.73 mode and a 1.30" for F/7-F/10 planetary mode. I will make two upper tubes when I do the tube construction.


Edited by careysub, 20 January 2019 - 11:14 AM.

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#107 Starlease

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Posted 19 January 2019 - 11:30 PM

11 inch mirror in a 12 inch tube might not be good.

Had a 12.5 inch mirror in 14 inch tube and found tube wall currents getting in the optical path.


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#108 Starlease

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Posted 20 January 2019 - 12:40 AM

Going to do an 8 inch f8 and will use a 12 inch tube to avoid having tube currents in the light path or at least minimize them.


Edited by Starlease, 20 January 2019 - 12:40 AM.

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#109 Deep13

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Posted 20 January 2019 - 12:57 AM

I've got an 8" f/6 Dob in a 9.5" tube, and that is fine. I do use a single fan behind the primary.

#110 Jon Isaacs

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Posted 20 January 2019 - 01:19 AM

What seems strange to me is that longer, smaller aperture Newtonians would be good in Dobsonian mounts but shorter FL ones are instead. Sure 6 inch F8 are commonplace, but 8 inch F8 would be fine at 64 inches. 10 inch F7 is about it for that size, feet on ground, and not too heavy. Glad to have a 10 F6 (as well as F5).

 

A 48 inch long scope will fit in all but the smallest cars.  It only takes a few inches and the scope will no longer fit in many cars .  The 6 inch F/8, the 8 inch F/6 and the 10 inch F/5 are all optimized for transporting to a dark site in a passenger car .  My 10 inch F/5 traveled thousands of miles around the back roads of the southwest . Many of those miles we in the backseat and trunk of a 1989 Nissan Sentra .

 

Jon



#111 careysub

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Posted 20 January 2019 - 11:23 AM

11 inch mirror in a 12 inch tube might not be good.

Had a 12.5 inch mirror in 14 inch tube and found tube wall currents getting in the optical path.

 

One problem is getting decent forms with arbitrary diameters for making the tubes (I will be doing a composite construction with 1/4" end grain balsa, with carbon fiber, and 1 mm baltic birch). 12" tubes are easy to get (Sakcrete forms) and the actual ID with that as a form would be 12.5" (using a 0.25" foam sheet taped on to it for easy removal of the lay-up).

 

So it would have a 0.75" gap for the 11" mirror. But in F/7 planetary mode the gap would really be 1.75".

 

Also notice I said "insulated", the balsa is an excellent insulator. I expect this would suppress tube currents from tube cooling, and I would have a fan at the bottom also.

 

Given all this, are tube currents likely to be a problem in either F/5.7 of F/7 modes?

 

Any suggestions for increasing the form diameter another inch or so would be welcome. I will have a year to think on this.



#112 starcanoe

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Posted 20 January 2019 - 12:00 PM

 

IMO there is almost NO downside to an oversized tube....perhaps maybe a smidge in weight....and the "well...no the focal point is way out there now and you need a bigger diagonal"...

 

But that need not be an issue... where the focuser is you can engineer a recessed area for the focuser to get closer to the optical axis...


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#113 Galicapernistein

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Posted 20 January 2019 - 01:26 PM

IMO there is almost NO downside to an oversized tube....perhaps maybe a smidge in weight....and the "well...no the focal point is way out there now and you need a bigger diagonal"...

 

But that need not be an issue... where the focuser is you can engineer a recessed area for the focuser to get closer to the optical axis...

A low profile focuser would probably be good enough to compensate for the wider tube.


Edited by Galicapernistein, 20 January 2019 - 01:27 PM.


#114 spencerj

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Posted 20 January 2019 - 02:13 PM

I would be interested in observing with one of these scopes. To see if I felt the benefit was there or not. To me, right now, an 8” F8 in a 12” tube sounds a lot like dealing with a 12” Newtonian, but only getting 8” of aperture. Just doesn’t seem like a good deal to me.
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#115 Don H

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Posted 20 January 2019 - 02:33 PM

I am putting money down this week on an 11" Zambuto FL 63" (F 5.73) quartz mirror , which I should get next year some time.

 

My plan is to mount it in a 12" ID insulated tube as a split tube Dobsonian, with the lower connecting ring acting as a rotating ring, resting against PTFE pads on the cradle.

 

When putting it together for viewing it will have the option of inserting a 9" aperture diaphragm over the mirror to turn it into a 9" F/7 (of course I can also do a 6.3" F/10 if I like).

 

The purpose is to build the perfect "apo killer" planetary scope, which will be comfortable to use: no climbing up on things, rotating for best EP position. I am 70" tall and figure that 63" is about the maximum for comfortable viewing close to the zenith (since I am going to use it on other objects besides planets).

 

I determined the maximum convenient FL and then looked at how wide a tube I was willing to work with, to get the most Zambuto glass consistent with the other objectives.

 

Another factor was that I already have a 13.1" FL 57.4" Swayze mirror truss Dob (very nice), so it did not make sense to crowd that in aperture.

 

PS: An additional advantage of the split tube is that I can have two upper tubes with different secondaries, a 2.14" for F/5.73 mode and a 1.30" for F/7-F/10 planetary mode. I will make two upper tubes when I do the tube construction.

I think when you put the 9 inch mask on your 11 inch mirror, you will find that 11 inches will provide superior views and detail every time...



#116 Deep13

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Posted 20 January 2019 - 05:00 PM

I would be interested in observing with one of these scopes. To see if I felt the benefit was there or not. To me, right now, an 8” F8 in a 12” tube sounds a lot like dealing with a 12” Newtonian, but only getting 8” of aperture. Just doesn’t seem like a good deal to me.

If one is that worried about tube currents, an open design might be the better option.

#117 25585

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Posted 20 January 2019 - 06:38 PM

A 48 inch long scope will fit in all but the smallest cars.  It only takes a few inches and the scope will no longer fit in many cars .  The 6 inch F/8, the 8 inch F/6 and the 10 inch F/5 are all optimized for transporting to a dark site in a passenger car .  My 10 inch F/5 traveled thousands of miles around the back roads of the southwest . Many of those miles we in the backseat and trunk of a 1989 Nissan Sentra .

 

Jon

Having a 10 F5 I agree on the size mattering for solid tubes. But shorter faster seems to be used for truss scopes too.



#118 careysub

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Posted 20 January 2019 - 07:03 PM

I think when you put the 9 inch mask on your 11 inch mirror, you will find that 11 inches will provide superior views and detail every time...

I may very well indeed. One purpose of the project is to investigate this very issue.



#119 CollinofAlabama

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Posted 22 January 2019 - 01:44 AM

Gwlee,

I have the SkyWatcher 6. I measured the primary at 147.5mm, so 6”-ish is more accurate, but I’m rather happy with mine. I easily upgraded the horrible rack and pinion (even sold it here on CN Classifieds for $30 or so) to a nice GSO Crayford. I bought the Orion magnet counterweight, and it works like a top. No eyepieces too heavy (even tho I own mostly lightweights anyway for my refractors). This thing is amazingly easy to set up, check collimation (which stays put fairly well) and always has folks saying “that’s only a six incher?”

I had to drill two small holes to install the focuser. Didn’t even remove the secondary, just covered it in plastic; did remove the primary and measure it and blacken the edges with a sharpie (and installed some flocking paper around the primary and across from the secondary). But that was it. I used the top two original holes of the Synta r&p focuser. The GSO is longer, thus the two holes drilled on the bottom. C’est ça.

I glued a Rigel base near the focuser and use an Orion 6x30 RA visual finder. I also purchased an Agena 1.25” collimation cap and one of their Blue Fireball 2” extenders (already have a FarPoint secondary laser collimator). This thing is built for speed and the effort to make it so was genuinely minimal. It’s my favorite scope, and I just spent nearly a thousand bucks on a 110mm ED refactor I use less often frown.gif The SW6 is easily upgraded. Don’t let Orion’s XT 6 with its 1.25” focuser hole constrain you. Step up to 2” reality. It’s way better.

I caught most of the eclipse with the Explore Scientific 28mm 68°, but finished up with the Meade 25mm HD-60. That Meade in the SW6 even split Almach. Didn’t really expect it to, but 48x was enough for the SW6, even on a turbulent, windy West Texas night. And the color rendition was excellent, orange-yellow primary with the small blue secondary — beautiful! It was a tight split, but not a snowman, two stars. This baby’s a keeper. It has ease of use and portability written all over it, and although I can see coma in it, it’s really negligible at F/8. I see all kinds of coma in my 8” F/6, but it’s just not a big deal at F/8. The F/8 parabola throws up one nice image, and that’s a fact!

Edited by CollinofAlabama, 22 January 2019 - 03:34 AM.

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#120 Jeff Morgan

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Posted 22 January 2019 - 11:34 AM

IMO there is almost NO downside to an oversized tube....perhaps maybe a smidge in weight....and the "well...no the focal point is way out there now and you need a bigger diagonal"...

 

But that need not be an issue... where the focuser is you can engineer a recessed area for the focuser to get closer to the optical axis...

 

Yes, trading central obstruction for tube currents is a bad trade. I built my last scope using Daniel Mounsey's 1.5" air gap rule, and the results were noticeable. Much more so than fretting over a couple of percentage points of CO. Tube currents never sleep.

 

The bulk is a bit more, but if we are talking about laying a tube across the back seat it is length that matters. Diameter could be 6", 12", or 24" and still fit.

 

Regarding recessing the focuser, I would not get hopes too high on that one. That will of course vary by manufacturer, but I just took a look at my FeatherTouch LW and the space between the focuser knob and the base is maybe 1/4". I could not fit my little finger into the gap.

 

OTOH, this is a thread about long focus Newtonians, and one of the upsides is that skinny light cone has a lot of "throw". That is to say, getting the focal plane away from the longitudinal axis of the scope has a much lower opportunity cost than it would with a faster mirror.


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#121 25585

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Posted 22 January 2019 - 03:29 PM

With oversize tubes, a good idea in theory, that means needing primary cels with longer reach to your tube's inner walls, unless the appropriate cel can be found that can hold a smaller mirror.

 

Better maybe to use tube material and design rather than increasing dimensions. My very first was like the one below, which had a mirror inspection hatch, thath could be opened to help cooling etc.

 

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#122 Jeff Morgan

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Posted 22 January 2019 - 07:04 PM

With oversize tubes, a good idea in theory, that means needing primary cels with longer reach to your tube's inner walls, unless the appropriate cel can be found that can hold a smaller mirror.

 

Better maybe to use tube material and design rather than increasing dimensions. My very first was like the one below, which had a mirror inspection hatch, thath could be opened to help cooling etc.

 

I would not be comfortable with longer commonly used 1/4-20 bolts either.

 

What I did was make the tube stiffening ring interior to the tube so the cell "saw" a smaller ID.


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#123 TOMDEY

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Posted 22 January 2019 - 08:31 PM

I just re-realized that the many of us who already have Big Fat Dobs, can slap an off-axis stop-cap on and instantly see the effect of a long, slow, obstruction-free reflector. I actually had that on my old, trusty 17.5-inch Coulter. A 6-inch thru-hole stop right on the front, operating at F/13. The planetary images were wonderful. And I could spin the cap around, while looking thru the eyepiece, to find the best-performing part of the mirror/thermals.

 

Now I have a 36-inch, where I could do similar, with unobstructed stop-holes up to 14.5 inches!

 

Unobstructed               F#

   Aperture

 

    6-inch                      22

    8                             17

  10                             13

  12                             11

  14                             10

 

I know, I know...that's rather stupid. But, given the conditional that I am already there, in the observatory, using the 36... no harm in slapping on a cheap foam plate, spin it while looking... and see what I get!    Tom

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#124 Jeff B

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Posted 22 January 2019 - 11:28 PM

My 11" F7.2 resides in a 14" tube so I too comply with Dan's 1.5" rule of thumb. waytogo.gif    And I've no real complaints about tube currents.   So, another data point.  My 16% obstruction by diameter still gives a decent fully illuminated FOV too, another advantage of the slower FR.

 

As an aside, the OTA, built by Parallax, also has three very nice, tall pegs installed circumferentially about a foot back from the eyepiece.  They act as handles to position and rotate the tube, keeping my hands off of the bare metal tube.

 

Now my old AP pre-ED triplets have tube diameters that match that of the aperture and they do suffer from tube currents for a while.  



#125 CHASLX200

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Posted 23 January 2019 - 07:11 AM

Seems most tubes were 1.5 to 2" bigger in OD than the mirror.  Parks did make a tube for the 12.5" mirrors and also a 14.5" as it had a OD of 16".  But it seems Parks also changed the OD's of their tubes over the years.  My 10" Cave is about 12" OD on the tube.




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