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

Long slow Newtonians

equipment mirror making observing planet reflector
  • Please log in to reply
186 replies to this topic

#151 CHASLX200

CHASLX200

    Hubble

  • *****
  • Posts: 17377
  • Joined: 29 Sep 2007
  • Loc: Tampa area Florida

Posted 05 February 2019 - 07:49 PM

This RV6 from 1960 is just a killer.

20j21qg.jpg


Edited by CHASLX200, 05 February 2019 - 07:49 PM.

  • Deep13, Illinois, PXR-5 and 3 others like this

#152 Illinois

Illinois

    Gemini

  • *****
  • Posts: 3015
  • Joined: 18 Dec 2006
  • Loc: near Dixon, Illinois USA

Posted 06 February 2019 - 08:04 AM

CHASLX200..... Wow! Long slow finder! lol
  • Galicapernistein and happylimpet like this

#153 Galicapernistein

Galicapernistein

    Viking 1

  • *****
  • Posts: 522
  • Joined: 24 Sep 2007

Posted 06 February 2019 - 08:31 AM

Here's my 8" f8 on a dob mount.  I've gotten some not so nice comments on the classics forum for putting it on a dob mount. It's so much better than the inadequate gem it came with.  This is my largest aperture telescope. It doesn't fit in any vehicle I own but not because of the OTA, because of the mount. I have to transport it in my dirt bike cargo trailer. 

A long slow newt is an ideal scope for a dob mount. The wider arc the scope moves through makes it much easier to put objects at the edge of the field for manual tracking, instead of the delicate nudging shorter scopes can require. If anybody has a problem with a slow newt on a dob mount, they can take it up with Archimedes.


  • Astrojensen, happylimpet, 25585 and 1 other like this

#154 25585

25585

    Gemini

  • *****
  • topic starter
  • Posts: 3405
  • Joined: 29 Aug 2017
  • Loc: In a valley, in the UK.

Posted 06 February 2019 - 09:02 AM

A long slow newt is an ideal scope for a dob mount. The wider arc the scope moves through makes it much easier to put objects at the edge of the field for manual tracking, instead of the delicate nudging shorter scopes can require. If anybody has a problem with a slow newt on a dob mount, they can take it up with Archimedes.

Yup. My F6 10 is easier to use than the F5. And I am less likely to kick or trip over its stand. A F7 10 , or F6 12 would be perfect.


  • Galicapernistein and Earthbound1 like this

#155 CollinofAlabama

CollinofAlabama

    Mercury-Atlas

  • *****
  • Posts: 2798
  • Joined: 24 Nov 2003
  • Loc: Lubbock, Texas, USA

Posted 06 February 2019 - 06:25 PM

Unfortunately, there aren’t any options for non-mass produced slow dobs above 6” anymore, that I know of.

 

Well, that's not entirely true.  There is the 8" F/7 Discovery.  Now, it costs twice as much as anybody's 8" F/6, but it might have half as much coma?  Haven't used an F/7 parabolic mirror, myself, so I have no idea what coma would be like in this guy.  And even though twice as much money isn't fun, it's still pretty inexpensive compared to other custom dob options I've seen on the web.  It's definitely got me interested.

 

I notice coma in my 8" F/6 in the outer 1/3 of the field, when using a widefield eyepiece.  I suppose it's in the smaller fields, too, but I try and keep things centered when I'm at high power, anyway, so that's a little less of a concern.  If I were a big-time lunar or solar observer, that would be unacceptable, but since my views of both are usually pretty wide field-ish, and not too high powered, it's again pretty easy to keep them inside the inner 2/3's of the Field of View.  BTW, I notice coma even in my SW 6" F/8, but it's just not bothersome.  There it really is a small annoyance.  F/7?  That is literally the thousand dollar question.



#156 CHASLX200

CHASLX200

    Hubble

  • *****
  • Posts: 17377
  • Joined: 29 Sep 2007
  • Loc: Tampa area Florida

Posted 06 February 2019 - 06:46 PM

CHASLX200..... Wow! Long slow finder! lol

It is long for a 6x30.



#157 CHASLX200

CHASLX200

    Hubble

  • *****
  • Posts: 17377
  • Joined: 29 Sep 2007
  • Loc: Tampa area Florida

Posted 06 February 2019 - 06:48 PM

Well, that's not entirely true.  There is the 8" F/7 Discovery.  Now, it costs twice as much as anybody's 8" F/6, but it might have half as much coma?  Haven't used an F/7 parabolic mirror, myself, so I have no idea what coma would be like in this guy.  And even though twice as much money isn't fun, it's still pretty inexpensive compared to other custom dob options I've seen on the web.  It's definitely got me interested.

 

I notice coma in my 8" F/6 in the outer 1/3 of the field, when using a widefield eyepiece.  I suppose it's in the smaller fields, too, but I try and keep things centered when I'm at high power, anyway, so that's a little less of a concern.  If I were a big-time lunar or solar observer, that would be unacceptable, but since my views of both are usually pretty wide field-ish, and not too high powered, it's again pretty easy to keep them inside the inner 2/3's of the Field of View.  BTW, I notice coma even in my SW 6" F/8, but it's just not bothersome.  There it really is a small annoyance.  F/7?  That is literally the thousand dollar question.

I hate any coma and at F/7 i am fine with no Paracorr. Once you get faster than F/6 it starts to bother me. Other people will vary on Coma.


  • CollinofAlabama likes this

#158 izar187

izar187

    Soyuz

  • -----
  • Posts: 3597
  • Joined: 02 Sep 2006
  • Loc: 43N

Posted 07 February 2019 - 12:37 AM

Well, that's not entirely true.  There is the 8" F/7 Discovery.  Now, it costs twice as much as anybody's 8" F/6, but it might have half as much coma?  Haven't used an F/7 parabolic mirror, myself, so I have no idea what coma would be like in this guy.  And even though twice as much money isn't fun, it's still pretty inexpensive compared to other custom dob options I've seen on the web.  It's definitely got me interested.

 

I notice coma in my 8" F/6 in the outer 1/3 of the field, when using a widefield eyepiece.  I suppose it's in the smaller fields, too, but I try and keep things centered when I'm at high power, anyway, so that's a little less of a concern.  If I were a big-time lunar or solar observer, that would be unacceptable, but since my views of both are usually pretty wide field-ish, and not too high powered, it's again pretty easy to keep them inside the inner 2/3's of the Field of View.  BTW, I notice coma even in my SW 6" F/8, but it's just not bothersome.  There it really is a small annoyance.  F/7?  That is literally the thousand dollar question.

It's a beauty, with lots of things going for it.

 

But it can be built from parts for less.

Though perhaps not that pretty purple and birch.



#159 careysub

careysub

    Mercury-Atlas

  • *****
  • Posts: 2500
  • Joined: 18 Feb 2011
  • Loc: Rancho Cucamonga, CA

Posted 07 February 2019 - 10:40 AM

I’ve been using my “Orchid”, a 10” f-8 since I built it back in 1969. This is its last days as a f-8 though. In a few weeks ZOC will be finishing a new 10” f6.4 (gasp) primary for it. I have always loved long slow dobs in fact I was tutored by the man himself who thought anything below f-6 was well...a pig. Don’t get me wrong though, the fast scopes have theirs own attractions and strong points. I just like to keep the flame going for the slow ones....if f 6.4 can still be considered slow....

You are thinking of a scope (but for the tube arrangement) very similar to the one I mentioned upthread, wherein I just placed an order for an 11" FL=63" ZOC. But you are making me think now that maybe I pushed the aperture up a little too high, and I should go with a 10" instead.

 

I mentioned that I intended to arrange for an aperture mask to push it to higher F ratios (9" F/7), but maybe the base configuration is a bit too low F-wise, and a 10" FL=63" would be better. Aperture appetite makes one tend toward gluttony. Not having compared F/5.73 and F/6.3 scopes I can't say how much difference there might be.

 

BTW, I have dropped the idea of using end-grain balsa for the split tube after thinking through the problems of connecting rings and such like with the relatively fragile core. Instead I will make using plywood core and carbon fiber, either two layers of 3mm lite ply or baltic birch.



#160 Tyson M

Tyson M

    Gemini

  • *****
  • Posts: 3101
  • Joined: 22 Jan 2015
  • Loc: 53 degrees North

Posted 07 February 2019 - 07:59 PM

For backyard viewing I can see the application of these scopes. 

I just LOVE the looks of the old Claves and mounts.  I simply cannot see these and their mounts being remotely transportable with their extra long tubes, unless in a minivan with no seats in the back at a minimum. 

Those mounts almost look worse to move then the OTA that attach to them....



#161 csrlice12

csrlice12

    ISS

  • *****
  • Posts: 23821
  • Joined: 22 May 2012
  • Loc: Denver, CO

Posted 07 February 2019 - 08:31 PM

Knew a physicist in Albuquerque (Sandia Labs)who had built a 12" f10 dob.  This was in the late 80s/early 90s.  Saw him at another friends funeral about a year ago, alzheimers has taken its toll but he remembered the scope....the scope now resides with his son.


Edited by csrlice12, 07 February 2019 - 08:32 PM.


#162 Starlease

Starlease

    Vostok 1

  • -----
  • Posts: 140
  • Joined: 11 Aug 2018
  • Loc: Rocky mountains.

Posted 07 February 2019 - 09:18 PM

Focal length does not really matter for planetary. Have an Antares 8 inch f5 with refigured mirror to 1\14 wave that beats the heck out of my old 8 inch f7 claimed 1\8 wave Optical Mechanics scope. Quality of the main mirror is everything.

 

When Mars came by spring 2014 the 8 inch f5 was showing the exact same views as my recently acquired APM 6 inch f8 double. Did not keep the APM very long.


Edited by Starlease, 07 February 2019 - 09:27 PM.

  • CollinofAlabama and happylimpet like this

#163 CHASLX200

CHASLX200

    Hubble

  • *****
  • Posts: 17377
  • Joined: 29 Sep 2007
  • Loc: Tampa area Florida

Posted 08 February 2019 - 06:18 AM

Focal length does not really matter for planetary. Have an Antares 8 inch f5 with refigured mirror to 1\14 wave that beats the heck out of my old 8 inch f7 claimed 1\8 wave Optical Mechanics scope. Quality of the main mirror is everything.

 

When Mars came by spring 2014 the 8 inch f5 was showing the exact same views as my recently acquired APM 6 inch f8 double. Did not keep the APM very long.

True. My best planet views came with F/5 or faster Zambuto and OMI optics in the 11 to 18" range.


Edited by CHASLX200, 08 February 2019 - 06:18 AM.


#164 CollinofAlabama

CollinofAlabama

    Mercury-Atlas

  • *****
  • Posts: 2798
  • Joined: 24 Nov 2003
  • Loc: Lubbock, Texas, USA

Posted 08 February 2019 - 12:58 PM

True. My best planet views came with F/5 or faster Zambuto and OMI optics in the 11 to 18" range.

 

And that's my experience, too -- that a well made newtonian will beat anyone's refractor 2" or less of aperture.  Heck, I had the same experience with far from "prime" optics.  About 13-odd years ago, a friend of mine, God rest his soul, had one of the original Orion 120mm ED scopes, the one that came with the single-speed Synta Crayford, and its image of Saturn simply couldn't keep up with my Meade LightBridge 8" dob (GSO).  So 4.7" of unobstructed Chinese doublet lens couldn't match the Taiwanese parabolic mirror's 8" optical capacity.  People who claim otherwise, I guess the dob owners either had particularly bad mirrors (possible) or simply didn't know how to collimate them (likely), or the mechanicals in their scopes can't hold collimation well (as likely).  I can't explain refractor people with their APM's and the like, when most reasonably well made 8" dobs (and especially anything larger) will clean them up on the planets, or most anything else.  We've got a guy in town with an XT10i that has an exceptionally good mirror, keeping up with custom scopes, and that thing shows the planets, not to mention DSO's, better than anyone's sub observatory sized refractor.

 

But it's not just the figure of the mirror, it's the mechanicals to maintain collimation, and the maintenance will and ability of the scope owner to keep and maintain his scope in top functioning order.  Reflectors do require all three of these things, excellent optics, good mechanicals, and will and skill of the owner.  Refractors only require good mounts smile.gif -- of course they require good lenses, too, but they seem to (mostly) come with pretty decent ones these days.  It's goes without saying that achromats have other issues, but with ED scopes, it's usually true that they function quite close to their capacity.  Given the three variables for reflectors, many out in the field are not up to par, giving people the impression of the superior refractor.


  • happylimpet, Stephen Kennedy and LFORLEESEE like this

#165 Stephen Kennedy

Stephen Kennedy

    Surveyor 1

  • *****
  • Posts: 1639
  • Joined: 03 Jul 2014
  • Loc: California

Posted 08 February 2019 - 06:22 PM

ColinofAlabama makes some excellent points.  A good Newtonian can perform as well or better than a refractor or any other type of telescope of the same aperture and since you normally get considerably more aperture with a Newtonian it is the optimal choice for a telescope.  I have been using my Mikage 210 mm F/7.7 Newtonian since I purchased it in 1988 while stationed in Japan.  Those of us who use Newtonians understand and accept that they require more effort than some other types of telescopes to keep them performing at their best.  However, it pays of with excellent results whether it is being used for visual or astrophotography.  


  • CollinofAlabama and 25585 like this

#166 Don H

Don H

    Viking 1

  • -----
  • Posts: 904
  • Joined: 28 Dec 2006
  • Loc: Desert SW

Posted 08 February 2019 - 06:49 PM

You are thinking of a scope (but for the tube arrangement) very similar to the one I mentioned upthread, wherein I just placed an order for an 11" FL=63" ZOC. But you are making me think now that maybe I pushed the aperture up a little too high, and I should go with a 10" instead...

 

I think your ZOC 11 inch mirror will be incredible for viewing pretty much everything. The f/5.7  focal ratio should provide a nice blend of fov and mag. When you are looking at things in the future, you will not wonder how much you might be missing if you went to 10 inch instead. FWIW, I have built 10 inch f/6.3, 11 inch f/4.5 and f/6, and 12.5 inch f/5.9 (among other larger and smaller mirrors/scopes). 11 is a great size, nowhere near the bulk of a 12.5, and not a whole lot more than a 10. And 11 adds a bit extra to globs, galaxies, planetaries, galaxies and details on planets. You could always bump the FL up to f/6, as 66 is still an excellent height for your eyepiece and it might be a compromise on your doubts between f/6.3 and f/5.7. But 5.7 may give you just a bit wider fov for things like M81/82, double cluster, M31/32/110, Veil, and many other neat 2 for one images...



#167 CHASLX200

CHASLX200

    Hubble

  • *****
  • Posts: 17377
  • Joined: 29 Sep 2007
  • Loc: Tampa area Florida

Posted 08 February 2019 - 07:15 PM

Just got this 8" F/6 Meade today.  Not super slow or fast since it is F/6. Needs some collimation work, but the moon looked pretty good a hour ago.

 

168tw7p.jpg


  • Deep13, Bomber Bob, happylimpet and 2 others like this

#168 25585

25585

    Gemini

  • *****
  • topic starter
  • Posts: 3405
  • Joined: 29 Aug 2017
  • Loc: In a valley, in the UK.

Posted 08 February 2019 - 10:30 PM

So as a sort of counterpoint, when a same-aperture Newt gets to the same FL and FR as a Cassegrain, who would stick with Newts, who would switch to Cassegrains? 


Edited by 25585, 08 February 2019 - 10:32 PM.


#169 Stephen Kennedy

Stephen Kennedy

    Surveyor 1

  • *****
  • Posts: 1639
  • Joined: 03 Jul 2014
  • Loc: California

Posted 08 February 2019 - 10:59 PM

In that scenario the Cassegrain would have a much shorter OTA making it easier to handle and able to be put on a less costly mount. You could also observe sitting down in a normal chair.
  • 25585 likes this

#170 25585

25585

    Gemini

  • *****
  • topic starter
  • Posts: 3405
  • Joined: 29 Aug 2017
  • Loc: In a valley, in the UK.

Posted 09 February 2019 - 09:19 PM

In that scenario the Cassegrain would have a much shorter OTA making it easier to handle and able to be put on a less costly mount. You could also observe sitting down in a normal chair.

Yes, I think there is a type switch point. An 8 inch F9 is about tallest Newt, if not F8.

 

Then it's a C8. Even sooner for larger apertures , over 10 F7, 11 F6 etc.  I am sure such long Newts would be better optically though.



#171 Jeff Morgan

Jeff Morgan

    Voyager 1

  • *****
  • Posts: 10548
  • Joined: 28 Sep 2003
  • Loc: Prescott, AZ

Posted 10 February 2019 - 11:03 AM

In that scenario the Cassegrain would have a much shorter OTA making it easier to handle and able to be put on a less costly mount. You could also observe sitting down in a normal chair.

 

I know that today's Chinese GEMs are not very expensive, but don't discount the weight and hassle factor of a GEM. Ballpark an extra 50 pounds for tripod, head, counterweights - and they can get much heavier. The head and the tripod generally can't be (easily) moved together. Extra trips. Then there is tube balancing and polar alignment. Not rocket science, but it is extra tasks and time.

 

In contrast, a 8" Dob, even a long focus one, is a self-contained package. One trip. Plop and go. (That stays true even into the larger apertures when wheelbarrow handles are employed.) If you use DSCs, even the alignment part is faster and easier than the GEM counterpart.

 

I am not saying one is better than the other - pluses and minuses to each. But the Dob's self-contained nature and absence of cantilevered loads make it a much easier logistical proposition.


  • Astrojensen likes this

#172 dusty99

dusty99

    Mariner 2

  • *****
  • Posts: 210
  • Joined: 13 Jun 2009

Posted 10 February 2019 - 12:47 PM

Not to spin this thread off course, but how would a contemporary mass-produced 6" f/8 Newt (almost all Dobs) compare to an RV6 or other, more hand-made (as much as they were) classic Newt?  

 

I wish this was available in the US: https://www.harrison...-telescope.html (although it might be a bit wobbly on that mount)


Edited by dusty99, 10 February 2019 - 12:47 PM.


#173 Astrojensen

Astrojensen

    Voyager 1

  • *****
  • Posts: 11697
  • Joined: 05 Oct 2008
  • Loc: Bornholm, Denmark

Posted 10 February 2019 - 01:20 PM

Not to spin this thread off course, but how would a contemporary mass-produced 6" f/8 Newt (almost all Dobs) compare to an RV6 or other, more hand-made (as much as they were) classic Newt?  

 

I wish this was available in the US: https://www.harrison...-telescope.html (although it might be a bit wobbly on that mount)

I have a Sky-Watcher 6" f/8 newtonian and I must say, it's not bad at all, in fact it's very good, but I've never looked through a RV-6 or a Cave or any of those, so I don't know how it compares. It must be pretty close, though, because my S-W is easily better than diffraction limited and shows a wealth of details on Jupiter and the Moon in good seeing. 

 

And yes, the EQ-3-2 is completely inadequate. I bought mine used as OTA only. The 1.25" focuser, while quite good, has been replaced with a 2" crayford.

 

gallery_55742_4249_83221.jpg

 

 

Clear skies!
Thomas, Denmark


  • Deep13, siriusandthepup, SandyHouTex and 3 others like this

#174 Astrojensen

Astrojensen

    Voyager 1

  • *****
  • Posts: 11697
  • Joined: 05 Oct 2008
  • Loc: Bornholm, Denmark

Posted 12 February 2019 - 04:14 PM

Inspired by this thread, I took the 6" f/8 out this evening, to look at the Moon and compare it with my Altair 102/1100mm ED. I intentionally only gave it as long cooldown time as it took to set up the EQ-6 mount and get ready for observing, about fifteen minutes or so. Using a magnification of around 140x (not exactly sure, since I was using a Baader 2.6x GPC in the 2" nosepiece of my Zeiss binoviewer and 25mm Zeiss eyepieces), the image was quite surprisingly good. There was a fair amount of tube currents/boundary layer seeing, but it frequently allowed superbly crisp views behind the blur. Meanwhile, the ED was razor sharp, showing that the seeing itself was fairly good. After about half an hour, perhaps less, the 6" seemed to have cooled down and gave very good views, although never as steady as the 4" ED, but, again, frequently gave razor sharp views that left the four times more expensive ED standing in the dust. The 50% better resolution was VERY noticeable, as was the brighter image. At 82x (using a Baader 1.7x coma correcting GPC and 25mm Zeiss eyepieces), the image was just about as steady and crisp as in the ED at more or less the same magnification. 

 

On the other hand, the ED gives a sharp image MUCH sooner after being taken outside and is actually quicker to set up. This makes it the superior grab and go scope and will probably remain my preferred "weekday evening scope". It is also vastly superior for solar observing. 

 

But if you can live with the slightly longer cooldown time and slightly more unsteady images, the $200 6" f/8 Sky-Watcher has a LOT to offer. 

 

 

Clear skies!
Thomas, Denmark


Edited by Astrojensen, 12 February 2019 - 04:16 PM.


#175 CHASLX200

CHASLX200

    Hubble

  • *****
  • Posts: 17377
  • Joined: 29 Sep 2007
  • Loc: Tampa area Florida

Posted 12 February 2019 - 06:59 PM

I have a Sky-Watcher 6" f/8 newtonian and I must say, it's not bad at all, in fact it's very good, but I've never looked through a RV-6 or a Cave or any of those, so I don't know how it compares. It must be pretty close, though, because my S-W is easily better than diffraction limited and shows a wealth of details on Jupiter and the Moon in good seeing. 

 

And yes, the EQ-3-2 is completely inadequate. I bought mine used as OTA only. The 1.25" focuser, while quite good, has been replaced with a 2" crayford.

 

gallery_55742_4249_83221.jpg

 

 

Clear skies!
Thomas, Denmark

Stubby looks good with the short legs to keep out of the way of the OTA.

 

10" is the biggest i want to deal with in a Newt. I have a 10,8 and 6" Newts and the 8" OTA is less than half the weight of the 10" F/6 Cave.  12.5" and up are just too big to grab and mount up by yourself.  If it were not for size and weight then a 16" F/8 to around 20" F/6 would be by dream planet Newt.


  • Astrojensen likes this


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





Also tagged with one or more of these keywords: equipment, mirror making, observing, planet, reflector



Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics