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Long slow Newtonians

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#76 gwlee

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Posted 23 September 2018 - 10:07 PM

Have you considered letting someone else do it for you? There are both skilled ATMs that could do it, but there are also some professional telescope makers that are willing to build one-offs. New Moon Telescopes comes to mind, as an example. 

 

I have considered it, but haven’t found anyone that I am comfortable asking to do the work. Unless a person has built something very similar recently, its very difficult to predict the outcome with any certainty. It’s a crapshoot especially regarding mechanical aspects such weight, balance, smoothness, and stability (freedom from shakes), which are just as important to me as the optics. It’s easy to clearly and objectively specify a parameter, such as weight, but specifying such things as smoothness and stability is very difficult. 

 

On the other hand, I have methodically gathered and evaluated every component I need for a smaller premium refractor rig except the the OTA itself, and I have evaluated the mount, tripod, DSCs, and all the other accessories working together using an inexpensive Chinese OTA, so the final outcome is much more predictable than would be for a bespoke 6”f8 Dob.

 

At this point, I just need to select and aquire the OTA. If any component in the final system fails to please for any reason except for limited aperture, it’s easily replaced with another off-the-shelf component. 



#77 Starlease

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Posted 23 September 2018 - 10:22 PM

Buy any old 6 inch f8 scope.

Then get replacement mirrors from Edmund optics.

You can even get silver coatings on both mirrors.

Edmund does not make the f10 anymore.

The f8 brand new 40 year old mirror I got on eBay is optically great.

The ones they sell now for 500 dollars are probably even better.


Edited by Starlease, 23 September 2018 - 10:22 PM.

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#78 gwlee

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Posted 23 September 2018 - 11:08 PM

Buy any old 6 inch f8 scope.

Then get replacement mirrors from Edmund optics.

You can even get silver coatings on both mirrors.

Edmund does not make the f10 anymore.

The f8 brand new 40 year old mirror I got on eBay is optically great.

The ones they sell now for 500 dollars are probably even better.

I had two 6”f8 scopes in use here until a few weeks ago. I found they offered an excellent balance between portability and optical performance for my use at this site. The original optics were adequate, but the mechanical aspects, especially the focusers, were poor and not easily upgraded, so I let them go. 


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#79 25585

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Posted 24 September 2018 - 07:05 AM

Buy any old 6 inch f8 scope.

Then get replacement mirrors from Edmund optics.

You can even get silver coatings on both mirrors.

Edmund does not make the f10 anymore.

The f8 brand new 40 year old mirror I got on eBay is optically great.

The ones they sell now for 500 dollars are probably even better.

That would be my solution as well. Upgrading a focuser is easy enough and need not be too expensive these days.


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#80 gwlee

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Posted 24 September 2018 - 12:07 PM

That would be my solution as well. Upgrading a focuser is easy enough and need not be too expensive these days.

I have replaced focusers in other telescopes that I have owned, and I spent quite a bit of time carefully looking into focuser replacement options for these two 6”f8 scopes and didn’t find a solution that I wanted to persue.

 

Each approach required modifying the OTA, adding a significant amount of weight to the top end OTA, which required adding additional weight to bottom end of the OTA to rebalance the scope, or other more complex rebalancing options that would have turned this into a full blown ATM project. 

 

The light weight of these 6”f8 scopes (34#) has proven to be essential for the incessant tree dodging required on this observing site. Replacing the focuser and doing it right, would result in them weighing almost as much as the 8”f6 scope that already own, and the focuser was only one of the mechanical problems with these scopes. I decided it made more sense for me to just use them as is or let them go to a new home, so I sold them both. 

 

For my use on this site, I still believe a 6”f8 Dob provides the most suitable balance between aperture for performance and light weight for portability, but I want a more refined 6”f8 scope than those that can be purchased today, and I don’t want to build it from scratch myself. 



#81 barbie

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Posted 24 September 2018 - 02:50 PM

I don't see why you should have any problems with the commercial 6" dobs!!  The focuser on mine works perfectly and the optics are fantastic.  Perhaps you are asking a little too much of a 6"f8 dob!!  My instrument provides razor sharp images at high magnification on the planets, splits close doubles, and has enough light grasp to show me all the Messiers and brighter NGC's.   It does all of these things CONSISTENTLY  well, every time I use it!!grin.gif

You seem to want a telescope that does everything well and still be portable enough to use at your site but you already have had them and keep selling them!!  I don't think you are going to find anything better than a 6"F8 Dob from Orion or Skywatcher unless you build one yourself or have an ATM'er do it for you. FWIW, I've had two mirrors from Edmund and both were pretty lousy compared to what I have in my Orion XT6.  They BOTH failed numerous times to split doubles that should have been easy to split with a 6" aperture and didn't have the snap to focus that my XT6 has. The focuser on my XT6 is very smooth with no play or backlash and works perfectly for my intended use so no complaints there.  Collimation is easy and it cools very efficiently.


Edited by barbie, 24 September 2018 - 03:01 PM.

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#82 gwlee

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Posted 24 September 2018 - 05:38 PM

I don't see why you should have any problems with the commercial 6" dobs!!  The focuser on mine works perfectly and the optics are fantastic.  Perhaps you are asking a little too much of a 6"f8 dob!!  My instrument provides razor sharp images at high magnification on the planets, splits close doubles, and has enough light grasp to show me all the Messiers and brighter NGC's.   It does all of these things CONSISTENTLY  well, every time I use it!!grin.gif

You seem to want a telescope that does everything well and still be portable enough to use at your site but you already have had them and keep selling them!!  I don't think you are going to find anything better than a 6"F8 Dob from Orion or Skywatcher unless you build one yourself or have an ATM'er do it for you. FWIW, I've had two mirrors from Edmund and both were pretty lousy compared to what I have in my Orion XT6.  They BOTH failed numerous times to split doubles that should have been easy to split with a 6" aperture and didn't have the snap to focus that my XT6 has. The focuser on my XT6 is very smooth with no play or backlash and works perfectly for my intended use so no complaints there.  Collimation is easy and it cools very efficiently.

Barbie,

 

Here’s how I see it:

 

i have owned four XT6 scopes over about 30 years. All had very good optics by my standards, but none had what I consider to be a good focuser, and I find the mounts a bit too shaky to be entirely satisfactory to me. I am glad that yours satisfies you though. 

 

I usually prefer to observe from home and just own and use one telescope for everything. The XT6 strikes an excellent balance between aperture and portability for my use at some sites. At these sites, I use the XT6.  At other sites with different opportunities and challenges, other telescopes work better for me, so I use other telescopes at these sites, which explains why my scopes come and go as my home address changes.

 

This is one of those sites for which a 6”f8 Dob is particularly well suited, and I have been using a 6”f8 Dob as my primary scope here for the last two years. However, none of the readily available 6”f8 scopes on the market, including mine, offer the degree of mechanical refinement that I want these days, and I don’t want to build a scope myself, so I sold my last remaining 6”f8 Dob to try using two complimentary scopes: an 8” Dob that offers more performance than a 6” scope, but less portability, and a small refractor that offers more portability than a 6” Dob, but much less performance. 

 

Both of these rigs are inexpensive, both are more refined than my 6”f8, and both are easily upgraded or replaced with a more refined scope if I want to do so. If this experiment doesn’t work after giving it a fair trial, it will be easy to find another 6”f8 Dob to replace the one I just sold. 

 

Gary


Edited by gwlee, 24 September 2018 - 09:28 PM.

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#83 Jon Isaacs

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Posted 24 September 2018 - 05:49 PM

Gary:

 

I agree the standard Orion/Synta rack and pinion focuser leaves something to be desired. It can be worked over and improved but it still could be smoother and lighter.

The 1.25 inch GSO Crayford seems like it would have been a reasonable upgrade and could have been made to fit without too much effort.

https://agenaastro.c...ngle-speed.html

 

jon



#84 gwlee

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Posted 24 September 2018 - 08:01 PM

Gary:

 

I agree the standard Orion/Synta rack and pinion focuser leaves something to be desired. It can be worked over and improved but it still could be smoother and lighter.

The 1.25 inch GSO Crayford seems like it would have been a reasonable upgrade and could have been made to fit without too much effort.

https://agenaastro.c...ngle-speed.html

 

jon

Jon,

 

I had two XT6s here for a while, so over the last two years, I put all the best components into the scope I preferred using. Then, I reworked the best focuser to make it lighter, smoother, and elimate backlash. I used some of the techniques that you developed and posted along with some of my own techniques. When I was done, it worked much better, it was serviceable, but not great, and it took frequent tuneups to keep it working at its best. 

 

I looked at replacing it with a 1.25” GSO or other 1.25” focusers, which wouldn’t have required as much OTA surgery, but I really wanted a higher quality 2.0” focuser with compression bands for securing both 1.25” and 2.0” eyepieces if I was going to do it.

 

Moonlight and FeatherTouch both have focusers with the quality and features I wanted, but both were much heavier, which would have required adding significant weight to the other end of the OTA to rebalance it, and light weight is the principle advantage of this scope to me. Neither was a drop in replacement, so required OTA surgery I didn’t want to undertake.

 

In contrast, both Moonlight and FT offer replacement focusers that just bolt onto the XT8 that I own without requiring any modifications to the OTA, allowing me to reinstall the stock focuser in the stock OTA If I ever want to move the ML/FT to another scope and sell the XT8. 

 

Given all these factors, I decided the XT6 was best used as is or replaced with a better candidate for up grading, such as the XT8, which I own and has a Baltic Birch base than Ken Fiscus built for me. 

 

6”f8 Dobs are about the longest focal ratio newts still in mass production. For my purposes, the XT6 seems to be the best of the bunch. It’s stock optics are pretty good, which along with its light weight are its greastest strengths, but it has some mechanical worts that are not as easily fixed as they are on slightly larger and faster Dobs that better supported with aftermarket parts.


Edited by gwlee, 24 September 2018 - 08:51 PM.

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#85 joe nastasi

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Posted 25 September 2018 - 10:57 AM

If Someone is considering building a large, long focus Newtonian for planetary work, then I would say go for it. Jupiter and Saturn are going to be quite low in declination for the next 5 years, so you won't need a ladder at all to reach the eyepiece.


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#86 CHASLX200

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Posted 25 September 2018 - 06:08 PM

If Someone is considering building a large, long focus Newtonian for planetary work, then I would say go for it. Jupiter and Saturn are going to be quite low in declination for the next 5 years, so you won't need a ladder at all to reach the eyepiece.

They are gonna be in the murk for a long time. At least Jupiter moves pretty fast compared to slow Saturn.



#87 barbie

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Posted 25 September 2018 - 09:57 PM

Despite their low altitudes, I've had some of my best Jupiter and Saturn observations in over 3 decades through my 6"f8 Newtonian and 4" F9 apo.


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#88 daquad

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Posted 26 September 2018 - 06:58 PM

If Someone is considering building a large, long focus Newtonian for planetary work, then I would say go for it. Jupiter and Saturn are going to be quite low in declination for the next 5 years, so you won't need a ladder at all to reach the eyepiece.

Not for those that live down under.


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#89 Pinbout

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Posted 26 September 2018 - 07:36 PM

one of my best views of Jupiter...in my 8in f3.5...5/8" thk plate glass primary

 

under a street light.

 

gallery_106859_4364_36856.jpg


Edited by Pinbout, 26 September 2018 - 07:36 PM.

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#90 LFORLEESEE

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Posted 26 September 2018 - 10:00 PM

Despite their low altitudes, I've had some of my best Jupiter and Saturn observations in over 3 decades through my 6"f8 Newtonian and 4" F9 apo.

Same here barbie,   This is my view, we are 300 metres above sea level with a clear view to the horizon and 10 kms to the Coral Sea. 

 

This is looking east and I can be seated and then standing flat footed for 2 hours after planet rise with my 20 inch Obsession and then it is

 

one step at a time as they rise overhead. 

 

I will be standing flat footed with my 20 foot long Herschel in a few months time. smile.gif

 

Just started the mirror box yesterday with 25mm black form ply.

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#91 25585

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Posted 29 September 2018 - 11:22 AM

one of my best views of Jupiter...in my 8in f3.5...5/8" thk plate glass primary

 

under a street light.

 

gallery_106859_4364_36856.jpg

That is a real cutie!


Edited by 25585, 29 September 2018 - 11:22 AM.

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#92 Pinbout

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Posted 29 September 2018 - 01:19 PM

And it’s sold



#93 25585

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Posted 29 September 2018 - 05:18 PM

If you got money http://www.scopecity...truments PI320 


Edited by 25585, 29 September 2018 - 05:19 PM.


#94 CHASLX200

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Posted 29 September 2018 - 06:52 PM

That hurts my body just looking at it.


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#95 junomike

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Posted 29 September 2018 - 09:06 PM

And a Time Machine as ScopeCity has been gone for close to ten years (maybe 9).


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#96 Astrojensen

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Posted 30 September 2018 - 02:35 AM

And a Time Machine as ScopeCity has been gone for close to ten years (maybe 9).

But Parrallax still exists and doing fine (as far as I know), so they could probably still build you one, if you so desired. 

 

 

Clear skies!
Thomas, Denmark


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#97 Jon Isaacs

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Posted 30 September 2018 - 03:00 AM

And a Time Machine as ScopeCity has been gone for close to ten years (maybe 9).

 

And scopes like the one in the photo are a big reason why Scope City did go out of business.  When I moved to our present house in San Diego in 1999, the Scope City was a 10 minute walk from the house.  Their showroom was stocked with old school Newtonians on Gems and acromatic refractors.   No one was buying those big old monsters, people were buying Dobs but they didn't have any.  

 

When the closed down, I think they still had those same scopes in the showroom.  

 

Jon



#98 CHASLX200

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Posted 30 September 2018 - 05:47 AM

And scopes like the one in the photo are a big reason why Scope City did go out of business.  When I moved to our present house in San Diego in 1999, the Scope City was a 10 minute walk from the house.  Their showroom was stocked with old school Newtonians on Gems and acromatic refractors.   No one was buying those big old monsters, people were buying Dobs but they didn't have any.  

 

When the closed down, I think they still had those same scopes in the showroom.  

 

Jon

But no matter how you cut it, a good old school 8" F/8 Newt is the best scope going. Just the 1.5" shaft mounts from all the makers were just total garbage in my book.  Slap that 8" F/8 on a AP 1200 and you are as good as gold.


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#99 gwlee

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Posted 30 September 2018 - 11:26 AM

But no matter how you cut it, a good old school 8" F/8 Newt is the best scope going. Just the 1.5" shaft mounts from all the makers were just total garbage in my book.  Slap that 8" F/8 on a AP 1200 and you are as good as gold.

Don’t think there would be many takers.


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#100 CHASLX200

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Posted 30 September 2018 - 12:39 PM

Don’t think there would be many takers.

I could never pay for a AP1200 at what they cost unless i found a old non GO-TO version for like 3.5k and that is as high as i would ever go.  But the older OTA's are pretty cheap


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