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Retirement Refractor?

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#51 drollere

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Posted 22 August 2014 - 08:29 PM

i don't understand why you can't wander with an equatorial mount ... you might one day wish for equatorial, and there it will be, ready to do your bidding.



#52 BillP

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Posted 22 August 2014 - 08:57 PM

I hate to disappoint but the diagonal prisms still have an internal reflection. 

 

 

Not off a mirrored surface...just polished glass...no aluminum...no silver...no gold...no dielectric phase reflective scatter rich yuk....just pure and simple glass, so I am not disappointed in the slightest  :grin:


Edited by BillP, 22 August 2014 - 09:00 PM.


#53 Bakes

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Posted 22 August 2014 - 09:08 PM

My first real telescope was a Criterion RV-6. So I am somewhat nostalgic for 6" F8 optics(reflector or refractor).

 

I'd get Bill's Lunt 152ED mounted on a Half Hitch FTX. Not sure if I would get the DSCs. I don't really need them except to identify the occasional "find" when I am casually scanning the Milky Way regions. There would be a 2" Herschel wedge and binoviewer setup. I'd keep my Ethoses for mono viewing.

 

If I had the resources for an observatory in dark sky country I'd go with an Admiral Smyth setup. A D&G 6" F15 on an equatorial mount. Or maybe even a custom build Skylight 6" F15 to get that classical ambiance.



#54 Peter Natscher

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Posted 22 August 2014 - 10:11 PM

 

I hate to disappoint but the diagonal prisms still have an internal reflection. 

 

 

Not off a mirrored surface...just polished glass...no aluminum...no silver...no gold...no dielectric phase reflective scatter rich yuk....just pure and simple glass, so I am not disappointed in the slightest  :grin:

 

Prism diagonals do produce aberrations.  Read this excerp from Roland Christen from the Astro-Physics User Group talking about the then new Baader Mark V Binoviewer and its prism:

 

Astro-Physics User Group

The Glasspath corrector is used to correct the prism color and spherical aberrations when you wish to have minimal focal length increase for wide field as well as high power observing. It is the only unit that actually corrects for the glass path error introduced by the prisms when they are used with short focal ratios and higher magnification eyepieces where the prism errors would be more pronounced. However, adding any kind of Barlow element of 1.75x or higher will essentially eliminate the errors introduced by the prisms because the focal ratio that the prisms now see will be very long. No glass-path correction is needed. There is no real difference in the final result between any of the dedicated Baader screw-in Barlows and the Barcon or Baader FFC Barlow. There are no optical advantages to any of these schemes. Pick your method. ...

 

... Recommended eyepieces (of Mark V Binoviewer). I have no particular eyepiece recommendations as to type, except to say that the longer focal length eyepieces are going to give you the best view. To explain a bit further, if you want high power, it is better to produce this before the prism set than afterwards. The rewards are many, including longer eyerelief, less sensitivity to tiny miscollimations and much lower amounts of optical aberrations due to the long prism path itself. If you have a fast scope, F7, F6, F5 or even shorter, it makes no sense to use 5mm, 4mm or 3mm eyepieces to get up to planetary powers. You will magnify not only all the prism aberrations, but also any image offsets that might be in the eyepiece barrels or the prisms themselves. You may have trouble merging the image. Even if you could adjust the collimation precisely by moving the eyecups
around, the next time you insert the eyepieces, they won't be in the exact same orientation and all your collimation efforts will be for naught.

My rule of thumb for any Binoviewer is to use focal lengths no shorter than 10mm, preferably 12mm for high power viewing. Insert a Barlow element ahead of the Bino to achieve a long focal length. This will narrow the beam considerably over prime focus, with the result that the active area for each object beam will occupy only a tiny portion of the prism faces. This will really cut down on aberrations in the optical path. This applies to every Binoviewer out there. While the Baader prism are top notch, and better in my opinion than any others, they are not perfect. They will, however produce a perfect Airy disc when used with a Barlow ahead of the optical path. If you are looking for that last 2% for those subtle Mars features, think long eyepiece, long F-ratio, and you will dazzle them.

Roland Christen



#55 Peter Natscher

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Posted 22 August 2014 - 11:17 PM

 

I hate to disappoint but the diagonal prisms still have an internal reflection. 

 

 

Not off a mirrored surface...just polished glass...no aluminum...no silver...no gold...no dielectric phase reflective scatter rich yuk....just pure and simple glass, so I am not disappointed in the slightest  :grin:

 

After more reading on this subject, I now see where Bill is coming from.  Read:

 

http://www.cloudynig...omparison-r2877

 

°  Simulations and seasoned observational experience show a prism can be preferred as a diagonal in moderate focal ratio telescopes:  Indeed. a prism will add its own aberration (overcorrected spherical and color), but until you raytrace a system with a prism (I have – with ZEMAX), your just guessing at the aberrational residuals. I own a 2" multi-coated prism (Badder Planetarium) and at f/9, it works superbly. With 99% transmission, and the lack of light scatter, this diagonal beats any mirror diagonal I've ever used. Of course, a well made prism will cost more, but until interference coated supersmooth diagonals are available, the high quality prism for moderate to long focal length refractors will reign supreme. …  In an achromatic or apochromatic lens, the variations of spherochromatic aberration and secondary and tertiary color make the use of a prism very interesting indeed. Raytracing in ZEMAX -- various APO designs with 25mm and 50mm prisms with f/ratio's of f/6 to f/12 -- shows that the OPD spherical levels for different wavelengths change in different amounts, sometimes improving a system at some wavelength at the cost of others. The ONLY way to know what is really happening is to know the exact design, raytrace the total system, or star test the system, and check to see if the contrast is higher with the prism, or the mirror diagonal. But even with these spherical and color variations, the aberration levels do get excessive at around f/7 or f/8. (Ref: groups.google on 12/1996, Thomas Back, Subj: Prism Diagonals Pros & Cons)
°  Simulations support the observations that at f/8, any color or spherical aberration that might be induced by a prism should be non-detectable:  If you do use a prism, the small 1.25" won't make a difference that you can see or measure. The 2" will introduce a very small amount of color and spherical, but again it probably won't be visible under normal circumstances. I don't think it will hurt planetary performance at all.  I just did a simulation in ATMOS lens design on a perfectly corrected lens. The spherical correction with 50mm of prism inserted into the optical path changes by only 1/40 wave. The color correction changes only minutely to the point where you cannot see any difference at all in the focused star image.  I did the simulation at F8.  (Ref: Astromart Forums on 2/2005, Roland Christen, Subj: Prism vs mirror diagonal in APQ's - Msg: 306947, 306950, 307121)
°  Refractive surfaces produce less scatter than reflective surfaces given a same surface smoothness: “A surface irregularity on a refracting surface produces a much smaller wavefront error than on a reflecting surface, by something like a factor of six [countered to be a factor of four]. Thus unless reflecting surfaces are extremely well polished, and have coatings that are extremely regular, they will scatter more light than refracting surfaces.” (Ref: yahoogroups on 3/2002, Subj: Prism Diagonals, Msg: 12138)



#56 BillP

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Posted 23 August 2014 - 11:33 AM

I have no doubt that they do produce aberrations at some levels, just as mirrored surfaces definitely do as well.  Interestingly in the cooldown process, I found that the AP Maxbright produced astigmatism on-axis until it was fully thermally acclimated (about 15 minutes) - prisms showed no such behavior.  And of course other stress factors in the construction can result in permanent aberrations.  Every optical element in the telescope produces unwanted issues at some level. 

 

The excerpt on the binoviewer is interesting.  Also interesting is that I actually have never tried short focal length eyepieces in a binoviewer!  I've always settled on using long focal length EPs and using various Barlows to achieve magnifications because easier to get a bunch of Barlows than doubles of eyepieces.  So now about all I use in the binoviewer is a pair of Meade RG 20mm Widefields.  The view they put up is astoundingly sharp.  Barlows range from 1.6x GPC to TV 2x to BCO 2.2x to APM 2.7x, all of which of course produce more magnification than their stated value due to the longer path thru the bino.  So guess I fell into the better way by accident :lol:

 

But regardless of all these posts on the technical reasons, in the end I let the observation be the ruling factor.  So if observation goes against someone's theory...I don't particularly care :p  as all it means is that the theoretical model someone is using is either wrong, or is leaving out critical other factors that are not modeled and therefore make their model incomplete (probably 100% of what is generally offered falls into this category).

 

Thanks for digging up those various posts...certainly interesting!

 

 

 

I hate to disappoint but the diagonal prisms still have an internal reflection. 

 

 

Not off a mirrored surface...just polished glass...no aluminum...no silver...no gold...no dielectric phase reflective scatter rich yuk....just pure and simple glass, so I am not disappointed in the slightest  :grin:

 

Prism diagonals do produce aberrations.  ...


Edited by BillP, 23 August 2014 - 11:36 AM.


#57 Moonglum

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Posted 23 August 2014 - 12:51 PM

Bill you are lucky to have obtained two Meade RG widefields with fieldstops the same size, same location.  This is not a given even with currently produced orthos...

 

Kirk



#58 BillP

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Posted 23 August 2014 - 01:18 PM

Nope...they are slightly off...I just seat one a little higher...not much.



#59 Scott99

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Posted 23 August 2014 - 03:33 PM

I thought RC's statements about prism-introduced aberrations refered to the beam splitter prism in the BV, not the right-angle one in the diagonal.  

 

I used to have a Japanese-made 2" prism diagonal from University Optics, I sold it when the AP Maxbright came out.  I put my trust into RC and figured he would not use the Maxbright design unless it was the best possible diagonal. 

 

However, I miss the way the UO prism diagonal sealed the tube, it seemed to get less dirty and I could clean it without taking it apart.  

 

I see that the current Zeiss 2-inch prism is lighter than Maxbright and also has shorter light path.  It should be good for binoviewing.  I'd love to try one.  *edit - actually the Zeiss prism light path is 110mm which is similar to AP Maxbright.


Edited by Scott99, 23 August 2014 - 04:06 PM.


#60 AustinAstronomer

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Posted 23 August 2014 - 07:30 PM

From whom can I purchase a new Zeiss 2" prism diagonal?

 

Thank you.

 

AA


Edited by AustinAstronomer, 23 August 2014 - 07:31 PM.


#61 tomcody

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Posted 23 August 2014 - 07:49 PM

From whom can I purchase a new Zeiss 2" prism diagonal?

 

Thank you.

 

AA

Apine astro.com part # PRISM-2



#62 pdxmoon

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Posted 23 August 2014 - 09:50 PM

Mine would be a classic Unitron 160 4". The problem is, I wouldn't be able to lift it for very long after retirement!

 

As a practical matter, I guess I already have the upper limits of a retirement scope. I've always wanted that 4" Skywatcher, and it's light enough that the Polaris could probably handle it, so I'd probably buy that.

 

But I already have the AT72 waiting for those days when I don't have the umph for more. Plus, my many classic 60-76mm scopes that are ready with rings to mount on the Porta II. And a Unitron 3" 140, which is a big scope.

 

Finally--I have probably one of the best Lunar scopes for a retired guy--that C102 that I bought two of last year for $59 each. Amazing views, very lightweight.

 

Now that I think of it--I really would like a Unitron 114. Superior engineering, and I'd be able to move it around for a good long while--I hope!

 

I just want to retire! (5 years or so...)


Edited by pdxmoon, 23 August 2014 - 09:53 PM.


#63 george tatsis

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Posted 24 August 2014 - 08:26 AM

The refractors found in my backyard equipment signature are, and will be my retirement scopes. They can all be used with the Takahashi Teegul without a hitch, stored in the limited available space I have, and above all, they deliver what they were designed for .  :)  :)  :)

 

George


Edited by george tatsis, 24 August 2014 - 08:27 AM.


#64 opticsguy

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Posted 24 August 2014 - 08:55 AM

I chose my retirement scope almost 20 years ago and still happy with my choice.  An 8" f/13 D&G on Dob/Berry styled mounting. Portability is always relative for each individual and the 8" is portable but easier with 2 persons.  I just designed up a new jig for lifting the OTA into place and plan to build this design sometime in winter.  My 5" D@G is a really nice very portable OTA and will I be buried with this one.  Eyepieces are 41 pan and the 9, 14, 20 ES 100's and a few spares.

 

The 10" f/13 I just built is not portable without a lot of set up and take down time. Definately a 2 person set up. Again, the jig I designed for lifting the 8" can be used on this scope, however 2 persons needed to carry.  Ultimately, having an observatory with everything ready to use would be the best.



#65 Peter Besenbruch

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Posted 26 August 2014 - 04:28 PM

Retirement is a few years off, but not that many. I have not planned my retirement scope, but the trend of my last scope purchases is "interesting." My oldest telescope is a put together 10", f7 Newtonian, undersized secondary, Losmandy GM-100. Wonderful instrument. I have not seen better planetary views in any other scope of any other size.

 

I eventually sold the van I used for transporting the beast, and bought a 6", f6 Parks tube assembly with a wonderful mirror. I put it on a Great Polaris mount with a wooden, short legged tripod. Eventually, I replaced the GP mount when the hand controller blew out yet again. I got a CG-5.  Sadly, it raised the eyepiece height a bit too high, and the scopes lower end argued with the tripod over the same spot in the space-time continuum. That meant re-aligning the mount every time they argued.

 

Eventually, I purchased the Celestron/Skywatcher 4.7" ED refractor when it went on sale for $1400. Shipping was free, and my credit card gave me triple points on the purchase. A mount extension eliminated the tripod leg collisions.

 

That's a drop from 10" to 4.7" in 15-20 years. It's also quite the climb down in light gathering and resolution. Still, my back pain went away when I started using the 6". Observing is downright comfortable. I view through the 4.7" seated.

 

What will the future hold? I posted a not so stellar review on the site I purchased the refractor from, and got a very nice reply from someone who said I should have bought a Takahashi. He even suggested a very nice, 2.4" inch model at roughly the same price I paid for the 4.7" Hmmm... If I reach 85, I may be down to a 30mm monocular. If so, I'll be sure to come back to Cloudy Nights to ask how to mount the thing. ;)



#66 t.r.

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Posted 27 August 2014 - 09:19 AM

 An 8" f/13 D&G on Dob/Berry styled mounting

 

Any chance we can see a pic of this retirement setup, Opticsguy?!?!

 

At one time, I too had targeted a D&G 8" f/12 as my "retirement scope". Now Istar is on the scene offering big glass with "improved" color correction in less time as an option too. But, me thinks I will be happiest with a TEC180FL as "The One" and last. In achromats I think I need a minimum CA ratio of 2 to 2.5.  Barry of D&G has said in the past that visually, there isn't anything really different to see between f/12 and f/15 anyway.  We'll see what I end up with.


Edited by t.r., 27 August 2014 - 09:28 AM.


#67 The Ardent

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Posted 27 August 2014 - 09:44 AM

Dear OpticsGuy,

Wanna trade? Custom D&G  5.375",  f/22 only weighs 20lbs. APO-like

 

Attached Files


Edited by The Ardent, 27 August 2014 - 09:45 AM.


#68 jrbarnett

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Posted 27 August 2014 - 11:49 AM

For me retirement will be more about whole system and location than it will any single scope.

 

I'd like either an observatory, or at least a covered permanent pier, with a higher capacity mount in or on it (likely a used AP900 GTO or similar).

 

That opens the door for use of different telescopes for different purposes.  I want DSOs one week?  Put a C14 OTA on the mount and have out it.  I want to image the next week?  Swap the C14 off in favor of the TEC 140 or similar.

 

To me a permanent set up goes hand in hand with retirement in that you have a scope always ready to go at a moment's notice, and also the option of not being married to a single scope for all targets.

 

Regards,

 

Jim



#69 BillP

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Posted 27 August 2014 - 11:55 AM

... and also the option of not being married to a single scope for all targets.

 

 

So...you're not a one scope guy :lol:



#70 jrbarnett

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Posted 27 August 2014 - 02:10 PM

 

... and also the option of not being married to a single scope for all targets.

 

 

So...you're not a one scope guy :lol:

 

I would be a one scope guy if I could find one single scope that did all of the things that I like to do in all of the places that I like to do them, well.  But I'm kind of thinking that the "One Scope" doesn't exist, or if it does, it would be prohibitively expensive (i.e., like an f/12-ish, 12" to 14" well-mounted and domed triplet).

 

Regards,

 

Jim



#71 jrbarnett

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Posted 27 August 2014 - 02:24 PM

Of course, there's another strategy for retirement in keeping with the "simplify your affairs" mantra.

 

That is to embrace the fact that for most of us there is no "One Scope" that does absolutely everything we like to do well, and instead adopt the conciliatory, compromise tack - choose a "One Scope" that doesn't do anything at top-of-the-class levels, but instead does pretty much everything in an "ok to good" fashion.

 

Enter the Schmidt-Cassegrain, a Jack of All Trades, Master of None design, that is relatively affordable per unit of aperture, readily available from dealer stock, easily and affordably serviced or replaced in the event of loss, easy to mount on a tracking system at only moderate cost, and manageable in apertures much larger than the largest manageable refractor apertures.  A 9.25" SCT will give a 180mm refractor a run for its money on many classes of targets, and with a suitable mount included, cost less than the focuser, tube rings, finder and diagonal on a 180mm APO.

 

An every-man scope has some appeal, particularly once you're on a relatively static and reduced income and expense program.

 

:thinking:

 

- Jim



#72 madcity

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Posted 27 August 2014 - 02:37 PM

For me retirement will be more about whole system and location than it will any single scope.

 

I'd like either an observatory, or at least a covered permanent pier, with a higher capacity mount in or on it (likely a used AP900 GTO or similar).

 

That opens the door for use of different telescopes for different purposes.  I want DSOs one week?  Put a C14 OTA on the mount and have out it.  I want to image the next week?  Swap the C14 off in favor of the TEC 140 or similar.

 

To me a permanent set up goes hand in hand with retirement in that you have a scope always ready to go at a moment's notice, and also the option of not being married to a single scope for all targets.

 

Regards,

 

Jim

 

I like the general principle, but have minor quibbles with the details  ;-)).

 

Having said that ... This would be my close-to-ideal retirement setup ... except all those beautiful mother-nature-provided trees in our neighborhood. All I see is a relatively narrow E/W slice spanning maybe 90 degrees. Not bad for the moon, but not much else.

 

Short of selling the house and moving (ain't gonna happen!) or cutting the trees (ditto!), that environment forces me to be (almost) totally portable. Thus the Mach1 mount for photography and alt/az for visual, and limiting myself to <=5" scopes (5" is on order). (The orange tube C8 is a now-rarely-used 35-year-old scope.)

 

For me, identifying suitable outdoor, publicly-late-night-open-and-accessible, easily-drivable, dark-ish observing sites (other than our club site) becomes important.



#73 jrbarnett

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Posted 27 August 2014 - 02:45 PM



Mine would be a classic Unitron 160 4". The problem is, I wouldn't be able to lift it for very long after retirement!

 

As a practical matter, I guess I already have the upper limits of a retirement scope. I've always wanted that 4" Skywatcher, and it's light enough that the Polaris could probably handle it, so I'd probably buy that.

 

But I already have the AT72 waiting for those days when I don't have the umph for more. Plus, my many classic 60-76mm scopes that are ready with rings to mount on the Porta II. And a Unitron 3" 140, which is a big scope.

 

Finally--I have probably one of the best Lunar scopes for a retired guy--that C102 that I bought two of last year for $59 each. Amazing views, very lightweight.

 

Now that I think of it--I really would like a Unitron 114. Superior engineering, and I'd be able to move it around for a good long while--I hope!

 

I just want to retire! (5 years or so...)

Thom, you'd be welcome to my Antares 105/1500 when you're ready for one.  She's easier to mount than a Unitron and has better optics than either of the 4" Unitrons I've used.  You'd just need to figure out how to get her to Portland from Petaluma, as I wouldn't want to ship her.  I use it primarily for outreach (the scope looks like a telescope should look in the eyes and minds of the public) on a CG5-GT mount atop and Antares Pier.  Basically a "you haul it" deal.  It's in great shape and performs flawlessly.

 

jimscope4.jpg

 

Regards,

 

Jim


Edited by jrbarnett, 27 August 2014 - 03:05 PM.


#74 jrbarnett

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Posted 27 August 2014 - 02:48 PM

Dear OpticsGuy,

Wanna trade? Custom D&G  5.375",  f/22 only weighs 20lbs. APO-like

Uh...no.  Just no.  ::shakes head vigorously::

 

I'd never want to look at any target that low on the horizon, and I'd not want to lay on my belly in the grass to look at targets above 2x airmass, which is where I try and do all of my observing.

 

Been there, done that.  Never again for this cowboy.

 

- Jim



#75 BillP

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Posted 27 August 2014 - 03:49 PM

  

That is to embrace the fact that for most of us there is no "One Scope" that does absolutely everything we like to do well, and instead adopt the conciliatory, compromise tack - choose a "One Scope" that doesn't do anything at top-of-the-class levels, but instead does pretty much everything in an "ok to good" fashion.

 

 

I've settle on my retirement trio...

 

6" Lunt 152 ED-APO

...then when can no longer lift that recommission my TSA-102

....then when I can no longer ligt that recommissionmy Onyx 80mm APO

.....then when I can no longer lift that recommission the bottle of Scotch

......then when I can no longer lift that go to the six-foot hotel  :lol:


Edited by BillP, 27 August 2014 - 03:50 PM.







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