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Ideal planet scope.

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#76 Deep13

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Posted 28 December 2018 - 05:28 PM

Here it is.

8 in f8 1.jpg


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#77 Deep13

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Posted 28 December 2018 - 05:32 PM

8 in f8 2.jpg

 

It is most definitely NOT under-mounted. That mount weighs a metric shot tonne (the same as 2200 lbs. or 1000 static kg of lead shot, or anything else). Seriously, it makes an EQ6 look like a cheap toy.



#78 Deep13

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Posted 28 December 2018 - 05:36 PM

8 in f8 4.jpg

 

I'm going to have to replace the focuser with something lower and move the mirror back a couple of inches. As it is, the secondary is just a tiny bit too small.



#79 Deep13

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Posted 28 December 2018 - 05:39 PM

8 in f8 5.jpg

 

These castors are all right for the driveway, but I'll need a cart thing with bigger wheels to get it to the back yard where I have a clear view of the south.


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#80 Deep13

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Posted 28 December 2018 - 05:51 PM

So, needed upgrades:

--mirrors need to be recoated

--new focuser

--mirror cell pulled back

--fan installed

--cart built

--a bit of flocking opposite focuser

--replace plastic focuser on guide scope (eventually)

-- X-hair EP for guide scope

 

I can attend to most of this when the mirrors are at the coater.

 

I'm going to use the guide scope to find Uranus since I'm not going to shoot photos and f/8 isn't suited for it anyway. Also, no "go to," so I have to find it visually.



#81 PaulEK

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Posted 28 December 2018 - 07:50 PM

So, needed upgrades:

--mirrors need to be recoated

--new focuser

--mirror cell pulled back

--fan installed

--cart built

--a bit of flocking opposite focuser

--replace plastic focuser on guide scope (eventually)

-- X-hair EP for guide scope

 

I can attend to most of this when the mirrors are at the coater.

 

I'm going to use the guide scope to find Uranus since I'm not going to shoot photos and f/8 isn't suited for it anyway. Also, no "go to," so I have to find it visually.

I'm glad you got the scope home safely. If I had kept it, I would have done all the things you have listed here. You could also add lots of accessories to that monster mount, up to and including a 12.5-inch f/6 guide scope!

 

A note on fans: At our public observatory, we have a small house fan on a tripod. We use it 'afocally': it sits behind the 14-inch reflector to do initial cooling of the primary, without actually touching the scope in any way. The scope also has a much smaller, quieter 12-volt fan blowing the rest of the time. If the temperature is falling quickly, I keep the big fan going toward the back of the scope.


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#82 Deep13

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Posted 29 December 2018 - 04:17 PM

I'm thinking a T shape made of 2x4 lumber (48mm by 96mm, pre-milling). Each end of the T can have a 6" castor. I can put the EQ mount on it and pull it with a rope.

#83 Deep13

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Posted 29 December 2018 - 07:14 PM

I'll have to make like a Z with right angles on the ends so the mount doesn't need to be an extra 10 inches (24 cm) higher than it is now. Need an unobtrusive way to keep the joints non-flexible.

#84 Deep13

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Posted 06 January 2019 - 01:15 AM

Sent to mirror off to the coaters and ordered a new secondary.



#85 Deep13

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Posted 07 January 2019 - 06:48 PM

Antares secondary coming and the Chinese super deluxe focuser. Will paint tube interior tonight and finish the cart for the mount. I'm going to keep the scope in an unheated garage. Does it really need a fan?

#86 ltha

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Posted 11 January 2019 - 09:20 AM

Many of you may have read this thread, but if not it is very informative:

https://www.cloudyni...r-vs-reflector/

In my own search for the “best planetary scope” I bought and sold over 100 telescopes of most every design. As I would pick a winner, I would find a new challenger and do yet another side-by-side comp. Having the scopes under identical seeing conditions and owning dual sets of Pentax SMC orthos, and now Zeiss Abbe Orthos (4-34mm) helped keep the comps as fair as possible. While I read the theoretical differences I personally prefer seeing the images.

Many of my best views have been through Newtonians. To me the 8” Newtonian is the unsung hero of backyard astronomy. To this day my most memorable view of Mars was through a 10” Portaball. As Jon said, seeing is the key and I happened to hit a night of near perfect seeing with the Portaball. After that experience I bought a larger Portaball because I agree there is no substitute for aperture IF the optics are very good to excellent and supported by seeing. At the same time I was climbing the aperture ladder with refractors which topped out at a D&G 8” f/12 for achromats, and my current TEC200ED though I did get a chance to view through Al George’s 15” D&G along the way.

Over the years I found I prefer refractors. Please do not read that as stating they are better because I have had simply stunning views through Newtonians, Dall-Kirkhams and Maks. But ergonomics factor into my preference too and I prefer being seated with a binoviewer to standing on a ladder. For the last two months I have been comparing the TEC to a a 1960s Cave 12 3/4” Newtonian with arguably the finest mirror Cave ever produced, and Quartz to boot. When the seeing permits, and the big Newt is cooled to near ambient it clearly beats the TEC. Deeper color saturation and finer detail on Jupiter and Saturn. But I am up a ladder three steps at zenith and have forgotten that fact once or twice....

While aperture - with supporting seeing - wins, people who have never used big scopes do not see the downside. Big scopes are heavy, at some point exceed one-person set up, and demand correspondingly large mounts and, preferably, permanent installation.

My most used scope this last year has been a Takahashi FC-125. Why? Superb images, easy set up, and it matches seeing consistently. At the moment I do not have a 8” Newtonian, but if I did it would be right in there too, especially one of f/7 - f/8 focal length. I am going to build an observatory this year and will permanently mount the TEC with a smaller refractor piggybacked so I can cover all seeing. And the 18” Starmaster will be the deep sky partner.
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#87 Deep13

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Posted 11 January 2019 - 02:46 PM

Well, that's all good information. 100 scopes? Wow, this really is an affluent person's hobby. No complaints with refractors except they're big for their size, expensive, and generally smaller aperture than Newts. For practical reasons, I wouldn't want to go much bigger than 8" for an EQ Newt. I have a 12.5" Dob as my dark site scope. My favorite all around scope is an 8" f/6 Dob from Discovery. Gives really sharp views. I think the mirror was made by elves.
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#88 ltha

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Posted 11 January 2019 - 04:57 PM

Hi Deep13,

 

With the advent of CloudyNights and Astromart buying and selling scopes became pretty easy. When I was a kid the Criterion RV-6, Unitrons, and various Japanese refractors were far beyond my means, but as an adult, at least before the interest in classic scopes hit, you could buy them at very reasonable prices. I would buy a scope or two, use them, compare them and either sell/trade for a different scope or give one away to friends whose children had become interested in astronomy. Nearly two dozen scopes became gifts to friends and families. The rest came and went as I figured out which I liked best.

 

The Discovery 8” DOB was one I used and loved. Great optics, easy to use and an absolute blast. It was given to friends whose boys developed an interest in the night sky and kept at their mountain cabin. Still in use today. 

 

I know many astro people who have bought and sold dozens of scope and I doubt any of them see themselves as affluent. And I have run into extremely wealthy amateurs, but one need not be affluent to enjoy the hobby. A few visits to star parties will allow anyone interested the chance to look through a wide variety of scopes. And there are so many scopes today that offer high performance at very reasonable prices. To me the 8” Newtonian is the best of that group. Years back I bought a Cave 8” f/8 Model B and it was simply excellent optically. Paid something like $900 for it complete with the original mount. In many ways I wish I had never sold it!

 

larry


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#89 Deep13

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Posted 14 January 2019 - 12:36 AM

Here's the carriage I made for the Meade RG mount. The mount is too big to carry around the yard, so I made this wheeled platform. I removed the leveling screws and castors and used the hole for the castors to attach the mount to the carriage. The wheels are each 8 inch (20 cm) pneumatic castors. The front one is on a swivel and needs extra space to swing around.

 

scope cart 1.jpg

 


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#90 Deep13

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Posted 14 January 2019 - 12:50 AM

scope cart2.jpg



#91 Deep13

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Posted 14 January 2019 - 12:58 AM

The biggest problem was maintaining rigidity at the corners around the wheels. I wanted the 8" (20 cm) diameter wheels to be able to move over unpaved ground, but I did not want to raise the whole platform the 10 inches (25 cm) of the castors.

 

scope cart 3.jpg


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#92 Deep13

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Posted 14 January 2019 - 01:03 AM

scope cart 4.jpg

The end of each leg on the pier is 36 inches (91 cm) from the ends of each of the other legs.



#93 bobhen

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Posted 14 January 2019 - 08:47 AM

Many of you may have read this thread, but if not it is very informative:

https://www.cloudyni...r-vs-reflector/

In my own search for the “best planetary scope” I bought and sold over 100 telescopes of most every design. As I would pick a winner, I would find a new challenger and do yet another side-by-side comp. Having the scopes under identical seeing conditions and owning dual sets of Pentax SMC orthos, and now Zeiss Abbe Orthos (4-34mm) helped keep the comps as fair as possible. While I read the theoretical differences I personally prefer seeing the images.

Many of my best views have been through Newtonians. To me the 8” Newtonian is the unsung hero of backyard astronomy. To this day my most memorable view of Mars was through a 10” Portaball. As Jon said, seeing is the key and I happened to hit a night of near perfect seeing with the Portaball. After that experience I bought a larger Portaball because I agree there is no substitute for aperture IF the optics are very good to excellent and supported by seeing. At the same time I was climbing the aperture ladder with refractors which topped out at a D&G 8” f/12 for achromats, and my current TEC200ED though I did get a chance to view through Al George’s 15” D&G along the way.

Over the years I found I prefer refractors. Please do not read that as stating they are better because I have had simply stunning views through Newtonians, Dall-Kirkhams and Maks. But ergonomics factor into my preference too and I prefer being seated with a binoviewer to standing on a ladder. For the last two months I have been comparing the TEC to a a 1960s Cave 12 3/4” Newtonian with arguably the finest mirror Cave ever produced, and Quartz to boot. When the seeing permits, and the big Newt is cooled to near ambient it clearly beats the TEC. Deeper color saturation and finer detail on Jupiter and Saturn. But I am up a ladder three steps at zenith and have forgotten that fact once or twice....

While aperture - with supporting seeing - wins, people who have never used big scopes do not see the downside. Big scopes are heavy, at some point exceed one-person set up, and demand correspondingly large mounts and, preferably, permanent installation.

My most used scope this last year has been a Takahashi FC-125. Why? Superb images, easy set up, and it matches seeing consistently. At the moment I do not have a 8” Newtonian, but if I did it would be right in there too, especially one of f/7 - f/8 focal length. I am going to build an observatory this year and will permanently mount the TEC with a smaller refractor piggybacked so I can cover all seeing. And the 18” Starmaster will be the deep sky partner.

I agree.

 

I live in PA just outside of Philadelphia in the land of cloudy nights, fast moving cold and warm fronts with the planets rarely getting high in the sky and with the jet stream nearby or right overhead – ouch. Mediocre seeing is the rule.

 

In the 40-years I have observed from this or near this location…

 

My best view of Saturn was with my C-11 at 600x. The rings showed groves like a phonograph record.

 

My best view of Jupiter was with my 15” F5 Newtonian with a Galaxy mirror. The detail was etched from pole to pole.

 

My best view of Mars was with my Astro-Physics 152 F9 triplet refractor. The details were hard-edged and “photographic”. To this day I marvel at the drawing I did.

 

The C-11 never came close to delivering to that power on Saturn again and on most nights the highest power used on Saturn with the C-11 was 225 to around 300, give or take. The views were always softer in the C-11 than the Newtonian or refractor.

 

The 15” reflector delivered those best Jupiter views on back-to-back nights with the dew dripping off the telescope. On the planets the 15” did better than the C-11 but on most nights thermal acclimation, size and the seeing at this location worked against the big Newtonian.

 

The AP 152 refractor was (and refractors remain my go-to scopes for planetary viewing from “my location”) less “fussy” and on the vast majority of nights it just delivered better views (sharper, steadier) in my mediocre conditions than did the larger Newtonian and C-11. The Newtonian would “tease” but its much greater potential just couldn’t be realized. If I lived in Florida, then a 10” or larger mirror scope would be my scope of choice.

 

So yes in near perfect conditions any scope worth its salt will or should deliver the goods. But I need a scope that can come on-line quickly, stay on-line as conditions/temperatures change and can deal with mediocre seeing. For “my” needs at “my” location, high quality refractors (for the reasons listed) have been my planetary scopes of choice.

 

I currently have a Tak TSA 120 and a Mewlon 210 and the above still applies.

 

Bob


Edited by bobhen, 14 January 2019 - 08:50 AM.

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#94 Deep13

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Posted 14 January 2019 - 09:57 PM

By the forgoing logic, I should just stick with my 5" f/12 refractor with D&G lenses. It's a really nice DIY scope, but it's aperture limited. I can really blow up Saturn, but it's dim. I think 8" (Newt) is just the right size.
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#95 Deep13

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Posted 14 January 2019 - 10:01 PM

Oh, I got my secondary and my fan with battery holders today. I should get the focuser tomorrow.

#96 bobhen

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Posted 15 January 2019 - 08:08 AM

By the forgoing logic, I should just stick with my 5" f/12 refractor with D&G lenses. It's a really nice DIY scope, but it's aperture limited. I can really blow up Saturn, but it's dim. I think 8" (Newt) is just the right size.

With the seeing, weather and temperatures in Ohio I think a 6 or 7-inch refractor would be the scope of choice on 99-percent of nights. Of course, an apo is expensive and an F15 achromat is long and unwieldy. So for ergonomic and budgetary considerations, I think an 8-inch Newtonian “optimized” for planetary observation is an excellent choice. Just don’t sell that 5” D & G!

 

Bob


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#97 Deep13

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Posted 15 January 2019 - 04:23 PM

With the seeing, weather and temperatures in Ohio I think a 6 or 7-inch refractor would be the scope of choice on 99-percent of nights. Of course, an apo is expensive and an F15 achromat is long and unwieldy. So for ergonomic and budgetary considerations, I think an 8-inch Newtonian “optimized” for planetary observation is an excellent choice. Just don’t sell that 5” D & G!

Bob


Sorry, but I think I will. The justification for the 8" f/8 is that it will replace the 5" and the C9.25. The 5" rides on an EQ6 that I don't really need for anything else. I'll get rid of the SCT first, since I've never really liked it. I'll still have a TV101 if I really want a refractor view.

The 5" refractor is in a 6" aluminum tube with flocking and baffles. It has an odd brass R&P focuser with a section of 2" copper tubing. So it's heavy for its size and the EQ6 can just handle it.
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#98 Deep13

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Posted 15 January 2019 - 09:19 PM

Okay, my focuser arrived. It's one of these:

https://www.telescop...AiABEgJ4MPD_BwE

It's rather heavy.


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#99 dhferguson

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Posted 16 January 2019 - 02:13 AM

Cheers,

 

Let us consider seeing, optics, mount stability, eye relief and exit pupil, and comfort:

 

(1) seeing. Seeing is too often the limiting factor. The "best" planetary scope will have an Airy disk small enough that the view is dominated by seeing. In mathematical terms, the system (optics+seeing) Airy disk FWHM ("full width at half maximum for the central peak) will be ~ < 110% of the optics Airy disk, or the Airy disk FWHM s/b about x0.45 your best-case seeing or less. Now when we talk seeing in visual terms, we talk about seeing over the time scale of the persistence of the eye, about 1/15th of a second. If the best seeing you generally encounter is about 1 arcsec, you need an Airy disk FWHM ~ lambda/D ~0.4 arcsec (8" aperture), and so on, where lambda is the wavelength (0.55um is about right) and D is the mirror (yes, mirror!) diameter. This will set the optimum size for your optics. In general, for the vast majority of us, this would be an aperture of about 8"-16",

 

(2) the way to think about optics quality for the planets is by examining the optics system modulation transfer function, or MTF. The MTF is the Fourier transform of the optics point spread function. Graphed, the abscissa  is spatial frequency (the inverse of resolution). That is, zero spatial frequency, 0 lines/mm, is a resolution of infinity while high spatial frequency represents very fine resolution. The ordinate is contrast, which is always a value of between 1 and zero. At zero spatial frequency, the contrast is unity for any optical system, so the curve starts at unity in the upper-left hand corner. At very high spatial frequency, the contrast asymptotically approaches zero. Contrast of features for extended sources such as planets (as opposed to point sources like stars) can then be determined by examining the equivalent spatial frequency for that feature.

 

Now for a perfect, unobstructed Airy disk, there is a curve descending from unity at zero spatial frequency to zero as you move to the right along the abscissa. All optical systems are imperfect to some degree, however, and thus the curve for your system will lie slightly below the "perfect" MTF curve for most spatial frequencies. Now to get more quantitative: the MTF decrease for a circular central obstruction of 15% is hardly noticeable, at 20% it is noticeable but small, and decreases rapidly from there with increasing central obscuration. Few observers would notice much difference between the 15% and the 20% obscuration but, for a planet killer, that should be the limit. Note that this is slightly larger than the secondary size due to the slightly larger diameter secondary holder. For example, my 10" f/6/6 telescope utilizes a 1.83" minor axis (which installed at a 45 deg angle corresponds to a circular obscuration of the primary) but the secondary holder is in fact 0.193" in diameter. Thus, my telescope meets the 20% criterion. Could I instead install the next smaller 1.52" "standard" size secondary for even smaller obscuration? Sure, but I would be vignetting significantly at the field edge for powers lower than about x180. I also like to look at DSOs, so this is would not be a good trade for me.

 

What about geometrical aberrations, such as spherical aberration and coma? These can rapidly drop the MTF curve. This is why it is important to have very good optics, and I mean a total system peak-to-valley wavefront error of less than 1/4 lambda (again, 0.55 um is a good visual wavelength average). The secondary mirror will contribute too, of course, as would any corrector plate. Note that, due to the tilt, the secondary mirror aberrations of a Newtonian can be reduced by 1/SQRT(2). To get the total system peak-to-valley wavefront error, RSS the optical component errors. For example, suppose the primary is 1/8 wave at the (HeNe laser) interferometric wavelength of 0.63um, and the secondary is similarly 1/10 wave. We then have a total peak-to-valley wavefront error of 0.63um/0.55un * [SQRT ( 1/8^2  + (1/SQRT(2) * 1/10)^2 ] = 1/7 wave. "Diffraction limited" is often considered to be 1/4 wave for the total system, so the system in this example is diffraction limited.  However, 1/7 wave of aberration will drop the MTF curve noticeably, so it is not ideal. What is ideal? A total system wavefront error of better than 1/10 wave comes very close, and is probably indistinguishable from perfect. My personal 10" Newt has a total system peak-to-valley wavefront error at 0.55 um of 1/16 wave, making the deviation due to geometrical aberrations from the diffraction-only MTF curve indiscernible. Incidentally, I am assuming you know how to properly collimate your scope.

 

(3) mounts and comfort: a poor mount is a PITA, right? A good mount for planets will do the following:

  --hold the image steady at high power (x50 the aperture size in inches),

  --place the eyepiece in a comfortable viewing position,

  --not break your back to set it up,

  --for those of us who hate "nudge-nudge-nudge," track well enough to keep the planet in the field of view, and

  --for those of us who hate ladders, not place the eyepiece position above standing.

 

A GEM with a decent drive (like those equipped with Byers or Opti-Craft machining gears) can satisfy all these needs provided the telescope focal length is short enough to keep the eyepiece position at or below eye level. As luck would have it, most of us cannot reach the eyepiece at telescope focal lengths of about 65" (GEM) or 70" (Dobsonian). For the latter, subtract the d'Artume tracking table height, so you also end up with about 65". Now it is very difficult to obtain and properly secure a mirror while maintaining system optical quality of 1/10 wave P-V (peak-to-valley) or better at f/ratio < f/4.5. Dividing 65" by f/4.5 yields a maximum mirror diameter of about 14". Happily, this happens to about match the largest telescope we can use even in "ideal" seeing conditions.

 

Thus, for those who wish to manhandle a 14" telescope (or mount it permanently), who have occasional excellent seeing at their primary observing site, pay up for excellent optics (or make your own), and who also take great care with their mirror cell and system collimation, a 14" Dob telescope with a tracking table (d'Artume table)  is about the maximum aperture for the best planetary viewing experience.

 

A GEM with 2" and above axes will also work for such a telescope but, really, it needs to be permanently mounted. I've found a 10" is about the maximum one can mount on a GEM with 1.5" axes (the maximum mount size that can be rolled and is transportable) w/o stability problems, and an electric focuser helps, too. I am easily able to roll mine out on casters from my garage to my backyard concrete patio.

 

So there you have it: for those of us with pretty good but not great seeing, a high quality 10" telescope on a GEM with rotating rings (mandatory for comfort) would be ideal. A 12.5"-to-14" tracking Dob would be even slightly  better if seeing permits.

 

OK, why not a 14" Cass or a SCT? Both have larger central obstructions that diminish MTF, and the commercially readily available SCTs rarely have good enough optics. What about a refractor? Aperture-for-aperture, a high-quality APO (almost no lateral color) refractor will be the best of all BUT the Airy disk of a 12" scope is HALF the width of a 6" refractor and trounces the 6" perfect refractor MTF curve. Go larger and refractors have problems: with lateral color correction, with mechanical distortion of the heavy lens elements, and should I mention ... cost? 

 

(4) let us not forget our eyes. For those of us who are a bit older, we've probably accumulated a number of floaters in our eyeball fluid over many years. The result: these can become really annoying when the system exit pupil (= focal length of eyepiece/focal ratio of telescope) decreases below about 0.5 mm. Choose your magnification accordingly. Also, good eye relief is wonderful. For these reasons, my favorite high power eyepiece for my f/6.6 telescope is the 20mm eye relief 3.5mm Pentax XW.

 

Happy observing always,

 

Don


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#100 Deep13

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Posted 16 January 2019 - 02:29 AM

Terry Ostahowski emailed me saying the main mirror has no discernable aberations. Huzzah!

I built a cart for my GEM. My observing chair is pretty tall. Seeing here comes and goes, but like I said before, I find a lot of "seeing" problems vanish with active scope ventilation. I don't mind short eye relief EPs.


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