164 replies to this topic

[aa6ww]

  If  your a Dob man, the AR6 wont need a minus violet filter. You will still mostly be locked into the fun of deep space observing. If you look at lunar and planetary objects, that would be 3 main objects, the Moon, Venus and Jupiter, the Minus violet will help but its not a necessity.

 

  The rest of the sky will be great. Probably just sharper at the edges then a dob, and dimmer over all.

 

..Ralph

 

 

It's an achromat, so needs some serious minus violet filtration.  Otherwise, everything has a purple fringe around it.

A 6" scope needs to be f/18 to f/30 to be free from false color (notably, purple), and that scope is much faster.

So, just figure a strong minus violet filter will be required, and you're good to go.

If it's also in bad mechanical condition, it should be very inexpensive.

[Deep13]

Vanilla!  No, chocolate!

 

Beatles! No, the Stones!

 

Ginger!  No, Mary Ann!

Oh come on! How can you even compare them?! Mary Ann has it all over Ginger in every possible way.

[SandyHouTex]

Monetarily a reflector has it all over an APO.  A 6 inch APO costs the same as a 16 inch Teeter-Zambuto.

[Steeveaux]

Oh come on! How can you even compare them?! Mary Ann has it all over Ginger in every possible way.

Just recently caught an episode. I don't remember Lovey looking so young.

Back to the topic: I have one mount and two great scopes-an APO and a reflector. I usually go a month or so with one and then a month or so with the other.
After a month with the reflector I'll switch to the refractor and notice how pretty the stars look all across the field.
After a month with the refractor I'll switch to the reflector and achieve higher mag than is possible with the APO.


I'm not sure I could call one a favorite but I like the refractor for any outreach situation. It just seems easier for the uninitiated.

Steve O.

Edited by Steeveaux, 19 April 2018 - 09:35 AM.

[starcanoe]

Just recently caught an episode. I don't remember Lovey looking so young.

 

 

 

Just wait until you think she looks like your grandaughter !

[gwlee]

 I used to think the same way till I got my 6"F8 dob with excellent optics.  It absolutely blows away ANY 4 inch scope no matter how well refined it may be.  BTW my 6" dob gives me views that are just as fine as my former 4" and 5" apos and is much easier to set up.  Collimation is also extremely easy as is thermal equillibrium.  No triplet Apo refractor will cool as quickly.  The views in the 6 inch reflector are superior, in every way.  Mechanicals are also quite excellent!

While my former large Apos were nice, they were not convenient to set up which is why I sold them and am no longer a fan of such scopes. YMMV.

My inexpensive, garden variety, Synta 6”f8 Dob also gives better views of anything that will fit within its 1.3 degree fov than my sensibly perfect 4” refractor. Although its optics could be improved, its mechanical systems are its weakest link, focuser, alt/az bearings, etc.

 

The three 6”f8 Dobs that I owned also cooled very quickly, but not faster than my 4” refractor. They didn’t set up any faster than my alt/az-mounted refractor, but they were easier to safely carry through a doorway fully assembled.  I have not owned or compared them to 5” refractors, so I can’t comment from experience. 

 

I have never owned one scope that comes closer to meeting all of my needs for an all around telescope than a 6”f8 Dob. Unfortunately, current 6”f8 Dobs are considered entry level scopes that are being continually cheapened (plastic focusers for example)  to keep the price as low as possible. Unlike 8”f6 Dobs, there are no boutique manufacturers that I am aware of that offer premium versions.

 

Stock 6”f8 scopes are more difficult to upgrade because fewer high quality, drop-in, aftermarket parts are available. For example, a high quality, drop-in, 2-speed 1.25” focusers isn’t available; its OTA can be modified to accept a high quality 2” focuser, but this creates weight and balance problems. Correcting these problems pushes the scope’s total weight into 8”f6 territory, where it can’t compete optically. 

 

Of course ATMs can build anything, but I prefer to purchase scopes rather than build them whenever possible. However, If I ever build a scope from scratch, it will probably be a high quality 6”f8 because I can’t buy one. What 6”f8 Dob do you have, and how did you aquire it? 

[Starman1]

Here you go: premium 1.25" focusers (you buy the base plate you need):

http://starlightinst...ory&path=37_109

[gwlee]

Here you go: premium 1.25" focusers (you buy the base plate you need):

http://starlightinst...ory&path=37_109

Thanks. When I contacted Starlight and Moonlight, neither had a direct, drop-in replacement for the XT6. I called Wayne at Starlight again this AM and sent him some photos of the XT6 to see if anything has changed. 

 

Starlight did a custom adapter for my Mewlon 210 that has become a standard item, but they weren’t interested in supporting the XT6. Both supported the XT8. 

Edited by gwlee, 19 April 2018 - 02:55 PM.

[barbie]

My inexpensive, garden variety, Synta 6”f8 Dob also gives better views of anything that will fit within its 1.3 degree fov than my sensibly perfect 4” refractor. Although its optics could be improved, its mechanical systems are its weakest link, focuser, alt/az bearings, etc.

 

The three 6”f8 Dobs that I owned also cooled very quickly, but not faster than my 4” refractor. They didn’t set up any faster than my alt/az-mounted refractor, but they were easier to safely carry through a doorway fully assembled.  I have not owned or compared them to 5” refractors, so I can’t comment from experience. 

 

I have never owned one scope that comes closer to meeting all of my needs for an all around telescope than a 6”f8 Dob. Unfortunately, current 6”f8 Dobs are considered entry level scopes that are being continually cheapened (plastic focusers for example)  to keep the price as low as possible. Unlike 8”f6 Dobs, there are no boutique manufacturers that I am aware of that offer premium versions.

 

Stock 6”f8 scopes are more difficult to upgrade because fewer high quality, drop-in, aftermarket parts are available. For example, a high quality, drop-in, 2-speed 1.25” focusers isn’t available; its OTA can be modified to accept a high quality 2” focuser, but this creates weight and balance problems. Correcting these problems pushes the scope’s total weight into 8”f6 territory, where it can’t compete optically. 

 

Of course ATMs can build anything, but I prefer to purchase scopes rather than build them whenever possible. However, If I ever build a scope from scratch, it will probably be a high quality 6”f8 because I can’t buy one. What 6”f8 Dob do you have, and how did you aquire it? 

 The focuser on my 6"F8 dob is just fine as are the bearings.  There is no slop or backlash in the focuser so I don't care if it's plastic or not.  My 6" F8 dob cools far faster than any 4 inch refractor I've had. Optically, my XT6 dob is excellent and there are no problems with it in the mechanics either so you may have gotten a less satisfactory sample.  I also don't consider a 6" dob an entry level scope.  A 4 inch refractor is a far less capable entry level scope. Again, as far as the mechanicals are concerned, they are just fine and do everything required by me so I see no need for upgrades at this point.  I've used the focuser to 300x magnification without any problems, the bearings are smooth and the mirror cell is excellent as is the spider and secondary holder.  I've seen "premium" scopes that were far less capable.

Edited by barbie, 19 April 2018 - 03:15 PM.

[Jeff B]

I have a superb TEC200ED and equally superb (optically, mechanically and coolingly...did I just make up a word?) Parallax/Zambuto 11" F 7 Newt.  Other than image brightness and a slight warmth in the TEC's color tone, there is little difference between them for solar system viewing.  Sometimes I prefer the TEC, others the Newt.  The only "glaring" difference is the Newt's diffraction spikes, especially on Jupiter and Mars.   But I've learned to live with the spikes and ignore them much like I can ignore CA in achromats ( if it's not too severe anyway), however there are also solutions for that too (curved spider vanes or, preferably but very expensively, an optical window, like Ed did). 

 

For me, the key to really enjoying my newts has been great optics & great mechanical and cooling designs.  I want my newts to behave like an excellent APO and I've found that it is easily done if I pay attention to the big three: optical quality, mechanical design & execution, and cooling design & execution.  Everything else is "sauce for the goose" for me (however, I freely admit to being one of those people who have sub-F5-phobia and yes, I am considering seeking therapy for it).

 

Jeff

[gwlee]

 The focuser on my 6"F8 dob is just fine as are the bearings.  There is no slop or backlash in the focuser so I don't care if it's plastic or not.  My 6" F8 dob cools far faster than any 4 inch refractor I've had. Optically, my XT6 dob is excellent and there are no problems with it in the mechanics either so you may have gotten a less satisfactory sample.  I also don't consider a 6" dob an entry level scope.  A 4 inch refractor is a far less capable entry level scope.

 

I have owned three XT6s and two XT8s in 25 years and still own one of each. All of my XT6s had marginal focusers and marginal alt/az motions when compared side-by-side to my XT8s, which are also very inexpensive scopes. Despite these mechanical shortcomings, an XT6 is still the most used of my three scopes because it offers the best tradeoff between portability and optical performance for my needs at this site and everything works well enough to do the job.

 

The optics of all my XT6s have all been very good, no complaints, but they all fell short of the "sensibly perfect" standard that's expected from very expensive optics, but the XT6 is only a $300 scope. Nevertheless, the XT6's 6-inch optics were good enough to best the optical performance of my very expensive and sensibly perfect 4" refractor, but the 4"refractor was a much more refined scope in every other way, only limited by its aperture. 

 

I don't consider a 6"f8 Dob an entry level scope either, but as long as I've been in the hobby, manufactures have treated 6"f8 Dobs as entry level scopes to be sold newcomers at the lowest possible price, so the quality of 6"Dobs have steadily declined while the quality of slightly larger Dobs have steadily improved. I wish it weren't so because a 6"f8 Dob built to higher optical and mechanical standards would meet all my requirements for a single life-time scope, but I would probably have to build it to get it, which I would like to avoid, so I am auditioning possible replacements. 

[barbie]

I couldn't disagree more with many of the points in your post.  My XT6 beat out all of my refractors which were also "sensibly perfect".  As for the mechanics of the XT6, they are excellent as well and meet all the criteria for what is needed on a scope, inexpensive or not.  The XT6, as is, is my idea of a sensibly perfect scope and will handily beat a 4 inch anything.  The scope is easy to collimate, has excellent optics, holds collimation well, has an excellent focuser, has a well designed and executed mirror cell which promotes efficient cooling of the optics and is generally very well constructed, the ONLY modifiction I made was flocking opposite the focuser which improved contrast quite well.   At this point, I no longer see any benefit in going back and forth with you as these points have been stressed and re-stressed over and over again.  I also suggest you read the reviews of this scope, if you can!

Edited by barbie, 19 April 2018 - 10:00 PM.

[Jon Isaacs]

 I used to think the same way till I got my 6"F8 dob with excellent optics.  It absolutely blows away ANY 4 inch scope no matter how well refined it may be.  BTW my 6" dob gives me views that are just as fine as my former 4" and 5" apos and is much easier to set up.  Collimation is also extremely easy as is thermal equillibrium.  No triplet Apo refractor will cool as quickly.  The views in the 6 inch reflector are superior, in every way.  Mechanicals are also quite excellent!

While my former large Apos were nice, they were not convenient to set up which is why I sold them and am no longer a fan of such scopes. YMMV.

Superior in every way? 

 

How about a 4.9 degree TFoV?   Or even just a 3.0 degree TFoV? 

 

The virtue of a 4 inch apo is it's versatility.   From low power wide field of view to high power, both astronomical and terrestrial.. 

 

Jon

[starcanoe]

My eyes have about a 180 degree field of view.

 

Just saying .

[Jon Isaacs]

My eyes have about a 180 degree field of view.

 

Just saying .

 

That means you could possibly use an eyepiece with a 180 degree AFoV.   But this is about comparing telescopes and their capabilities.  True field of view is one defining parameter. 

 

For me it's important because it's one of the major advantages/reasons that i use small (6 inches and under) telescopes.  

 

Jon

[barbie]

Superior in every way? 

 

How about a 4.9 degree TFoV?   Or even just a 3.0 degree TFoV? 

 

The virtue of a 4 inch apo is it's versatility.   From low power wide field of view to high power, both astronomical and terrestrial.. 

 

Jon

I can get the same FOV with my large binoculars. A 4 inch apo is still only 4 inches.  6 is better than 4.

As for the wide field of view to high power and terrestrial, been there, done that!!  That no longer does anything  for me. As far as I'm concerned and after 50 plus years of observing and telescope making, there is no such thing as an all in one telescope.  Different tools for different jobs.

Edited by barbie, 20 April 2018 - 01:56 PM.

[johngwheeler]

Thanks for the answers to my question. Some of them drifted a little off-topic into a "refractors vs reflectors" debate, but all good to hear.

My take-away is that it *is* possible for a reflecting 'scope to get very close to or even exceed the image quality of an Apo refractor, and that this may be easier to achieve at larger apertures where the refractor will also start to be more affected by seeing.

The key "ingredients" seem to be:

1) High quality optics - mostly from smaller custom manufacturers

2) Thermal management - correct tube / structure materials, cooling & ventilation

3) Small secondary mirrors (& hence longer f-ratios) - there may be some debate about the real effect of mirror size on image quality.


My question is not just restricted to Newtonian reflectors. I'm also interested in how this applies to various Cassegrain designs, such as Ritchey-Chretian or Dall-Kirkham etc.

[barbie]

Well John, my other scope is a 90mm F14 Mak-Cass and it performs as well as my small apo refractors on the planets without the need to resort to using very short focal length eyepieces to achieve higher magnifications.  I have since sold the apos and strongly prefer the Maksutov for planets for the above mentioned reason.

[Jon Isaacs]

 

Superior in every way?

 

How about a 4.9 degree TFoV?   Or even just a 3.0 degree TFoV?

 

The virtue of a 4 inch apo is it's versatility.   From low power wide field of view to high power, both astronomical and terrestrial..

 

Jon

 

I can get the same FOV with my large binoculars. A 4 inch apo is still only 4 inches.  6 is better than 4

As for the wide field of view to high power and terrestrial, been there, done that!!  That no longer does anything  for me. As far as I'm concerned and after 50 plus years of observing and telescope making, there is no such thing as an all in one telescope.  Different tools for different jobs.

 

 

Your statement was that your 6 inch was superior in every way to a 4 inch apo.  I was merely pointing out one of ways in which a 4 inch apo is superior 6 inch F/8 Newtonian.  

 

I very much agree there is no such thing as an all in one telescope, different tools for different jobs, that's why I refrain from claiming that one telescope is superior in every way to another. It all depends on the job.  A sledge hammer is not a good tool for putting together a fine piece of jewelry.  A jeweler's hammer is not very good for taking out a concrete slab.  

 

Jon

[barbie]

A 6 inch reflector is still superior to a 4 inch apo in terms of light gathering capability.

Since I observe mainly planets and double stars, I don't need the wide field  capabilitites of a 4 inch apo.

Like I said, I can get that with my big binoculars and I've already had too many 4 inch apos and see nothing extraordinary about them other than portability. To each their own!

Edited by barbie, 23 April 2018 - 11:58 PM.

[Jon Isaacs]

A 6 inch reflector is still superior to a 4 inch apo in terms of light gathering capability.

Since I observe mainly planets and double stars, I don't need the wide field  capabilitites of a 4 inch apo.

Like I said, I can get that with my big binoculars and I've already had too many 4 inch apos and see nothing extraordinary about them other than portability.

I use my 10 inch Dob (or something larger) for the planets and double stars.  For that, one might argue that a 10 inch is better than a 6 inch.  But a 10 inch is physically larger and heavier than a 6 inch so for some observers, the smaller scope is the better scope.  A couple of years ago, I wanted to lend my 10 inch to a friend  He's almost 20 years younger than I am but when I went to move it, it was too big.  

 

As far as big binoculars that will provide a 4.5 degree TFoV with the perfection and brightness of the NP-101+31mm Nagler I have not seen nor heard of those. 

 

Beauty is in the eye of the beholder.  To my eye what is extraordinary about a scope like the NP-101 is it's ability to do everything well within the limits of it's 4 inch aperture.  And with eyepieces like the 41mm Pan, the 31mm Nagler, the 21mm Ethos, it doesn't get any more perfect than that.  

 

I get it that some people are not interested all aspects of observing.  But that's not what this thread is about.  It's about the duplicating an apo with a Newtonian.  

 

Jon

[Cotts]

Hi, guys!   Here's my 12.5 " Teeter/Lockwood dob.  f/6.5 with a central obstruction of 15.5%...

 

 

I would like to ask those who prefer APO refractors:  what size triplet APO would be the equal of my telescope in terms of resolution, contrast transfer and light gathering power? And how much would this APO cost, including a suitable mount?

 

Dave

 

 

[Jon Isaacs]

Hi, guys!   Here's my 12.5 " Teeter/Lockwood dob.  f/6.5 with a central obstruction of 15.5%...

 

IMG_8114.jpg

 

I would like to ask those who prefer APO refractors:  what size triplet APO would be the equal of my telescope in terms of resolution, contrast transfer and light gathering power? And how much would this APO cost, including a suitable mount?

 

Dave

 

Dave:

 

I prefer the right tool for job.  For some jobs it's an apo refractor,  for some it's a larger aperture Newtonian on Dobsonian mount. 

 

If one wants to match ALL the capabilities of a 4 inch apo refractor with a Newtonian,  it ain't gonna happen.  if one wants to match ALL the capabilities of a 12 inch Newtonan with a refractor,  it ain't gonna happen.

 

That's why when i travel the southwest,  I take one of each. 

 

Big scopes for little objects,  little scopes for big objects.. 

 

Jon

[gwlee]

 

 

But it has got me to thinking about what’s invovled in creating a good quality reflecting telescope, and why we don’t seem to see many of them. I find myself consistently slightly disappointed with the images from standard Meade, Celestron and SkyWatcher reflectors (SCTs and Newts) and I’ve never looked through a reflector and thought “wow” - this is as sharp as my refractors. The best was a C8 Edge HD, which was very close and quite impressive.

 

Why is this? Is it just a market driven thing and that there is a limited desire for producing high quality premium reflectors outside of professional / research instruments?

I have also found that it’s much easier to find and purchase sensibly perfect (SP) refractors off the shelf than SP reflectors, which are usually only available from a few small custom shops. Custom SP reflectors are very expensive compared to off the shelf scopes. They have longer lead times, and some sizes, 6”f8 for example, are not available. 

 

Why? I believe that most people are satisfied with the optics and mechanics of production reflectors at 1/10 the cost and don’t want to wait months for delivery, so the market for SP reflectors too small to be attractive to large manufacturers who stay in business by selling people what they want to buy at a price they are willing to pay and do it efficiently enough to make money. 

 

For example, my factory 8”f6 Dob cost me $300 and was delivered to my front door by a big brown truck within 48 hours of placing my order. My custom 8”f6 reflector with sensibly perfect optics cost me $3,000 and delivery took a year. Its optics were better, but the improvement was subtle, usually requiring side-by-side testing in better than average seeing to confirm.

 

On the other hand, the optical improvements to be had from a 10” factory reflector costing $600 are immediately obvious, so more people are inclined to upsize their reflective optics rather than upgrade them.  Other people who are basically satisfied with their massed produced factory reflector optics might prefer to spend the same $3K on a SP refractor, not because it’s better than a reflector, but because it complements a reflector so well, it’s available off the shelf, and it scratches the SP itch too.

Edited by gwlee, 24 April 2018 - 12:25 PM.

[Jon Isaacs]

I have also found that it’s much easier to find and purchase sensibly perfect (SP) refractors off the shelf than SP reflectors, which are usually only available from a few small custom shops. Custom SP reflectors are very expensive compared to off the shelf scopes. They have longer lead times, and some sizes, 6”f8 for example, are not available. 

 

Why? I believe that most people are satisfied with the optics and mechanics of production reflectors at 1/10 the cost and don’t want to wait months for delivery, so the market for SP reflectors too small to be attractive to large manufacturers who stay in business by selling people what they want to buy at a price they are willing to pay and do it efficiently enough to make money. 

 

For example, my factory 8”f6 Dob cost me $300 and was delivered to my front door by a big brown truck within 48 hours of placing my order. My custom 8”f6 reflector with sensibly perfect optics cost me $3,000 and delivery took a year. Its optics were better, but the improvement was subtle, usually requiring side-by-side testing in better than average seeing to confirm.

 

On the other hand, the optical improvements to be had from a 10” factory reflector costing $600 are immediately obvious, so more people are inclined to upsize their reflective optics rather than upgrade them.  Other people who are basically satisfied with their massed produced factory reflector optics might prefer to spend the same $3K on a SP refractor, not because it’s better than a reflector, but because it complements a reflector so well, it’s available off the shelf, and it scratches the SP itch too.

 

 

A sensibly perfect post ..

 

Jon