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Cheap small Dob delight

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#26 Jon Isaacs

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Posted 26 June 2018 - 04:14 PM

I generally agree with you, but consider sharpness at high power only one test or aspect of sharpness and sharpness across the field another. If high power was my primary type of viewing, that might be my primary method of testing sharpness, but I prefer a scope that gives me a good balance of both.

 

 

In my experience, sharpness at high magnifications is about the telescope's optics, they're either sharp or they're not.   

 

Sharpness across the field is more about the eyepieces and, for a Newtonian, a good coma corrector.  The 31mm Nagler or the 20mm Ethos with the Paracorr 2 is sharp across the field in my 12.5 inch F/4.06, I don't think that has much to do with quality of the telescope's optics, it has to do with the quality of the eyepieces and the coma corrector. 

 

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#27 gwlee

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Posted 26 June 2018 - 08:07 PM

In my experience, sharpness at high magnifications is about the telescope's optics, they're either sharp or they're not.   

 

Sharpness across the field is more about the eyepieces and, for a Newtonian, a good coma corrector.  The 31mm Nagler or the 20mm Ethos with the Paracorr 2 is sharp across the field in my 12.5 inch F/4.06, I don't think that has much to do with quality of the telescope's optics, it has to do with the quality of the eyepieces and the coma corrector. 

 

Jon

Jon, I think our experience is similar, but we are thinking about sharpness a bit differently. Here’s the way I think about it. 

 

Some short FL refractors are plagued with field curvature. They can be focused sharply at the center, but not simultaneously focused sharply at the edges. In theory, EPs could be custom designed to provide some relief, but the problem is inherent with the telescope. Others short refractors can be simultaneously sharp across the full field of view, your NP101 for example, so I consider the later telescope sharper overall than the former. 

 

Some fast reflector optics are plagued with coma. Sure, a coma corrector can provide some relief for telescopes with this problem, but the problem is in the telescope. Others reflectors can be simultaneously much sharper across the full field of view, my 6”f8 newt for example, so I consider the later sharper overall than the former.

 

The designers of the former telescopes chose to tradeoff edge sharpness for other desireable features, so they are not defective telescopes; It’s not a quality issue. Their center sharpness is not diminished, and people who operate these scopes at high power won’t be disadvantaged, but their sharpness is definitely inferior when the whole field is considered and not just the center.

 

When I bought my 72mm f6, I expected a sharp center and fuzzy edges at low power with the best EPs, and that’s just what I got. If i wanted sharpness over the whole field, I would have bought a petzval or an F8+ scope of the same aperture. 


Edited by gwlee, 26 June 2018 - 09:01 PM.


#28 Jon Isaacs

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Posted 26 June 2018 - 08:56 PM

Some short FL refractors are plagued with feel curvature. They can be focused sharply at the center, but not simultaneously focused sharply at the edges. In theory, EPs could be custom designed to provide some relief, but the problem is is inherent to the telescope. Others short refractors can be simultaneously sharp across the full field of view, your NP101 for example, so I consider the later telescope sharper overall than the former.

 

 

It's still mostly about the eyepiece.  My 80mm F/6 ED/apo is quite sharp across the field with the 31mm Nagler, I have to look to see it.  It's pretty darn good and it's way better than the NP-101 with a so-so eyepiece.   

 

Some fast reflector optics are plagued with coma. Sure, a coma corrector can provide some relief for telescopes with this problem, but the problem is in the still telescope. Others reflectors can be simultaneously much sharper across the full field of view, my 6”f8 newt for example, so I consider the later sharper overall than the former.

 

A coma corrector like the Paracorr 2 does not just provide "some relief" for fast telescopes, my 12.5 inch F/4.06 plus the Paracorr 2 has considerably less coma than that 6 inch F/8.  

 

Field curvature and coma are not functions of the optical quality of the telescope, they are functions of the design.  And they can be dealt with.. 

 

What is a function of the optical quality is the image at the higher magnifications, this is where the resolution of the eye is no longer limiting the resolution of the telescope...  A rather poor mirror can be sharp across the field at low magnifications but at high magnifications the stars will show clear defects.. 

 

The main thing here is to simply realize that when you say one scope is sharper than another, it does not mean the same thing as when I say one telescope is sharper than another.  

 

And in general, I think it's better to define what is meant.  When I am discussing low power, wide field views, I try to make it clear that this is what I am discussing.  

 

Jon



#29 Pinbout

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Posted 26 June 2018 - 10:01 PM

 

"The best 6 inch F/8 is an 8 inch F/6."

you can't mount a 8inf6 on a tripod like ... for quick setups.

 

med_gallery_106859_3508_133994.jpeg


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#30 gwlee

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Posted 26 June 2018 - 10:27 PM

 

Field curvature and coma are not functions of the optical quality of the telescope, they are functions of the design.  And they can be dealt with.. 

 

 

I believe we are saying more or less the same thing above. 

 

From my point of view (definition), but clearly not yours, a telescope that inherently exhibits less coma or less field curvature, and therefore provides greater sharpness across the full field of view, so doesn’t require additional external components to “deal with it” is what I consider to be an “inherently sharper scope.”

 

I agree these two optical aberrations are sometimes the result of deliberate design tradeoffs too obtain other desireable optical characteristics that are a higher priority for some users, so can’t be considered optical defects in this context. My 72mm f6 is an example. The designer deliberately traded a wider field of view and greater portability for poor edge performance. 

 

I further agree that a telescope system comprised of a telescope with inherently poor edge performance, such as my 72mm f6, and additional external components of relatively high cost and bulk, such as a TV Nagler 31 or TV Comma Corrector, might be the equal or perhaps even superior in overall sharpness to an “inherently sharp scope” as defined in my first paragraph and previous posts. 

 

For me to consider a scope a “sharp scope,” it must have edge sharpness AND center sharpness at all magnifications up to its resolution limit. There are various ways to get there, some more expensive than others, but both must be there. 

 

For example, if you asked me if my 72mm is a sharp scope, I would say, yes, but would qualify my answer by saying the center is pretty good, but the edges are poor and detract from its overall sharpness, but probably no worse edge performance than any 432mm FL scope. Ask the same question about the NP101, and I would give you an unqualified yes. 


Edited by gwlee, 26 June 2018 - 11:30 PM.


#31 Jon Isaacs

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Posted 27 June 2018 - 06:18 AM

you can't mount a 8inf6 on a tripod like ... for quick setups.

 

 

Why bother with a tripod?  A Dob mount is quick and more stable.  

 

Jon


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#32 Jon Isaacs

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Posted 27 June 2018 - 06:40 AM

I believe we are saying more or less the same thing above.

 

From my point of view (definition), but clearly not yours, a telescope that inherently exhibits less coma or less field curvature, and therefore provides greater sharpness across the full field of view, so doesn’t require additional external components to “deal with it” is what I consider to be an “inherently sharper scope.”

 

 

So last night I had out my 10 inch F/5 Dob.  I was viewing Jupiter.  I was using the Paracorr II with my type 6 Naglers.  The seeing was quite good, I was able to get a good split on Antares even though it was still in the southeast and barely above the trees.  The views were crisp and I was seeing a lot of detail. And, across most of the field, Jupiter just about perfect. 

 

Eyepieces, you need eyepieces.  A coma corrector, they're out there.  If one has a larger aperture Newtonian, it's part of the admission price.  When I think of an F/8 scope, I think of it as being easier on eyepieces, more tolerant of eyepieces.  That does not translate to being a sharper scope.  For me, I consider my 10 inch F/5 as sharp across the field as any scope I have owned and that includes the NP-101.  With the Paracorr and the good eyepieces, it's pretty awesome. 

 

Jon


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#33 gwlee

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Posted 27 June 2018 - 10:43 AM

So last night I had out my 10 inch F/5 Dob.  I was viewing Jupiter.  I was using the Paracorr II with my type 6 Naglers.  The seeing was quite good, I was able to get a good split on Antares even though it was still in the southeast and barely above the trees.  The views were crisp and I was seeing a lot of detail. And, across most of the field, Jupiter just about perfect. 

 

Eyepieces, you need eyepieces.  A coma corrector, they're out there.  If one has a larger aperture Newtonian, it's part of the admission price.  When I think of an F/8 scope, I think of it as being easier on eyepieces, more tolerant of eyepieces.  That does not translate to being a sharper scope.  For me, I consider my 10 inch F/5 as sharp across the field as any scope I have owned and that includes the NP-101.  With the Paracorr and the good eyepieces, it's pretty awesome. 

 

Jon

Jon,

 

If I bought a 10 f5 Dob for $639, I would also a buy comma corrector for $486, and I would have a telescope SYSTEM with sharp views across the whole field of view, including low power (50x), at a system cost of $1,116 without EPs. Because the scope itself would have poor edge performance at low power, I wouldn’t consider it a sharp scope though; System yes, scope no. 

 

Instead, for various reasons besides sharp star fields across the full field of view at low power (50x), I bought a 6”f8 Dob telescope that sells for $279 that gives sharp views across the whole field of view without a comma corrector  It doesn’t have the resolution of the 10” scope, so no matter how perfect the optics might be, it will never match the high power sharpness of the larger scope, and it doesn’t do a lot of other desireable things the larger scope can do. 

 

To maximize performance in a portable package (34#) with acceptable edge performance at low power, I am considering buying (for evaluation) a 6”f5 newt along with a comma corrector and mounting it on a my DiskMounts DM4, so I understand your approach with your 10”f5, but don’t consider either of these scopes to be “sharp” scopes without the comma corrector. 

 

Gary


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#34 gwlee

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Posted 27 June 2018 - 10:53 AM

Why bother with a tripod?  A Dob mount is quick and more stable.  

 

Jon

Jon

 

Have you personally evaluated two similar configurations side-by-side? I ask because I am considering doing trying something similar?

 

Gary



#35 Jon Isaacs

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Posted 28 June 2018 - 07:05 AM

Jon

 

Have you personally evaluated two similar configurations side-by-side? I ask because I am considering doing trying something similar?

 

Gary

Gary:

 

I have a StellarVue MG-2 which is a version of the SkyTee-2 mount.  It's a robust alt-az mount with slow motion controls, I tried it out with my 8 inch F/5 Synta Newt.  It was OK but not as solid or as smooth as a Dob mount.  In your situation with the rough terrain, it might be useful.  

 

If I bought a 10 f5 Dob for $639, I would also a buy comma corrector for $486, and I would have a telescope SYSTEM with sharp views across the whole field of view, including low power (50x), at a system cost of $1,116 without EPs. Because the scope itself would have poor edge performance at low power, I wouldn’t consider it a sharp scope though; System yes, scope no.

 

The 10 inch F/5 is actually quite sharp across the field with the 31mm Nagler, definitely sharper than a 6 inch F/8 with BW-Optik 30mm 80 degree Widescan II clone.  

 

A telescope is always a system.  When talking about lower power/wide field edge correction, the eyepiece is important.  When talking about high power, on-axis views, it's the mirrors that are important, little else matters.

 

Jon


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#36 gwlee

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Posted 28 June 2018 - 12:08 PM

Gary:

 

I have a StellarVue MG-2 which is a version of the SkyTee-2 mount.  It's a robust alt-az mount with slow motion controls, I tried it out with my 8 inch F/5 Synta Newt.  It was OK but not as solid or as smooth as a Dob mount.  In your situation with the rough terrain, it might be useful.  

 

The 10 inch F/5 is actually quite sharp across the field with the 31mm Nagler, definitely sharper than a 6 inch F/8 with BW-Optik 30mm 80 degree Widescan II clone.  

 

A telescope is always a system.  When talking about lower power/wide field edge correction, the eyepiece is important.  When talking about high power, on-axis views, it's the mirrors that are important, little else matters.

 

Jon

 

Thanks Jon. I am not really satisfied with the stability (freedom from the shakes) of the two small Dobs that I own now, so wouldn't be satisfied with a 6" f5 on the DM4 unless it was at least as stable as my 6"f8 Dob. 

 

My grasp of optical theory is tenuous at best, but my understanding has alway been that TV EPs generally have a minuscule effect on field curvature. Knowing that I don't know what I am talking about, I had long phone conversation this morning with Al Nagler. We discussed tree dodging, night vision, field curvature, and TV EPs. Here's my take away regarding field curvature and TV EPs:

 

"Field curvature is due both to eyepiece and telescope design, and may add or cancel, depending on the eyepiece/telescope combination." Whether a given EP improved the field curvature or worsened it would be hit or miss and the effect would be "minuscule" at best. TV EPs are "designed to work best with flat field telescopes, such as the NP101," so this is a particularly good combination as you noted. 

 

An EP can be designed to compensate for the field curvature exhibited by a particular  objective, and that's done for binoculars with "Flat Field EPs," but it's not practical for refractors because each refractor FL would require a different EP design. 

 

If I understand Al correctly, if my short refractor has poor edge performance that’s caused by field curvature, there is no EP fix, I need another refractor with better edge performance, a longer FL or a petzval design for example. 

 

We also discussed my observing site and interests, and Al recommended that I look into night vision, which he thinks is a "game changer," but that's a subject for another discussion. 

 

Gary


Edited by gwlee, 28 June 2018 - 02:54 PM.


#37 CollinofAlabama

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Posted 29 June 2018 - 01:11 AM

f/6 isn't exactly "exacting". Even coma is almost non-existent at that focal ratio.


Not my experience, and I own both. An F/6 parabolic mirror puts up substantial coma. Not nearly as bad as F/5 and below, no, but definitely noticeable, and makes one desire a coma corrector. I notice coma even in my SkyWatcher 6” 2”-focuser F/8, but there it’s not particularly bothersome, and truly is somewhat negligible. I like the 6” size, much more portable and, for me at least, sharp as a tack. Jupiter was fantastic last Friday night, with Europa’s shadow transit quite apparent. I found Cor Coroli and Albireo both more colorful than an older 10” F/5.56 scope that was out that evening. Aatt wrote that Synta makes excellent mirrors at F/8. Seems true to me. I like my 8”, too, but if we’re doing public star parties, I’d just as soon take out the 6” than the 8”. Probably be using the 8” at dark sky sites, but I like them both, but the 6” throws up a substantially nicer maximum TFOV, due to the very small amount of coma present.

#38 gwlee

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Posted 29 June 2018 - 10:49 AM

Why bother with a tripod?  A Dob mount is quick and more stable.  

 

Jon

Snow


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#39 kfiscus

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Posted 29 June 2018 - 10:51 AM

Barefoot Jon doesn't see snow very often in San Diego.wink.gif


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#40 starcanoe

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Posted 29 June 2018 - 11:32 AM

Did a public star gaze on the beach last Friday and Sat night. Took my 6 inch f8 home built dob out there.

 

Haven't done any such gazes in years and back when I did I was hauling out the 10 f5.6 "big dob".

 

Turns out the 6 inch f 8 is the perfect outreach scope as well !

 

There was a C8 celestron. A 9.25 something or other cat. A 4 inch relatively fast ED refractor. Some other guy with another C8 ish type scope set up with a display screen and astrophotography.

 

Then, off to one side was my little 6 inch F8 Dob.

 

A fair fraction of the folks that looked through my scope made a point of saying that they were attracted to the scope and wanted to check it out.

 

And I can think of several reasons why. First obviously home built. Not nearly as impossing as the other scopes. Even the 4 inch frac had a serious looking mount. Not complicated looking. Those other scopes with all those fiddly bits and hand controllers and whirling motors are fairly intimidating to the general public I think. And needless to say the astrophoto/display screen scope took that to the next level.

 

Not only did it look simple...folks could see it was simple in use. Look at Jupiter say. Then swing around, sight along the side of the tube or use the laser pointer on the tube and bammmm....now we have Saturn....swing around again....Venus....swing around again...Alberio...swing around again...the moon...and so on and so on.

 

Need to move the scope to get an unobstructed view of X? Pick it up....move over...plop it back down....bam....done.

 

Most people expressed awe that I could "just find" things. But I explained things...simple landmarks in the sky. Albierio....end star of the easy to see Cygnus. M4...sorta between Antares and that other star. M22...forms a parallelogram with the handle and top of Scorpious the Tea Pot. M57....right between those two easy to find stars near easy to find Vega. Explained how Mars, Jupiter, and Venus are fairly obvious targets once you know what you are looking for. And even Saturn with a little care.

 

Then a fair number got fairly interested in the home built aspect. Hey, I just bought the optics and built a wooden box ! The side bearings are PCV flanges...look here the focuser is made of plumbing parts....you can do this too...especially with all the info and help on the internet these day....

 

Hey, how much does this cost? Ohhh, you can get something like this for around $300. A bit more and you can get an 8 incher ! The 4 inch frac cost about twice that (the tripod alone was $300). The other scope...well, more like $3000 rather than  $300. They probably didn't wanna know what the imager guy had invested.

 

As for views? The 9.25 showed a little bit more detail on Jupiter...but it was all a bit washed out to me (probably that large secondary mirror doing that).  The six f 8 dob beat everything else IMO. And this is just some random mirror I bought 25 years ago with a bog standard diagonal. Have never even star tested it. And the eyepieces....my $10 Vite 3 element/plastic lens 10 and 23mm plossls.

 

End of the night. Put the tube under one arm...grad the handle on the rocker box and walk to the car in the parking lot. Easy peasy.

 

I think a lot of people came away less intimdated about telescopes and costs and finding things in the night sky after seeing the little dob in action.


Edited by starcanoe, 29 June 2018 - 11:46 AM.

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#41 gwlee

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Posted 29 June 2018 - 03:20 PM

Barefoot Jon doesn't see snow very often in San Diego.wink.gif

I find comments about telescopes often reveal more about the observer’s physiology, observing style, and observing site than the telescopes. 



#42 starcanoe

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Posted 29 June 2018 - 03:39 PM

Don't forget disposable income !

 

Or lack of priorities :)



#43 Jon Isaacs

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Posted 01 July 2018 - 09:45 AM

Barefoot Jon doesn't see snow very often in San Diego.wink.gif

 

In town, they say it snowed in 1967 and there was some on the ground for about 20 minutes but I was away at college.  :)

 

On the other hand, San Diego county has a great variety of climates and geographies.  In the high desert, it typical snows a few times a year but it doesn't last long. In the mountains, snow is common.. 

 

5298954-Francis in the snow reduced and cropped.jpg
 
Jon


#44 NEOhio

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Posted 01 July 2018 - 12:58 PM

Jon, up north we don't call that "snow", we call that "a light dusting" :-)

 

It has to at least cover your shoes to qualify as "snow"...




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