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the F/6 reflector

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#26 BDS316

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Posted 25 November 2020 - 09:55 AM

<<  Photo please?  >>

 

My Starmaster 7 Oak Classic with Zambuto sans matching Tom O. platform.  My Zambuto calculates to an 5.4....this per Carl's focal length notation inscribed on the mirror edge.  Encoders not installed in this photo.  Also not shown is a stalk (slips into a Moonlite connector) I sometimes use to hold my 8 inch tablet.

very cool....Noticed you put liners on your altitude bearings.  Are they ebony star and did they help?  Rick S used to say they weren't necessary.

 

Thanks.


Edited by BDS316, 25 November 2020 - 09:58 AM.

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#27 Chucky

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Posted 25 November 2020 - 10:09 AM

<<  Noticed you put liners on your altitude bearings.  Are they ebony star and did they help?  >>

 

On my Oak Classic, that picture is deceiving.  Actually the only pads making contact on the Ebony Star are 4 small round nylon pieces (shown in the photo).  The longer showing white piece is just a piece of non touching velcro to cover up a few small holes (same thing on the other side). 

 

I've got the altitude movement just right.  I also can apply a variable VERY light dab of tension with the round teflon piece you can see in the photo.  That also adds a good way to keep the tube from sliding laterally as it goes up and down.  Improves digital setting circle accuracy.


Edited by Chucky, 25 November 2020 - 10:40 AM.

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#28 Keith Rivich

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Posted 25 November 2020 - 10:26 AM

I write to praise the F/6 reflector, which is becoming more scarce, just when it should become more important.

 

Right now, a person can pick up an F/5 150mm reflector rather easily.  But F/5 is not a very friendly place to be for anyone not wanting to spend the big bucks for a coma corrector, not to mention the heightened collimation requirements first for F/5, then for F/5-with-a-coma-corrector, if someone goes that route.

 

I am not trying to dismiss astrophotography needs.  I would be the first to agree that F/5 is probably better for them, but any F/5 reflector with a simplistic single speed Crayford isn't really meant for them in the first place.  If it's not for them, why try to force this on the visual world?

 

Certainly, an F/6 reflector, like the GSO 6, is a very large scope to mount on an EQ or other refractor-like mounts, but an F/6 6" dob would offer something different than the ubiquitous, but somewhat redundant 6" F/8 dobsonian for the visual observer.  Don't get me wrong, I like the 6" F/8 dob, but wouldn't an easily accessible 6" F/6 dob offer most folks a whole lot more, in terms of wider true field of view, without introducing intolerable amounts of coma.  The market is ready.

 

Likewise, a 130mm F/6 scope would be nice offering, too.  Again, no coma corrector, and still not too huge to fit on a reasonable EQ or alt-az mount.  Not a big fan of the tabletop mounts -- I will concede making a dob of this size would be slightly more difficult, but one could probably use the 6" F/6 base from the previoius paragraph with some minor modifications.  But even if it only came with tube rings, it's something I would be interested in, and think the market would like such a choice, too.  And a 2" focuser on this is a necessity, preferably at least a single speed crayford.  Both scopes should come with some kind of padded case.  Metal would be great, but even just a padded soft case would be fine.

 

What do you think?

I like my f/5's. They seem to hit a real sweet spot for the type of observing I like to do. Fast enough to make use of my 35mm pan and slow enough to get me high magnifications with moderate eyepieces, such as my 8mm Delos.

 

 I do not use a coma corrector. The edge of the FOV doesn't concern me as much as having the extra glass in the path. 



#29 Chucky

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Posted 25 November 2020 - 11:24 AM

((  What is the height of the altitude bearing axis?  ))

 

On my 7 Oak Classic the answer is 26 inches.



#30 stargazer193857

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Posted 25 November 2020 - 01:32 PM

I too agree f6 is nicely cleaner than f5. For 6", the improvement is clear. For 12" f5 vs 10" f6, I'd rather have more aperture when looking at galaxies.

I know people who think 8" f6 is too tall. It is, for a camping chair. It is a compromise height for tall chair vs standing vs fitting in the back seat. A lower mounted 8" f5.75 would fit better for a camping chair.


As for 10" f4.7 vs 8" f5.9, the 10" is brighter on M31 at 207x, making the stars direct vision. I'd stake the 10".


But for 6", I agree f6 is better. Give it an eq platform for extra height.

Edited by stargazer193857, 25 November 2020 - 01:34 PM.

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#31 Spikey131

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Posted 25 November 2020 - 07:25 PM

Too tall!

 

How about 12.5” f/7?
 

Just two short steps to the zenith.

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#32 stargazer193857

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Posted 25 November 2020 - 08:13 PM

Part of the reason we don't see more f6 scopes is people's desire for wide views or aperture, but part of it is shipping cost and the extra weight and width to balance it.

I think it certainly can be done though, with 10" f6 being one candidate.

A coma corrector and f5 is richer and sharper, but only if you get everything adjusted right. Part of the desire for f6 is simplicity and lower cost. I agree f6 coma is very tolerable.
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#33 CollinofAlabama

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Posted 25 November 2020 - 11:25 PM

Stargazer193857,

 

I agree completely about F/6, but really, F/5 only makes sense, in my opinion, when dealing with astrophotography or large apertures.  Yes, after about 10" (and many would say after 8"), reflectors at F/6 begin to have the same problems long tube refractors had before the advent of fluorite glass, the tubes just plain get too difficult to deal with.  But at 6" and smaller, F/5 for visual work is kind of ridiculous.  Yes, an F/5 150mm is shorter than an F/6 one, of course, but not by a lot, and if a coma corrector is employed  (adding about 1.15 times to 1.2 times to the focal length), then the slightly shorter tube is all you're really getting, since the difference in effective focal length is pretty small, indeed -- 862.5mm effective focal length not much less than 900mm.  So you've increased the complexity of operation dramatically, as well as introduced an expensive accessory, all for a nominally smaller tube.  Now for a 10", and especially larger mirror scope that can be all that matters, but at 150mm and less, this is nonsense.  Why dramatically increase the expense and complexity of an instrument for a nominal TFOV gain, post coma corrector?  Again, I'm discounting astrophotography here, which can certainly be worth it, but not served by inexpensive single speed Crayford or rack and pinion models currently on the market.  For visual use, F/6 makes the most sense for 150mm and smaller apertures.  And for portability, a 6" F/6 dobsonian is the cat's meow.  See my photos above.


Edited by CollinofAlabama, 26 November 2020 - 02:37 AM.


#34 CHASLX200

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Posted 26 November 2020 - 06:49 AM

Coma drives me nuts at F/5 so F/6 for me or slower.


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#35 stargazer193857

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Posted 26 November 2020 - 09:31 AM

I'm able to ignore coma at f5 and 82 degrees if I look at the center and use peripheral vision for the sides. But when I do look at a star near the edge, a brightish one, then the spikes are disappointing but don't ruin the whole view. At f6 though, I don't notice coma unless I look near the edge and am picky.

As for rich field, that only matters for some star fields, and maybe not even then. I find aperture is what gives the best view of dust lanes. That and transparency.
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#36 CHASLX200

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Posted 26 November 2020 - 09:53 AM

Too tall!

 

How about 12.5” f/7?
 

Just two short steps to the zenith.

Now that is my kind of scope. Always wanted a 16" F/6 Dob to a 20" F/6 Dob for planets.



#37 CHASLX200

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Posted 26 November 2020 - 09:54 AM

I'm able to ignore coma at f5 and 82 degrees if I look at the center and use peripheral vision for the sides. But when I do look at a star near the edge, a brightish one, then the spikes are disappointing but don't ruin the whole view. At f6 though, I don't notice coma unless I look near the edge and am picky.

As for rich field, that only matters for some star fields, and maybe not even then. I find aperture is what gives the best view of dust lanes. That and transparency.

FC also drives me nuts in fast Fracts as does coma in fast Newts while i am sweeping.  


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#38 Jon Isaacs

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Posted 26 November 2020 - 07:55 PM

But at 6" and smaller, F/5 for visual work is kind of ridiculous.  Yes, an F/5 150mm is shorter than an F/6 one, of course, but not by a lot, and if a coma corrector is employed  (adding about 1.15 times to 1.2 times to the focal length), then the slightly shorter tube is all you're really getting, since the difference in effective focal length is pretty small, indeed -- 862.5mm effective focal length not much less than 900mm.  So you've increased the complexity of operation dramatically, as well as introduced an expensive accessory, all for a nominally smaller tube.

 

 

Realistically, an F/5 with a Paracorr is not much more difficult to operate than an F/6 without. A Paracorr is an expensive accessory but it's a one time purchase, if you have one, you're set for life, every Newtonian you own can be fully corrected for coma.

 

F/5 + Paracorr + 31 mm Nagler = essentially perfect

 

F/6 + 31 mm Nagler = quite good

 

This thread has me thinking, comparing an 6 inch F/6 Newtonian to my 120 mm F/7.5 Orion Eon ED refractor.  A good 6 inch F/6 Newtonian could give it a run for the money. But I'd use it with a Paracorr 2 because I'm a guy who loves Newtonians and I have the tools and the skills to make Newtonians strut their stuff.

 

Jon


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#39 Bomber Bob

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Posted 27 November 2020 - 12:05 PM

I like F/6 or slower the best. I can skip a Paracorr at F/6.  Last Meade 8"f/6 i had was a 450x killer.

You mean... this one?  Yeah, it is a Planet Killer -- and ain't too shabby on the fuzzies, either:

 

Meade 826 Restore S01 - Lumicon 125 HF.jpg

 

My RV-6 got me into Reflector World, but this old Meade 826 sealed the deal.


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#40 25585

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Posted 27 November 2020 - 05:31 PM

I have the https://explorescien...ght-8-dobsonian

 

Its OTA can be removed from the Dob mount and out used on another mount.

 

Same goes for my Orion Optics VX10L F6 Dob's OTA.

 

But the most compact F6 reflector is a C8 with 0.63 reducer, making an 8" F6.3.



#41 CollinofAlabama

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Posted 27 November 2020 - 07:04 PM

 

This thread has me thinking, comparing an 6 inch F/6 Newtonian to my 120 mm F/7.5 Orion Eon ED refractor.  A good 6 inch F/6 Newtonian could give it a run for the money. But I'd use it with a Paracorr 2 because I'm a guy who loves Newtonians and I have the tools and the skills to make Newtonians strut their stuff.

 

Glad this thread has presented you with a possible comparison project.  CN is so nice about that, giving us all ideas to bounce around.  I, too, would be interested in hearing of your results in such a comparison and have wondered, myself, how a 6"reflector competes with a 120mm refractor.  I know from first-hand experience that an 8" dob can take a 120mm refractor, proved one night a long time ago in the first decade of this century with, of all things, a Meade 8" LightBridge (back when they made them!), and an Orion 120mm ED scope, the predecessor to the SkyWatcher 120mm ED Pro.  The target was Saturn, and the LightBridge was just plain better -- brighter, better defined details, all around better image.  But a 6" reflector?  I don't know and would be interested in your take on this, as a seasoned astronomer with a keen eye to comparison.

 

This reminds me of this old article by Joe Bergeron, a 2007 comparison of his Astro-Physics 155mm f/9 EDT, the Celestron 150 XLT F/5 reflector, and his AP 92mm Stowaway.  Unsurprisingly, the AP 155mm comes out on top with his assessment ...

 

"In overall performance, the [2007] $400 Omni was midway between these two fabled refractors, and closer to the big one than the little one"

 

How and where a 6" reflector might fit with a 120mm refractor would be a question I've wondered about for at least 13 years.  Good luck should you choose to make this comparison, or even if this only inspires someone else on CN to do it!


Edited by CollinofAlabama, 28 November 2020 - 02:50 PM.

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#42 CHASLX200

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Posted 27 November 2020 - 07:39 PM

You mean... this one?  Yeah, it is a Planet Killer -- and ain't too shabby on the fuzzies, either:

 

attachicon.gifMeade 826 Restore S01 - Lumicon 125 HF.jpg

 

My RV-6 got me into Reflector World, but this old Meade 826 sealed the deal.

I miss that scope.


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#43 stargazer193857

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Posted 27 November 2020 - 08:55 PM

What's that blasphemy? That's not a Dobsonian. Although, being that high up gets it out of the ground level thermals. That is more of a summer issue though.

#44 Abhat

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Posted 28 November 2020 - 10:24 AM

I have 6" F/6 GSO Dob which I mounted on a Very Old Hardin Dob base. This has become my default choice compared to other scopes I own even though its a compromise but a good one. The main reason being aperture and portability. 8" is too big for me to carry around in one piece all over my yard. Four inch is too small of an aperture. GSO 6" F/6  has 2 " single speed crayford focuser and most of my EPs work well. Also the Binoviewer works in it as well with 1.6X after I remove the extension tube. It has 10% CO by area but I have not noticed significant drop in contrast compared to the original 6" F/8 OTA probably because the Hardin mirrors had lost significant amount of coating. Also the build quality of this GSO OTA is fantastic.

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#45 stargazer193857

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Posted 28 November 2020 - 10:56 AM

I get better views of the milky way in an 8" f6 than in a 6" f5. Side by side. Not sure why. But a 6" f5 with coma corrector does much better. Also, at a dark site aimed at dense star fields, an 80mm refractor does better than the 6" f5.

For seated Dobsonian viewing, 6" f7 sounds better than f5, 6, or 8. But for a tripod, f5 is better but likely heavy regardless.

7" f6 would be nice. Or f6.3.

Faster mirrors (f4-4.5) with CC are nice for rich field views. Just costly and more skill to collimate.
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#46 CollinofAlabama

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Posted 28 November 2020 - 05:09 PM

For seated Dobsonian viewing, 6" f7 sounds better than f5, 6, or 8. But for a tripod, f5 is better but likely heavy regardless.

7" f6 would be nice. Or f6.3.

Faster mirrors (f4-4.5) with CC are nice for rich field views. Just costly and more skill to collimate.

Interesting observations about your Milky Way results.  Hard for me to comment on those.  I can say for sure that an F/5 120mm (Synta) and F/5 5” (Jaegers) achromats do better than a 4” F/7 ED scope for the Milky Way & Andromeda, at least to my eye.

 

But Stargazer 193867, my point about the beautiful aspect of the 6” F/6 dob is mostly focal length in absolute terms, F/whatever.   I can squeeze 2.42° out of my widest field ep (the APM 30mm UFF), but if one goes much above that, there goes the wider TFOV.  So a 130mm F/7 is fine (910mm, not much of a hit on TFOV), but a 6” F/7 will only have a tiny amount more field than my C102GT, which at 2-1/3° isn’t horrible, but under where I’d like to be.  Of course, 2.5° and better is preferable (why I’d love a 130mm F/6), but staying within spitting distance of that is critical to me.  The 6” F/6 certainly gets me there, but even at F/7, it’s too 4” F/10 refractor, if you see what I mean.

 

Look, I’ve been to dark sky sites with guys with 14” and larger dobs oogling over the TFOV my rather modest C102GT can yield, which can indeed seem considerable compared to those, but a 6” F/6 dob would have them oogling all the more, and certainly could be as portable with easier setup, even than my C102GT on the excellent GSO SkyView Deluxe alt-az mount (original AstroTech with genuine AstroTech Pier Extension) on Ho wooden legs, or about as easy as a good 4” F/10 refractor setup as it gets.  That’s what I’m talking about.

 

I’d never argue about F/7 being somehow worse, optically than F/6.  About that I’m with you from an optical perspective, but in terms of field of view and functionality, a 6” F/7 wouldn’t quite do it for me, taking me too far, no longer spitting distance, from 2.5° TFOV.


Edited by CollinofAlabama, 28 November 2020 - 05:13 PM.


#47 SteveG

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Posted 28 November 2020 - 11:18 PM

Glad this thread has presented you with a possible comparison project.  CN is so nice about that, giving us all ideas to bounce around.  I, too, would be interested in hearing of your results in such a comparison and have wondered, myself, how a 6"reflector competes with a 120mm refractor.  I know from first-hand experience that an 8" dob can take a 120mm refractor, proved one night a long time ago in the first decade of this century with, of all things, a Meade 8" LightBridge (back when they made them!), and an Orion 120mm ED scope, the predecessor to the SkyWatcher 120mm ED Pro.  The target was Saturn, and the LightBridge was just plain better -- brighter, better defined details, all around better image.  But a 6" reflector?  I don't know and would be interested in your take on this, as a seasoned astronomer with a keen eye to comparison.

 

This reminds me of this old article by Joe Bergeron, a 2007 comparison of his Astro-Physics 155mm f/9 EDT, the Celestron 150 XLT F/5 reflector, and his AP 92mm Stowaway.  Unsurprisingly, the AP 155mm comes out on top with his assessment ...

 

"In overall performance, the [2007] $400 Omni was midway between these two fabled refractors, and closer to the big one than the little one"

 

How and where a 6" reflector might fit with a 120mm refractor would be a question I've wondered about for at least 13 years.  Good luck should you choose to make this comparison, or even if this only inspires someone else on CN to do it!

I might be finding out myself. I've concluded this winter that I'm going to sell my ED120. It's just too close to my ED100, and I'm not liking the weight. I'm reading to try something really different, and have been considering the 6" f5. One thing I don't like about a lot of them is they're set up for imaging, and require long tube extensions for visual. I also wish you could buy a reflector with an aluminum tube.


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#48 Jon Isaacs

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Posted 28 November 2020 - 11:49 PM

I might be finding out myself. I've concluded this winter that I'm going to sell my ED120. It's just too close to my ED100, and I'm not liking the weight. I'm reading to try something really different, and have been considering the 6" f5. One thing I don't like about a lot of them is they're set up for imaging, and require long tube extensions for visual. I also wish you could buy a reflector with an aluminum tube.

I think there is a version of the Celestron XLT that comes with a 2 inch focuser that is setup for visual.  This one states it has a 2 inch Crayford.

 

https://www.highpoin...ly-ota-31057ota

 

Jon


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#49 Jon Isaacs

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Posted 28 November 2020 - 11:56 PM

I get better views of the milky way in an 8" f6 than in a 6" f5. Side by side. Not sure why. But a 6" f5 with coma corrector does much better. Also, at a dark site aimed at dense star fields, an 80mm refractor does better than the 6" f5.

For seated Dobsonian viewing, 6" f7 sounds better than f5, 6, or 8. But for a tripod, f5 is better but likely heavy regardless.

7" f6 would be nice. Or f6.3.

Faster mirrors (f4-4.5) with CC are nice for rich field views. Just costly and more skill to collimate.

 

What I find is a fast scope with a well corrected flat field provides impressive views of the Milky Way.  The aperture doesn't really matter, in fact in someways, larger is better, there's more to scan.  At F/5 with a Coma corrector, (F/4.4 without), scanning the summer Milky Way with the 31mm Nagler is pretty wonderful.  I love starting at M-8/M20 and slowly sweeping back and forth until I reach M6/M7.  

 

Jon


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#50 CHASLX200

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Posted 29 November 2020 - 07:27 AM

I think there is a version of the Celestron XLT that comes with a 2 inch focuser that is setup for visual.  This one states it has a 2 inch Crayford.

 

https://www.highpoin...ly-ota-31057ota

 

Jon

Send the optics to Zambuto and you got a first class killer.


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