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the F/6 reflector

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#1 CollinofAlabama

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Posted 23 November 2020 - 07:22 PM

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?


Edited by CollinofAlabama, 24 November 2020 - 03:06 AM.

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#2 jefffed

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Posted 23 November 2020 - 07:41 PM

I agree. At f/6 or even f/6.5 the secondary can be smaller with all the benefits that come along with that. Smaller obstruction = more light and more planetary detail. How about an 8" f/6?


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#3 CHASLX200

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Posted 24 November 2020 - 06:49 AM

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.


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#4 dmgriff

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Posted 24 November 2020 - 08:03 AM

A 6in f/6 (f~900mm) yields the same magnification as a 4.5in f/8, but, with more aperture and a sharper image (parabolic vs. spherical). The GSO f/6 steel tubes do have a little weight at about 14 lbs. My aluminum tubed f/8 newtonian is about 11+ lbs. The f/6 ota is shorter, therefore, less moment arm and transports a little easier. Priced at $200+, a good deal on a functional ota out of the box.
 
 
Nice and steady on a GEM....
DSC00165
 
 
Ready to go out the door on a Vixen Porta II...
DSC00174 (2)

Some dampening, but, works for me due to the Porta slomo (now knobs, instead of stalks as shown in photo) and a Borg 4317 helical and 7316 1.25in insert adapter.

 

I eventually replaced the stock GSO primary/secondary with a OWL/OSI pyrex primary and secondary.

 

Good viewing,

 

Dave


Edited by dmgriff, 24 November 2020 - 08:11 AM.

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#5 Eddgie

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Posted 24 November 2020 - 09:14 AM

Well, the think is that at 6" and f/6, you are not really getting anything that you would not get more of by going to an 8" f/6.

 

An 8" f/6 Dob tube weighs less than 20 lbs and is very easy to handle. The base is usually likewise very simple to handle.

 

The 8" is a considerable step up in performance over a 6".

 

Now I would myself never consider putting any kind of Newtonian on a GEM for visual use due to the poor ergonomics, but on an Alt-az mount, it is a really nice telescope.

 

Also, people say you can make the secondary smaller at f/6.  Well, that is only partially true because the problem is that in smaller scopes, the secondary has to be larger as a percentage of aperture than in a larger scope of the same focal ratio to get the same size fully illuminated field (if that matters). Many small reflectors have secondary mirrors that are actually so small that they only illuminate a very small image circle and I have seen really small reflectors that actually are not delivering full aperture because the secondary mirror is too small.  Anyway, with an 8" f/6, you can have an even smaller secondary and this means that along with the greater apertue there is a considerable step up in contrast. 

 

You can get around this by using a very low profile focuser like the KineOptics, but this has its own issues.

 

Now, in my opinion, the 6" f/6 telescope that everyone should try to own once is the Incredible MN66.  This is a 6" Apo rival that really shows how excellent a reflector can be when all the stops are pulled out, but the very small obstruction means that the field illumination is poor with modern long focal lenght 2" eyepieces. 


Edited by Eddgie, 24 November 2020 - 09:15 AM.

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#6 dmgriff

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Posted 24 November 2020 - 11:26 AM

 F/6 6" dob would offer something different

 

What do you think?

I thought that there was once a 6in f/6 Orion Intelliscope dob, but, that is probably wrong. They have always been f/5?

 

I think a 6in f/6 Intelliscope would be a good seller, better all around than a f/5. Easier on eps, less coma. Smaller than a 6in f/8 dob, but, a little easier to transport.

 

Good viewing,

 

Dave
 


Edited by dmgriff, 24 November 2020 - 11:42 AM.

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#7 CollinofAlabama

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Posted 24 November 2020 - 11:53 AM

Here's my reasoning, a 6" F/8, though a fine scope, really has little advantage over the 8" F/6, since an 8" F/6 delivers so much more light grasp, while still maintaining a reasonably easy collimation point and "deal-able" coma result.  Weight is a factor, and for someone with back issues, etc, the 6" F/8 is a genuine advantage, but for TFOV, they're exactly the same due to the same focal length.

 

But a 6" F/6 dob offers something truly different.  Unlike the F/5 scopes, coma isn't horrific.  It's bad, and, yes, I notice coma at F/6, but it's not like at F/5, where it really must be fixed, or you live with a crappy field of stars (what more expensive eyepieces are designed to fix, but cannot at F/5 due to the parabolic mirror's steep light curve).  At F/6, I can see the coma, but I really have to look for it, and just glancing, the field looks quite good.  But a 900mm focal length scope has SCADS more TFOV than a 1200mm one.  Them's just the optical facts.  And a 900mm focal length reflector is a REALLY small scope, dobsonian-wise.  If you have back issues, or just want something to slam down, kabang, for a quick look, you'd be hard pressed to beat a 6" F/6 dob, offering plenty of TFOV, yet all of 150mm of aperture.  It brings a lot more to the table than a 6" F/8 for TFOV and portability.  And if designed well, it doesn't have to be a tabletop mount.  It's still big enough to mount in the traditional dobsonian style.  Short, yes, but a small stool and you're in business.

 

I had a beautiful, custom made 6" F/6 once, but sold it to a friend.  Never should have done that, but ...  It was a wonderful scope, and cruising the Markarian Chain with that fellow was something!  The market's more than ready for this, with the GSO 6" OTA pointing the way.  Synta should get into the action, too.  A little competition goes a long way.

 

And a 130mm F/6 would be a serious challenge to the 4" APO market.  Might not be quite as good, but it'd be better than a 4" achromat, and should cost hardly any more.  And for DSO's, I'd bet a dime to a dollar a well made 130mm F/6 would trounce anyone's 4" APO.  Yes, planets and double stars might be slightly better in the APO, due to the lack of obstruction, I'll give you that, but just barely.  The extra light grasp of the 130mm mirror compared to a 4" lens would definitely make Andromeda and such better in the 130mm reflector.

 

The time has come for the 6" F/6 dob and 130mm F/6 reflector to be available in the market.  They've got something to show people, and the people will respond.


Edited by CollinofAlabama, 24 November 2020 - 11:56 AM.

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#8 howardcano

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Posted 24 November 2020 - 12:24 PM

I have both an SW 8" F/6 and a TPO-badged GSO 6" F/6, but the time is quickly approaching when I won't be able to move the 8", so the 6" will soon be the flagship.

 

I'd like to get a Dobsonian mount for my TPO, since it would be much easier to schlepp around than an EQ mount.  A couple of years ago I could have built one myself, but that's not really an option at this point.  A Dob mount that folds flat for transport would be great.

 

Oh, and despite its 50mm secondary, planetary performance of the TPO is better than my SV102 Access refractor!


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#9 planet earth

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Posted 24 November 2020 - 12:26 PM

3 year old thread whether it's useful or not?

 

https://www.cloudyni...-ota-for-16915/



#10 Jon Isaacs

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Posted 24 November 2020 - 12:51 PM

But a 6" F/6 dob offers something truly different.  Unlike the F/5 scopes, coma isn't horrific.  It's bad, and, yes, I notice coma at F/6, but it's not like at F/5, where it really must be fixed, or you live with a crappy field of stars (what more expensive eyepieces are designed to fix, but cannot at F/5 due to the parabolic mirror's steep light curve).  At F/6, I can see the coma, but I really have to look for it, and just glancing, the field looks quite good.

 

The difference in coma between F/5 and F/6 isn't that great and if one uses well corrected eyepieces, at F/5, the views are reasonable.  

 

F/8, coma is much less, the coma free circle at F/6 is 4.75mm, at F/8 it's 11 mm.. 

 

But I think the real issue with an 6 inch F/6 Dob is that it is too short. It needs a stand and a chair.  An 6 inch F/8 or an 8 inch F/6 just needs a chair.  

 

Jon


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#11 BDS316

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Posted 24 November 2020 - 01:12 PM

Does anyone on this thread remember the Starmaster Oak Classic?  If I recall correctly it was a 7 inch f/5.6 compact solid tube Dobsonian with fantastic mechanics and Zambuto optics.  

 

I envy those who own one of these.  Sadly I don't foresee anything like this on the horizon.... 


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#12 Chris Johnson

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Posted 24 November 2020 - 01:13 PM

A 6”f5 reflector with a 31mm Nagler gives you a 6.1mm exit pupil and a 3.4deg field of view. Perfect for cruising the Milky Way, veil nebula and M31.


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#13 Chucky

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Posted 24 November 2020 - 02:48 PM

(( Does anyone on this thread remember the Starmaster Oak Classic? If I recall correctly it was a 7 inch f/5.6 compact solid tube Dobsonian with fantastic mechanics and Zambuto optics. ))

I remember it everyday. It's on display as I type this note.

#14 BDS316

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Posted 24 November 2020 - 02:53 PM

(( Does anyone on this thread remember the Starmaster Oak Classic? If I recall correctly it was a 7 inch f/5.6 compact solid tube Dobsonian with fantastic mechanics and Zambuto optics. ))

I remember it everyday. It's on display as I type this note.

Photo please?



#15 CollinofAlabama

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Posted 24 November 2020 - 02:56 PM

Jon, I owned a 6” F/6 telescope for several years back in the first decade of this century.  It used a Russian mirror imported by a guy named Sunseri in Oregon.  It was designed by a disciple of Rick Singmaster, and a small chair was all that was required.  It used an 8” sized tube, but was much shorter, of course.  No stand required.  I’m only 5’10”, but my taller friends could look through it as well, utilizing my small fold up chair.  And that chair handled the ENTIRE sky, zenith to horizon.

 

Also, you have noted that coma is progressive.  Though plenty of the field of an F/6 is not coma free, the aberration starts later from the center and progresses more slowly.  The visual difference between a wide field view at F/5 vs F/6 is quite apparent, and dramatic.  Yes, F/8 is better still, for all the exact same reasons F/5 is worse than F/6.  But I find F/6 considerably more tolerable than F/5.  It’s a big difference, and you yourself have noted that F/5 coma is worse than the field curvature one might see in, say, an F/6 refractor, in terms of visual wide-field performance.  I agree F/6 isn’t like F/8, but it’s a whole lot closer than F/5.

 

I can see coma even at F/8, btw, but it’s truly a minor imperfection of the field.  At F/6 it’s worse, yes, but not something I’d bother with a coma corrector about.  At F/5? Better have a coma corrector handy, or suffer a rather lame outer field, with your expensive eyepieces performing like Kellners.


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#16 Jon Isaacs

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Posted 24 November 2020 - 03:15 PM

Jon, I owned a 6” F/6 telescope for several years back in the first decade of this century.

 

How tall was the base?

 

I had my 8 inch F/5 on a Dob mount for a short while.  It was just too darn short to use. A tall base would have been helpful but it would have represented something of a transportation challenge.

 

Since you have an aversion to eyepieces weighing even 16 ounces, I don't see the wide field, well corrected views coming into play for you, coma or no coma. I know of no 30 mm wide field eyepieces in your weight range that are decently corrected for astigmatism at F/6.. 

 

I think there's a place for a 6 inch F/6 but for me, it wouldn't be on a Dob mount, it would be on an alt-az mount and would replace my 120 mm ED refractor.. I wouldn't have the thermal advantages of the refractor but it would be otherwise very similar and much less expensive.

 

I'd use it with a coma corrector most of time. 

 

Jon


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#17 Chucky

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Posted 24 November 2020 - 03:24 PM

<<  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.

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  • Oak.jpg

Edited by Chucky, 24 November 2020 - 03:39 PM.

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#18 CollinofAlabama

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Posted 24 November 2020 - 04:32 PM

This was a cold March 4th public star party night in 2007 -- overview, target near zenith, and target near horizon ...

 

overview.jpg near-zenith.jpg near-horizon.jpg

 

Wish I owned the fantastic APM Ultra Flat Field 30mm back then, but it would be at least 11 years till that would appear on the market (and 12 years before I'd own one), but it would have produced a 2.42º field at just under 20 ounces.  That's not a huge field, I suppose, but was impressed with the 20mm Meade SWA perusing the Markarian Chain back then, imagine what it'd look like in the APM 30mm UFF!  Don't think anyone's 1200mm telescope can produce as big a field, regardless the fat, huge, overweight ocular one might stick in it.  The APM 30mm UFF produces a very nice field in my Orion XT-8 PLUS, btw.  It's a keeper.  Yes, I did have to break the one pound barrier, but not by much, and 1.25 lbs isn't a whole lot more.


Edited by CollinofAlabama, 24 November 2020 - 05:09 PM.

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#19 sw196060

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Posted 24 November 2020 - 05:11 PM

Chucky, 

That looks nice!   What is the height of the altitude bearing axis?   



#20 CHASLX200

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Posted 24 November 2020 - 07:16 PM

Does anyone on this thread remember the Starmaster Oak Classic?  If I recall correctly it was a 7 inch f/5.6 compact solid tube Dobsonian with fantastic mechanics and Zambuto optics.  

 

I envy those who own one of these.  Sadly I don't foresee anything like this on the horizon.... 

I used one once. Nice for sure.


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#21 Echolight

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Posted 24 November 2020 - 07:58 PM

I thought about buying a GSO 6 inch f6 and building a dob base. But I'd rather have one that size that was optimized for visual.


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#22 Jon Isaacs

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Posted 25 November 2020 - 06:07 AM

This was a cold March 4th public star party night in 2007 -- overview, target near zenith, and target near horizon ...

 

 Nice photos but it reminds me of my 8 inch F/5.  Doesn't look comfortable to me but posed pictures are often not the best.  A Starbound chair might and an even taller base.  This Starblast on a raised table worked quite well, the same thing could be done with a 6 inch F/6.

 

Starblast 6 inch CN.jpg

 

Jon


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#23 Illinois

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Posted 25 November 2020 - 07:18 AM

10 inch F6 Dobsonian would be nice! I would take it if I must have only one telescope! 


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#24 Chucky

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Posted 25 November 2020 - 08:01 AM

<<  10 inch F6 Dobsonian would be nice!  >>

 

Something like this?

 

 

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#25 Chucky

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Posted 25 November 2020 - 08:05 AM

For comparison, how about this?  All around F6.  Here we have a 7, and 8, and a 10.

 

 

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  • Collection.jpg

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