137 replies to this topic

[SteveG]

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

And while he's working on it, get an aluminum tube and swap the other components. Sounds like a great covid project!

[Cotts]

f/6, you say?  Here's my new (to me) 8-inch f/6 Maksutov Newtonian with Intes glass and structure by Matthias Wirth.

 

I'd tell you about its optical performance but it has been cloudy every ^Y()^%$# night since I got it!

 

I do expect it to perform as well as a 7.9 inch APO refractor on planets, doublestars, planetary nebulae, globs etc.......

 

 

It will ride on a Discmount DM6 on the pier in my observatory.  For Star Parties, a Losmandy HD tripod...

 

Dave

[CHASLX200]

f/6, you say?  Here's my new (to me) 8-inch f/6 Maksutov Newtonian with Intes glass and structure by Matthias Wirth.

 

I'd tell you about its optical performance but it has been cloudy every ^Y()^%$# night since I got it!

 

I do expect it to perform as well as a 7.9 inch APO refractor on planets, doublestars, planetary nebulae, globs etc.......

 

Tube.jpg

 

It will ride on a Discmount DM6 on the pier in my observatory.  For Star Parties, a Losmandy HD tripod...

 

Dave

It would be a easy 500x killer in my seeing i bet.

[Cotts]

It would be a easy 500x killer in my seeing i bet.

I'm hoping I will be able to bring it to WSP 2022...

 

Dave

[CHASLX200]

I'm hoping I will be able to bring it to WSP 2022...

 

Dave

Top notch optics can take crazy powers on some objects.  The seeing there should let you go to 500x on the moon easy. Even Jupiter is not bad at that power , but starts to get dim.  I don't think Jupiter will be in the sky for the WSP.  But Mars when close can take 500x better since it is so bright when close.

[Bomber Bob]

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.

Down here at The Swamp, we have Summer (85F & higher, with 60% & higher humidity) from April 15 to November 15...  I rate HOT nights by how many sweat rags I have to use to keep my drippings off the eyepieces...

Edited by Bomber Bob, 30 November 2020 - 09:49 PM.

[Don H]

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

Me too! After a number of years of mirror and telescope making in the late 90s and early 2000s, my keepers ended up being a 10" f/6.3 and a 12.5" f/5.9. I made a number of smaller and large scopes at different focal ratios, but favored f/6 overall. Some of the scopes I built but sold were 16" f/5, 14.3" f/5.25 and 14.3" f/4.7, 13.1" f/5.25, 11.3" f/4.5, an 11" f/6 which was nice, but heavy due to Baltic birch plywood, 8" f/4.5, and a 6" f/7. I originally liked f/6 for under my suburban patio sky, but found it also provided a nice contrast benefit per eyepiece under darker sky locations, too. Surely, one could darken the sky using shorter fl eps on the faster scopes, but f/6 seemed a nice fit between wide enough fov (compared to say f/10), and higher mag per ep. The 10 and 12.5" also had thinner mirrors with very nice wavefronts. If I were to have known I would eventually retire to AZ, I would have probably kept a 14.3". When I sold them, the views seemed close to the 12.5", which was quite a bit less weight to manage and cooled a lot faster. All in all, my 10" f/6.3 has been a great scope for me. It sits in the corner close to the patio door and can be carried out assembled, ready to view in minutes. The eyepiece height is 63" at the zenith, so it's very comfortable for viewing with my feet on the ground. For less elevated targets, I can just grab a patio chair for seated observations, too. The 1600mm focal length gives me 50x and 1.36 degree fov with a 32mm 68* eyepiece. My 24 Pan gives me 67x and one degree fov, with a 3.8mm exit pupil. This works out well for tracking down DSOs, although I initially designed the scope for planetary from my old patio. It really delivers in that regard as well. Maybe some years from now, a 6" or 130mm f/6 will be more in line with what I want to carry out to the back yard. I have a 6" f/5, but having compared it to my 4.5" f/4 (my g&g), I kind of like the 4.5" wider rft views and light weight alt/az mount, versus the eq mnt with counterweights on the 6.

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[stargazer193857]

My first reflector was an 8" f6, and I used a 25mm Plossl and then a 24mm ES 82 deg eyepiece. I thought I had never seen coma. Star fields were so nice. Jon must be too picky, I thought.

Then I got a 130mm f5. Using the 24mm, I saw spikes on stars outside the middle 2/3. The spikes were at 60 degrees to each other, not the dashes seen in Erfle eyepieces. But I could ignore them as I looked at stuff in the middle. The spiky stars did not look like galaxies or anything fuzzy, so at least they were not fooling me.


Actually I forgot. My first newtonian was a 4.5" f8. I recall not finding a coma difference between it and the f6. Neither seemed to have any.


Ok, so after I looked through the f5 and saw coma staring me in the face (and sometimes it disappeared), I looked back in the f6. There it was. Tiny, and far out to the sides, but I then knew the 60 degree angle spikes were not from excess eye juice. That pattern is coma, with the rest of the smear either too faint to see or cut off by the secondary. The star field in the f6 was spectacular compared to the f5.

But then compare M13 side by side in 8" vs 10", and 10" is nicely brighter. Same for the upgrade to 12". You then have to ask how much aperture or five you can sacrifice, or how big of an eyepiece you want to buy.


I recall seeing a really nice star field once, in a 6" f5. Just looked better somehow. Maybe it was the 35mm panoptic he owned. But as many green lettered eyepieces as he had in his case, I bet he had a Paracorr in the focuser.

[stargazer193857]

I think if someone plans to buy a Paracorr and learn how to position it and learn how to collimate well, they might as well go big, like 15" f4. F5 is useable without one, and so is f4.4, but star fields are not as pretty as f6. Definitely a trade-off.

[KI5CAW]

I have three f/5 Newtonians and I love them all. The coma is not objectionable to my eye. That being said......remember that coma varies with the cube of the focal ratio. Thus an f/4 has eight times the coma of an f/8 at a comparable distance from the center of the field. There is a balance point between tube length and coma  in terms of focal ratios, and I have always felt that the balance point is f/6. I own f/5 scopes because they were standard issue from the suppliers; f/6 would be custom optics with a higher price tag.

My f/8 Newts have no visible optical aberrations at any point in the visual field. The f/5s have a lot more but are quite usable. F/6 would be perfect. If I were to buy another Newtonian optical set it would be f/6.

[KI5CAW]

Oh, and one final point....John Dobson disliked focal ratios below f/6 because they had too much coma. Even his huge scopes were f/6 or f/7. The modern trend towards lower focal ratios began when Dob makers in the 70s wanted scopes that were easier to move around. Nowadays f/3 seems to be popular. Dobson used to sleep in his telescope tubes while on the road. Hard to do at f/3!

[Klaus_Lehmann]

Greetings @ all!

 

For a couple of years I only read at cloudy nights, now I am in and this is my first post here, I really appreciate!

 

My Dobsons are slightly slower than f/6, the 10" and even the 12" (Truss built) and I find them very versatile for both, planetary and deep sky observation. Of course, the 12" guy is a little long, but given a small Secondary it works pretty well when it comes to contrasty images.

 

Clear Skies,

Roman

Edited by Klaus_Lehmann, 02 December 2020 - 07:55 AM.

[Bomber Bob]

Welcome to Cloudy Nights!

 

We like Eye Candy -- please post pix of your DOBs when you can.

[Klaus_Lehmann]

Here you are:

don't mind the dissolute counterweights, they are just mounted for Binoviewing.

 

[cuzimthedad]

Nice! Welcome to Cloudy Nights Klaus!

[Woj2007]

My first 'real' telescope was a 6" f/6 Newtonian, hand-figured, back in 1993. After recoating the mirror and upgrading mechanics a year ago it still serves me well for low-power views, planets, and for double stars, showing 'tight' Airy disks. 

 

 

 

 

 

 

[Cotts]

Here you are:

don't mind the dissolute counterweights, they are just mounted for Binoviewing.

 

Wilkommen, Klaus.  Your information says you are from Jena.  Isn't that the location of the Zeiss company?

 

Dave

[Klaus_Lehmann]

Hello Dave,

 

indeed that's it, so its almost a sacrilege to have no Zeiss lensscope in my portfolio :-)

I've been member to the local observatory in the past and had to use some really nice (but also heavy) Zeiss scopes like the legendary APQ 100, a 150/2250 AS Coudé-refractor and the Meniscas (Mak) 180/1800 - very impressing! Where I first lived (kind of suburban district) lived some former engeneers from Zeiss so I had some historical discussions about Zeiss instruments right at the garden fence. pretty funny. When you talk to those people, you can almost breathe the spirit of accuracy on what everything they do depends on. Today I live at the country side near Jena and only go to work in the city.

 

CS Roman

Edited by Klaus_Lehmann, 03 December 2020 - 04:24 AM.

[ed_turco]

My "sweet spot" will always be f/6, but a 4.25" f/10 Newtonian from Edmund gave images that were a joy to behold!  An amateur who obtained such a telescope got a splendid introduction to astronomy!

[25585]

Photo of an Orion Optics UK F6 10" such as mine.

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[stargazer193857]

There are those who prefer f7. Those are the pickiest of those who stare at the edge stars for coma, or maybe use Erfle eyepieces where 6 vs 7 does make a difference for astigmatism.

Focal length being equal though, would you rather have 6" of aperture or 7"? Aperture is already given up going from f5 to f6. For me, f6 was more than tolerable enough. I'd almost go f5.5 to get another inch of aperture.





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What I said above is only holding focal length constant. Some people will take longer focal length to ease collimation and get more native magnification. They are happy with a 68 deg afov and use 41mm as their widest eyepiece. They don't state at edge stars so much as like to let planets drift across the full view.

So f7 can be better for some, depending on their goals. I'd rather be able to sit, and have a lighter weight scope, for a given aperture. Proper pads allow tracking at high power.

Edited by stargazer193857, 06 December 2020 - 08:31 PM.

[Jon Isaacs]

Since the thread has now moved from the 6 inch F/6 to F/6 in general..  I had a 12.5 inch F/6 Meade Research Grade for 8-10 years.  It was on a GEM and very heavy, too heavy to take anywhere, particularly when I had my 12.5 inch F/4.06 Dob.  It was a driveway only scope.  But I always had the idea of building a Dob mount for it, it would have been workable.

 

 

Then I saw an Astromart ad for a 12.5 inch F/6 Starsplitter at a very good price that included an equatorial platform.  It was in LA, within driving distance.  I made the deal, drove up, picked it up..

 

When I got home, I got to looking through the paper work and stuff and there was a template for center spotting the mirror, it's said 13.1 inch Royce.  I sent an Email to the seller and he said, "Oh yeah, that was a 13.1 inch) 

 

So, no 12.5 inch F/6 for Jon, just a 13.1 inch F/5.5.  I almost always use the Paracorr 2 with it.  It's quite good without the Paracorr but the Paracorr 2 fully corrects the coma across the field of the 31mm Nagler in my 12.5 inch F/4.06, it doesn't make sense to put up with poorer correction in the F/5.5.  It's mostly an issue viewing the planets.  The coma free field at F/5.5 is 3.7 mm, that's about 7 arc-minutes.,  The Paracorr increases the sweet spot so it's the full field at any magnification.

 

 

Jon

[CHASLX200]

I would love a Meade 12.5" RG if my house was set up for it. No way i am gonna deal with lifting that tube up on the mount.

Edited by CHASLX200, 07 December 2020 - 06:54 AM.

[Jon Isaacs]

I would love a Meade 12.5" RG if my house was set up for it. No way i am gonna deal with lifting that tube up on the mount.

 

I lifted it on once.  It was quite something, I really should have someone help.  

 

I lifted it off once, that was when I parted ways with it. I did have someone help me.

 

Jon

[25585]

Orion Optics UK do an 8" F8 and 6" F11 for planetary viewing.