14 replies to this topic

[Tom Duncan]

Working on a 'personally made' (I hate the term 'home made', implies poor design/construction...definitely not the case with this one) 12.5" f6 75.5"/1918mm focal length truss DOB, photo attached. 

 

It seems nowadays most truss DOB's are f5 or faster, higher end custom makers commonly offer f4.5 and faster, this one is f6. The secondary is quite small, only 1.5" in the minor axis. Without looking up all the makers of truss DOB's it seems to me such 'slow' DOB's are not popular. 

 

Is this what would be considered a planetary scope? When I got it and assembled it I thought it was quite tall for a 12.5" mirror and the secondary small, much smaller than my Meade Lightbridge 10" f5. 

 

Tom Duncan 

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[siriusandthepup]

Very interesting scope.

 

My 1st reaction - that secondary is too small.

 

Running the calculations  though, its possible with some extra special clearance attention. Does your scope have that?

 

Let's find out.

 

1.5" secondary on f/6 scope -   1.5" x f/6 = 9" maximum to focal plane from center of diagonal. (no vignetting, but no fully illuminated field either) No fully illuminated field is OK for a strictly planetary telescope.

From primary center to primary edge - 12.5"/2 = 6.25"

So of your available focal throw is now 9" - 6.25" = 2.75"

The 2.75" must accommodate the clearance from the inside of the cage to the clear aperture incoming light path and the focuser height to the final focal plane.

 

Translation: the gap you leave around the primary so as not to vignette the field of view and the eyepiece focal height above the outside of the cage baffle.

 

Now for a strictly planetary scope the total field of view can be sacrificed and you can close up the gap. Normally I would allow 1" on a scope like this. This would means 12.5" + 1" +1" = 14.5" for the diameter of the cage baffle. Let's go nuts and make the clearance 0.25" Now that puts us at 6.25" + 0.25" = 6.5" of the 9" taken up leaving 2.5" for focusing. That is doable with a low profile focuser with a little bit left over to allow actual focusing.

 

Tom, can you post a few more pics? My curiosity is peaked.

 

 Cage baffle inner diameter?

 

Focuser type - pic please.

 

Focal plane distance above cage baffle? You can either give the location of an eyepiece field stop at focus or just use a piece of paper above your empty focuser to give a sharp prime focus projection of the Moon.

 

Does it have high spec mirrors?

 

From the photo - this scope appears to be nicely constructed.   It's pretty!

[Astrojensen]

 

'home made', implies poor design/construction

Why do you think that? It sure doesn't to me. A homemade scope CAN be poorly made or constructed, but so can a commercially made one. 

 

 

Clear skies!

Thomas, Denmark

[Piero DP]

12.5" f6 75.5"/1918mm focal length truss DOB.

Is this what would be considered a planetary scope?

Yes, back in 90's.

I have a 12" f6 and 16" f4, the latter used with a TV paracorr 2. The secondary in my 12" f6 is 2.1", whereas in my 16" f4 is 3.5" (!).
My 16" f4 consistently outperforms my 12" f6 on planetary targets as well as the moon.

My personal take on this subject is that there is an obsession about tiny secondaries. Unless you have a substantially large CO, maybe >25-30%, I really doubt one would notice a difference. In contrast, a too small secondary can have detrimental effects in the sense that 1) it can reduce the full aperture of the primary mirror, 2) it can make collimation over critical, 3) the edge of the secondary, which is not as well corrected as the centre, is used.

In my experience, which is limited compared to some CN gurus but is not even at introductory level, what really counts for excellent planetary observation are:
- excellent seeing
- primary mirror cooled down AND maintained at ambient temperature (fan at the back)
- removal of the warm air boundary layer, 1-2" above the primary mirror surface (some people use side fans, but I find that if the mirror box is shallow enough, lifting up the light shroud 3" from the bottom is sufficient to get rid of this warm air - I typically gain 100-150x doing so).
- excellent primary, secondary, focuser collimation
- good baffling
- blackening the opposite side of the focuser with anti-reflection material
- planets above 45-50 degrees from the horizon to reduce atmospheric diffraction
- high quality eyepieces
- a good dose of patience / relaxation

Regarding the "theory" that less eyepiece glass makes a significant contribution for planetary observation, assuming the above points, my opinion is that it can be, but this is largely overstated. An increase in aperture has a way larger effect. For instance, all my dobsons including the 8" f6 Skywatcher I have in Italy outperform my 4" Tak f7.4, independently of whether I use eyepieces like TVs, orthos, Zeiss zoom, Docter, etc, and my 4" Tak always outperforms my TV-60 f6 (2.6").

Edited by Piero DP, 11 January 2025 - 11:53 AM.

[Garyth64]

1.5" secondary?  Without knowing more information, and seeing more pictures, it just doesn't seem right.  But I can see how it would work.

 

Playing with some of the math, the first goal is to have the secondary get all the light from the primary.

 

If this is considered to be a planetary scope, then the image plane used would be very small like maybe .1" in diameter or less (just a guess).  This scope has a Moon diameter of .68" at the image plane.  Just the cone of light for a single point of light, or a planet, is 1.49" in diameter at 9".  

 

So what is the distance you used between the secondary and the image plane?

Edited by Garyth64, 11 January 2025 - 09:33 AM.

[Scott E]

I have 12.4" f/5.5, so very close to yours. My secondary is 2.1" minor axis and I wouldn't recommend anything smaller. Not just because you may not fully illuminate even the very center with the whole primary, but also because below a certain percentage obstruction, the benefits are very small. Your spider design becomes more important for diffraction and contrast.

 

And I don't really understand your question. A planetary scope is any scope that performs well on the planets. The design of your scope should theoretically perform well on planets but mirror quality is probably most important of all. If your scope performs well on planets, then it's a planetary scope.

[MeridianStarGazer]

F6 is definitely planetary, and f5 is for dso. When looking at galaxies, some coma is not make or break, but extra aperture helps a lot. When looking at planets, sharp at the edge is important, and extra aperture does not always help. If one uses a coma corrector and fancy eyepieces, they can have it all at f5, but that is a lot more glass.

[Don H]

When I made my 12.5" f/5.9 (73.75"), I wanted it to be well suited for planetary observation. It has a 2.1" diagonal. The Pyrex primary mirror is only 1" thick, so it cools faster than those that are typically thicker. The lightweight mirror did result in a taller mirror box, but I preferred doing that versus adding counterweights. It does a fine job on planets, but is also very nice for DSO observing. Soon after I made a 10" f/6.3 with a 1.83" secondary, as I was enjoying the viewing comfort of the longer FLs. The 12.5" needs one step up to view at the zenith, but the 10 keeps my feet on the ground when looking straight up.

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Edited by Don H, 12 January 2025 - 01:24 AM.

[CHASLX200]

Working on a 'personally made' (I hate the term 'home made', implies poor design/construction...definitely not the case with this one) 12.5" f6 75.5"/1918mm focal length truss DOB, photo attached. 

 

It seems nowadays most truss DOB's are f5 or faster, higher end custom makers commonly offer f4.5 and faster, this one is f6. The secondary is quite small, only 1.5" in the minor axis. Without looking up all the makers of truss DOB's it seems to me such 'slow' DOB's are not popular. 

 

Is this what would be considered a planetary scope? When I got it and assembled it I thought it was quite tall for a 12.5" mirror and the secondary small, much smaller than my Meade Lightbridge 10" f5. 

 

Tom Duncan 

Dobs are the best planet scopes.

[Tom Stock]

I have a 12" f6 and 16" f4, the latter used with a TV paracorr 2. The secondary in my 12" f6 is 2.1", whereas in my 16" f4 is 3.5" (!).
My 16" f4 consistently outperforms my 12" f6 on planetary targets as well as the moon.

Glad you mentioned this.. I almost convinced myself to replace my 10" F4.5" OTA, mounted on a 2" shaft GEM, with a 10" F6.5 OTA which I saw for sale.

 

I thought about and decided for the length and weight of a 10" F6.5, I'd get much better results from a 12" F4.5.

 

I guess I made the right decision by passing on the 10" F6.5.

Edited by Tom Stock, 12 January 2025 - 08:59 AM.

[Rusty35]

Why do you think that? It sure doesn't to me. A homemade scope CAN be poorly made or constructed, but so can a commercially made one. 

 

 

Clear skies!

Thomas, Denmark

ATM'ers probably build the best of the best scopes,

[Tom Duncan]

Hey folks, thanks for all the input, I'll study it over the next few days and reply with specifics. 

 

Tom 

[MeridianStarGazer]

Glad you mentioned this.. I almost convinced myself to replace my 10" F4.5" OTA, mounted on a 2" shaft GEM, with a 10" F6.5 OTA which I saw for sale.

I thought about and decided for the length and weight of a 10" F6.5, I'd get much better results from a 12" F4.5.

I guess I made the right decision by passing on the 10" F6.5.

On an eq mount, the planet stays centered, so coma and astigmatism of f4.5 is not an issue. But for a non tracking dob, I'd take the f6.5, no cc needed.

[Tom Stock]

On an eq mount, the planet stays centered, so coma and astigmatism of f4.5 is not an issue. But for a non tracking dob, I'd take the f6.5, no cc needed.

Totally agree, I already own a CC but I agree I would prefer a slower scope over a faster scope. The problem becomes length for an EQ... length is almost worse than weight.

[Tom Duncan]

I've been fiddling with this scope for the last several nights but will have to pause for a week or two. I'll respond after I get back with some results, insights and more questions. 

 

Thanks

 

Tom