7 replies to this topic

[astrosully]

I dont know much about ATM (yet) but I was playing around with https://stellafane.o...b/newt-web.html and a 40" newtonian.  I found that the focuser would have to be 4 inches wide to accept all the light!  How would this work with only 2" eyepieces?  I'm not going to be building this but I would like to know.  

 

Clear Skies!

[havasman]

Could you not move the focuser further down the cone of light, further away from the primary? There are many very large aperture Dobs that use 2" focusers.

[luxo II]

The trick is to choose a "low-profile" 2"focuser with short draw tube, which will not cut off the light cone to the eyepiece. The downside is the available travel is quite limited (25 mm or less) meaning you might need to use adapter rings or extensions for some accessories eg cameras.

 

Just because the scope is big doesn't mean the focuser needs to be; the constraint is f/ratio, not aperture.

 

F/4 is f/4. regardless of whether the scope is 30cm or 100cm aperture.

Edited by luxo II, 05 August 2024 - 06:37 PM.

[Dale Eason]

Realistically there probably are only and handful of scopes that large.  Most ATM scopes are even under 30 inches.  

[Sean Cunneen]

Vignetting is a real problem when scopes get big. When I was actively building my 6" f15 refractors and calculating the baffles it struck me that anything shorter than f15 vignetted most focusers for most eyepieces. That meant that 6" f10, or all of those 6"f8 yard cannons were really only 4" scopes. Don't get me started on those f3 stubby newts! You need a 6" dia eyepiece to get anything out of those... A lot of builders get so focused on one feature of a scopes design they forget a scope is just one part of a complex optical system. If you do the math to optimize as many specs as you can you end up with an 8" f8 Newtonian using 1.25" eyepieces everything else is a mirage!

[Dale Eason]

I don't think you are correct.  I go by Mel Bartel's telescope calculator and his experience. https://www.bbastrod...wtDesigner.html

Edited by Dale Eason, 07 August 2024 - 11:25 PM.

[brebisson]

Hello,

 

I completely agree with the comment about lots of scope vignetting...

Lots of COMMERCIAL scopes! I have seen SO many 114*900 or 200 or 250 with draw tube so long/secondary so small that there is NOT EVEN A SINGLE SPOT where you get 100% of light!!!!

 

Now, in my experience, most ATM scopes are better than that with better seocndary/focal plan placement.

 

Cyrille

[Astrojensen]

Vignetting is a real problem when scopes get big. When I was actively building my 6" f15 refractors and calculating the baffles it struck me that anything shorter than f15 vignetted most focusers for most eyepieces. That meant that 6" f10, or all of those 6"f8 yard cannons were really only 4" scopes. Don't get me started on those f3 stubby newts! You need a 6" dia eyepiece to get anything out of those... A lot of builders get so focused on one feature of a scopes design they forget a scope is just one part of a complex optical system. If you do the math to optimize as many specs as you can you end up with an 8" f8 Newtonian using 1.25" eyepieces everything else is a mirage!

What absolute nonsense is this. I have one of those 6" f/8 Yard Cannons (Sky-Watcher, blue tube) and it most certainly worked at full aperture. I know, because you could see the foil spacers from the eyepiece end. 

 

There is a big difference between vignetting the off-axis rays and on-axis vignetting, which is more commonly referred to as stopping down. 

 

 

Don't get me started on those f3 stubby newts! You need a 6" dia eyepiece to get anything out of those...

????? What do you even base that on? 

 

 

Clear skies!

Thomas, Denmark