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nmoushon

Fast or slow F/Ratio for a DIY Dob?

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I'm thinking of making a Dob for a summer project. I'm not sure how big yet but probably going to be in the 8"-12" range. The basic design and construction principles seem easy enough through what I've read but the one thing I'm confused on is the F/Ratio. I see a lot of Dobs with fast f/5 ratios but I also see scopes with a slow f/10 ratio or even slower. For imaging I know what would be better but for visual purposes I have no clue. Whats the pros and cons of each?

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it's all about eyepiece height really I'd say. a 12" f10 would have a focal length of 120" and the eyepiece height would be about 9 feet off the ground. a 12" f5 would be about 6".

slower scopes (e.g. f10) are 'easier' on eyepieces showing less distortions generally in cheaper designs. although personally I prefer premium eyepieces even in my slow dob.

a longer tube is harder to balance, possibly has more flex and is harder to transport and make. that said, you use a smaller secondary so get less diffraction and more contrast. it's all a compromise based on your own preferences.

good luck - making your own scope is very rewarding.

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The maximum size is limited by roughly where your eye is when stood up.

Assuming about 5ft then 60 inches to your eye(s).

Slowest for a 12" would be about f/5, for a 10" about f/6.

Assumes that the mirror is fairly low down and that the height off of the ground and the lateral amount of the optical path from the secondary to the focuser more or less cancel out - seems a fair assumption for a finger in the air estimate.

I guess that most big Dobs are fast simply so that the observer can actually look through them all the time.

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I was fortunate enough to have conversations with John Dobson, both in the USA and the UK, on each occasion he bemoaned the fact that Dobsonians were using shorter and shorter F ratios, he felt that they should not be faster than F8!. From a practical point of view, accessibility of the eyepiece is a big factor. :smiley:

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For me its down to eyepiece height as im only short and dont want to be using a step ladder in the dark.alot can also be down to what you want to view as a nice f7 will give you a wider field of view than the same size primary at f5 due to less mag.im looking at an 18-22" dob next and it will be f4 if 18 and 3.6 if 20 or 22.

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if you are under 6" and want to avoid ladders or steps then really I think a 18" f4 is pushing it as are the others to be honest as you'd have to work hard to get the eyepiece height under 6 feet. roughly the focal length is 72" and the height off the ground to the mirror surface will negate any reduction across the tube walls. might be possible but do consider this. maybe a small step is OK though.

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Ok so basically its down to if you want to use a step latter or not and the quality of eye pieces you want to buy. The faster the scope the better the eye piece needs to be and slower the scope the more it can handle lower quality ones without it showing? Is that right? Other than that the quality of the imagine you see through the eye piece wont be better or worse based on the speed of the scope then? I hope I'm understanding this right.

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im currently making a 16inch f5 with my pal, grinding our own mirror. its a great project to do. hope you have fun doing yours

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if you are under 6" and want to avoid ladders or steps then really I think a 18" f4 is pushing it as are the others to be honest as you'd have to work hard to get the eyepiece height under 6 feet. roughly the focal length is 72" and the height off the ground to the mirror surface will negate any reduction across the tube walls. might be possible but do consider this. maybe a small step is OK though.

If you are under six inches it would be easier to build a little platform just below the focuser as you will really struggle moving a pair of steps.

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I'm thinking of making a Dob for a summer project. I'm not sure how big yet but probably going to be in the 8"-12" range. The basic design and construction principles seem easy enough through what I've read but the one thing I'm confused on is the F/Ratio. I see a lot of Dobs with fast f/5 ratios but I also see scopes with a slow f/10 ratio or even slower. For imaging I know what would be better but for visual purposes I have no clue. Whats the pros and cons of each?

The thread so far is very focused on height above ground (stepladders or not). I will add a different perspective, namely what you are most interested in looking at!

If you are a "solar system bum" and focus on planets and such then you want a slow scope, the longer the better, just get a bigger step ladder. On the other hand, for DSO and such you want wide views so the slower the better. Of course, a slower scope introduces its own restrictions (harsh demands on eyepices, collimation, coma control and so on).

Generally though, I'd say build a scope that fits your particular interests, not you height above ground. Of course, if you are an omnivore interested in everything then you want the scope to reflect that as well (avoid and F/4 configuration for example). Building your own scope is an opportunity to get one tailored to your needs, both in terms of optics and ergonomics. By all means use the opportunity to the max, standard aperture ratios are available in spades at your nearest astro vendor so don't be afraid to be radical.

HTH

Steve

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I can't edit so I'll correct my typo here:

"On the other hand, for DSO and such you want wide views so the slower the better."

should of course be:

"On the other hand, for DSO and such you want wide views so the faster the better."

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And naturally:

"Of course, a slower scope introduces its own restrictions"

shoudl obviously be:

"Of course, a faster scope introduces its own restrictions"

Sorry my brain locked up there.

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I don't agree. I use my 16" f4 scope, masked off to 170mm (creating a 170mm f11 unobstructed view) and this is far better on planets than anything else I have looked through. you can have the best of all worlds with a fast large newt.

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I don't agree. I use my 16" f4 scope, masked off to 170mm (creating a 170mm f11 unobstructed view) and this is far better on planets than anything else I have looked through. you can have the best of all worlds with a fast large newt.

....and that makes +2.

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I don't agree. I use my 16" f4 scope, masked off to 170mm (creating a 170mm f11 unobstructed view) and this is far better on planets than anything else I have looked through. you can have the best of all worlds with a fast large newt.

Fair enough, but you still have a 16" dob to lug around for what has in effect become a 170mm F/11. Which is fine if you want both, but not if you don't. So I think it's still relevant to consider what one is looking at.

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The use of stepladders is only going to be needed when observing up around the zenith in the "Dob hole". This isn't the best place to use a Dob anyway. Keep your observations down a few degrees and your aperture big :D

Sent from my iPhone using Tapatalk

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The thread so far is very focused on height above ground (stepladders or not). I will add a different perspective, namely what you are most interested in looking at!

If you are a "solar system bum" and focus on planets and such then you want a slow scope, the longer the better, just get a bigger step ladder. On the other hand, for DSO and such you want wide views so the slower the better. Of course, a slower scope introduces its own restrictions (harsh demands on eyepices, collimation, coma control and so on).

Generally though, I'd say build a scope that fits your particular interests, not you height above ground. Of course, if you are an omnivore interested in everything then you want the scope to reflect that as well (avoid and F/4 configuration for example). Building your own scope is an opportunity to get one tailored to your needs, both in terms of optics and ergonomics. By all means use the opportunity to the max, standard aperture ratios are available in spades at your nearest astro vendor so don't be afraid to be radical.

HTH

Steve

Thanks for the input Steve. Appreciate it. I do want to view both planetary and DSO objects....with maybe an attempt at an asteroid or two. Oh and with this comet of the century coming about I definitely want to be able to view that too.

Also a little confused on what Moonshane suggested. So basically I could make a 12" - 1200mm (F/4) Dob then build a dust cover with a hole in the middle that would bring it down to 6" of open aperture which would then make it a 6" - 1200mm (F/7.8) Dob? Would I make this mask to fit and cover up at the front of the scope or make one that I place over the mirror?

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Wow! Great write up! I dont know why more people do this. Will definitely be doing the mask. Now just to decided on primary mirror size....and to get the mrs not to look at the credit card statement for a couple months during the summer. Thanks for all the help and suggestions everyone.

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