budget eyepieces and fast scopes

[davekelley]

Perhaps more of a moan than a question but anyway here it is.  I have a Skyline 150p Dob and I've been using budget revelation 2 inch 30mm and 42mm ep's and i just love the views and I'm going to purchase the 2 inch barlow next.  I am told that as you go to faster ratios, these ep's start to perform less well.  My question is I would consider upgrading to an 8 or 10 inch dob, but why iis the 8 inch only an F5.9?  If it's easier to produce a mirror  with a longer focal length, and the extra height of the eyepiece/finder, would also be a great advantage, why do almost all 8 inch dobs stick with the 1200mm f/l ?   (I usually put mine on a small side table when in use).   

[Timebandit]

Hello. My understanding is different focal lengths of scope serve different purposes . That is a fast scope at around F5 or below is better for DSO. And a slow scope say F10 is better for planetary use. So taking the logic of this then I suppose something around F5.9 is a compromise and therefore a more all rounder. And yes when you start to go below about F5 it is more testing on eyepieces,so better quality needed. Where else in slow scopes around F10 then you can get away with not being as fussy with your eyepiece choice , but I still recommend a decent eyepiece 

I hope this helps and you like the reply. Thanks☺

[ronin]

Very likely it is just to sell scopes. A 10" (250mm) f/8 would put the eyepiece at something like 2 meters high. Could be a bit more as the mirror can not be at ground level just close, but I have assumed that the mirror height is cancelled out by the secondary to focuser distance. Top of my head is a miserable 1700mm off the ground and my eyes are less again by say 200mm so say about 1500mm. So to sell a scope to the genewral public the eyepiece really needs to be no more the 1500, maybe 1550mm off the ground. A longer focal length mirror 10" f/8 in effect would exclude people that are less then say 2.2mtr tall. Not a godd marketing ploy.

So larger diameter dobsonians scopes have little option then to be fast. A newtonian on a tripod is even worse as the mount height adds to much of this.

You are right a "slower" mirror is easier to make and more tolerant of aspects like collimation and not so costly eyepieces, but they first need to be sold to people and making a sort of "minimum height restriction" is not the best approach.

Similar in a nice car, you may never drive it at 130mph or anywhere even close, but you have by law to have tyres that are capable of matching the cars potential perfromance. So just accept that you will need reasonable eyepieces.

Likely less considered but a 10" f/8 OTA is some 80" long, you have bits above the secondary to consider that make up the tube length, that is not easy to transport.

[MarkVIIIMarc]

To my newb eyes I'm happy with eyepieces in the $50 to $80 range for my Z10.  I am sure the $,$$$ ones are better, but I have kids, a Mustang, other hobbies, healthcare and taxes to pay.

To borrow on an automotive analogy, that Bentley sure is nice, but is it worth the premium over the Cadillac?

Try some expensive eyepieces in your scope at a star party or meeting and see if they are worth it to you.

 

[wookie1965]

I have a 150p f5 and a 200p f5 I too wanted to update my eyepieces I took advice and punted for  an 8mm and 18mm BST ED Starguiders and I have never looked back they are brilliant I have 6 of them now, great for spectacle wearers as well. 

[rwilkey]

Hi Dave, my advise is to NOT buy the 2" Barlow, it's a bit pointless with widefield low power ep's such as the 30mm and the 42mm, also you may get viginetting which is not good, so it will be a waste of money.  If you want more power go for a 14mm 82 series ES or similar, the 68's are also good value and will work well in your scope, if you go for more power I suggest you go no higher than the 8mm = 150x.  Good luck with your choices.

[Moonshane]

The reason they use the same sort of tube length is probably to allow effective filling of space in the shipping container from China.

Any eyepiece will work in any scope. It's humans that are fussy

[Charic]

On 7/23/2016 at 09:38, davekelley said:

 would consider upgrading to an 8 or 10 inch dob

I have the 200P and its a good scope, and at f/6 (f/5.91) its pretty much ok with the eyepieces I use, and your present eyepieces would work just fine on the 200P Skyliner.
If I were to upgrade I would like a 300P solid tube at f/5 (f/4.9) over the 'faster'  f/4.8 10" scope? That's just my preference.
If my BST's were to fail my expectation on the 12", then maybe  the TeleVue Delos may be a better option, as their assured to work down to f/4? but on axis, all my eyepieces will still work well enough for my needs of visual only observations.