Stupid Happy Harry

[happyharry]

Who in here knows a whole lot about focusers for Newtonian scopes and can comment authoritatively on low profile focusers as opposed to focusers with more travel? I am building a 10" f/6 DOB with a thick (2"), beautiful Discovery mirror and protostar 2.14" secondary.

I bought a cheapie rack and pinnion, 2 inch focuser with 1 1/2" travel for experimentation on a test tube in that I anticipated its precice placement on the tube would be critical with this little amount of travel. Was I ever right! Run out of rack-in and you're totally screwed with a main mirror you cannot move up or down on that tube.

I have positioned this focuser at three different locations and have come to the conclusion that it's far safer to lengthen the focal plane from the secondary to the focuser top by placing the focuser hole closer to the main mirror for maximun in-travel.

I have also come to the conclusion that I had better buy a 2" focuser with 2.5" or more travel when I do the final installation of the optics. Being quite ignorant of the fine points of telescope optics, and the little information out there about precise focuser placement and how it relates to Barlows and different eyepieces, If I had it to do all over again I would have designed a jig to move that main mirror up and down the tube three inches in each direction. While I can't do the math, I am learning that different eyepieces and Barlows have a very direct effect on just where that total focal plane ends up.

The good news is I figured out how to get the collimation spot-on with just a Cheshire, and last night's view of the moon with a 6MM, 1.25" eyepiece was nothing short of spectacular! Of course, the moon is in our backyard, so I don't know how this will translate when I train this thing on far more distant planets.

Maybe I should have stuck to building fine chests and tables!

Not So Happy, Happy Harry

[Yigal]

Download Newt on
http://home.att.net/~dale.keller/atm/newtonians/newtsoft/newtsoft.htm

[happyharry]

Yigal, I know all about figuring the total focal plane of an objective, its integral relationship to the focal plane from the objective to the secondary, and the secondary to the eyepiece, which in "theory," at least, resides at the top of the focuser. You missed entirely the thrust of my concerns here, and rather assumed I was too stupid to figure this out. BTW, that download you referred me to, and I have seen many others refer to it, is the most convoluted piece of crap I have ever seen, both in its formatting and astonishing difficulty in unraveling.

Sir, I was here talking about focuser "travel" and its very crucial relationship as it bears on attaining maximum flexibility with respect to "in-and-out travel," particularly as it bears on the use of differeing eyepieces, and especially the use of Barlows. I can tell you right now that my very extensive fooling around with the longitudinal placement of this focuser on an identical test tube shows me without a doubt that the use of different eyepieces, and especially Barlows, dramatically affects the theoretical and/or actual focal plane of the objective and shoots these fancy theories all to hell.

BTW, I bench-tested the focal plane of this mirror to a fare-thee-well and it is spot on at precisely 60 inches. I also have this scope so well collimated that on bright star tests I get perfectly concentric rings out of focus even better than what I see in the books. They're picture perfect. Moreover, the Cheshire is spot-on, dead center with the primary center mark within two millimeters, absolute maximum. This is a near-perfect collimated scope. The tube is so straight, and the ends cut so square, that the primary reached perfect collimatarion at less than a 1/32 of an inch at the two opposing push-pull bolts.

Anyway, want an example of what can happen with Barlows and differing eyepieces, Yigal? The half-moon tonight was at about 45 degrees and offered perfect contrast for focusing details of craters and such. With the focuser on this tube placed precisey at 50 inches from the miiror, the distance from the secondary to the top of this focuser at half-rack out being exactly 10 inches, I got the following results with two eyepieces and a 2X Barlow:

Using a 6MM Plossl I attained perfect focus racked out to 5/8th of an inch. Not bad. Using a 26MM Plossl, I attained perfect focus racked out to 1 1/8th inch, close to the limits of this focuser.

Now using a 2X Barlow with this same 26 MM eyepiece, I had to rack all the way in to 1/4 inch to attain perfect focus. Using the 2X Barlow with the 6 MM eyepiece, I lost it all on the rack-in - not enough left to bring the moon into focus. See anything wrong with this picture?

Do you now see what I am talking about when I express "concern" about focuser travel and how it relates to Barlows and differing eyepieces? Sir, the "actual" as opposed to the theoretical focal plane is directly impacted by eyepieces and Barlows, and never resides at the top of that focuser - that much I can now tell you after many months of screwing around with this very, very real problem of rack-n and rack-out. All your assumptions rest entirely on that portion of the focal plane from the secondary to the focuser top as a mathematical constant irrespective of differing eyepieces and Barlows, and it just ain't so. And I knew it wasn't so the minute I started looking at these vastly different eyepieces and Barlows!. Nothing like testing "theories" as I always say.

Yigal, I posted in here almost a year ago as" Itscolderhere." For reasons of privacy and what can happen with internet searches, I now post as Happyharry. In any case, back then I expressed these same concerns, and they were souughed off, or not directly addressed. However, instinctively I knew there was something very wrong here, especially considering the very low margin of error for short-run focusers and what can happen with different eyepieces, adaptor collars and Barlows.

As a builder of fine furniture, and with a "compleat" workshop, I will figure it out on my own. One thing I DO know is that if you run out of "in" focus, you're totally screwed with an objective you cannot move. If I am going to err, even slightly, it will be on the side of extending the focal plane from the secondary to the focuser to assure maximum run in of the focuser. I can then always "add" something on the other end.

This is also why I will buy a 2 inch focuser with as much travel as I can find. And even then, I will reverse this tube, tape the old holes, and experiment again and again so as to maximize the point of run-in and run-out. Only then will I invest in a top-notch, 2 inch focuser and very expensive 2 inch eyepieces for this scope. I knew you guys were wrong from the get go. Always trust your isrtincts, I say, especilly if you have an unerring sense of what's wrong with this picture.

Yours truly,

Happytharry

[grendel1960]

yes it would be better to have a little too much in travel as that could accomodate a cameras focal plane at approx 42mm outside if the focuser, and for eyepieces that focus too far out a set of extenders could solve that one, a lot of it is dependant on the size of your secondary and wether you can get the full image size reflected back at the various distances - this can be where the newt software comes in as you can test various configurations for vignetting etc.
I do realise your concerns re barlows and different eyepieces, this is a perennial problem with newtonians as if you make too much travel on the focuser on full in travel the focuser tube can be blocking some of your aperture inside the tube (you see this a lot on factory made newts)
Grendel.

[pizwiz]

You're still obsessing sbout that ?
Get used to the fact that not all eyepiecees and eyepiece/Barlow combinations will work in your scope. The rest of us have to.
The solution is simple: Place your focuser so that the combination that requires the most in-travel comes to a good focus with just a bit to spare.
Then if some combination requires more out travel than the focuser has, use an extension. Although most situations could probably be corrected with a simle par-focal ring.
I don't care if your focuser has travel from here to the moon, you will always come across some combination that won't work sooner or later.
I also have a 10" Newtonian and there are indeed some eyepieces that won't come to focus in it. That's one of the reasons Dealers have return policies.
Quit trying to solve a problem that doesn't have a solution.

Adolph

[Yigal]

Harry, sorry I misunderstood your problem. Well, I don't know your mirrorcell design but some friends of mine put their mirror deliberately relatively far in the tube and make the mirror cell in such a way that in some cases the setscrews can be utrned a few turns so the mirror drops a few cms. Personally for my 10" Lurie telescope I calculated that the focus lies 75mm from the top part of the focusser. For photography I have then plenty of space for camera and equipment. For visual performance it might be just edge on and otherwise I elongate my focussertube.

[Boot]

So, ahhh, gentlemen, now that everyone's gotten that off their chest, lest we forget the forum category here, what exactly is "for sale?"

Boot

[andy123]

OK I have a helical style focuser with 2.5" of travel that I can sell you for about $55.00. It will be like the ones I sell on eBay except with a longer barrel.

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=330148074468


PM me for more details
Andy