Burgess binoviewer barlow barlow buster

[Mark Elkington]

Hi All,

I posted this at s.a.a. recently but have had only deafening silence so far.

Any help or comments on this appreciated. I'm thinking of modifying my GS 8 inch f/6 dob to use a binoviewer.

I want to avoid the need for a barlow, compensator, OCS etc, so plan to move the secondary & focuser back by the typical focuser in-travel requirement (110-130mm, S&T March 2005 p98), say 4.5 inches, and maybe use an extension tube for one-eyed viewing.

Punching the numbers into Newt V2.5, this results in no vignetting with my current 51mm secondary.

Some questions:

- Is this a dumb idea?!

- When using a binoviewer without a compensator or barlow, does the bino itself change the magnification at all? In particular, the Burgess Optical binoviewer? See http://www.astromart.com/articles/article.asp?article_id=180

- The cited mentions an ep focal length of around 20mm before noticeable vignetting with the Burgess 1.25 binoviewer with 18.5mm clear aperture - sound about right? Interestingly, at the moment they're for US$199 with a free pair of 17mm WA eps.

- Any other thoughts on binoviewing with a newt? (I guess I want to avoid if possible the $ and optical flaws of the Denk combo of BV+OCS+Power Switch, S&T March 2005 p92, i.e. image vignetting and off-axis distortions with power switch).

- Anyone had experience with the Saxon binoviewer (AU$399, e.g. http://www.andrewscom.com.au) ?

Thanks!
Mark (Blue Mountains)

[RAJAH235]

Hi Mark, & welcome. Sorry I can't help out with your questions on the binoviewer. Never come across one before. :D L. :welcome:

[gyro topple]

Gday Mark,
I have the Burgess Binoviewer (with 20mm EPs) and though I can get them to focus on my refractor and LX90, they just miss out on the Dob. I have not noticed any increase in mag when I have used them but I will take another look and see if there is any noticeable extra mag.

I found that the BV worked fine on the Dob if I used a 2x barlow though the extra weight up front needed a fair bit of counterbalancing.
Mine came with a little slip of paper (that I have subsequently lost) that mentioned a 21mm clear aperture and it was the prisms that were the limiting factor and that some prism clipping? could occur. I haven't seen any negligable light falloff off axis but you might notice something if you have a serious wide FOV eyepieces.

[gbeal]

Hi Mark,
I use the Denk, and OCS (2"), in a 10" f5 newt, as well as all my other scopes (some with and some without the OCS).
My gut feeling is leave the scope as is, and use an OCS, even if you buy one to try and eventually go the way you have described. Making such radical changes to the scope simply for bino viewing seems a little excessive to me.
I have the focus point on the newt quite a way out, and use about 50mm of extension to view with an eyepiece. This allows me to slip a camera or whatever into the optical path and come to focus. It still is not enough for the bino viewer sans OCS.
I haven't noticed any real change in magnification without the OCS (I sometimes use it like this on the Tak Mewlon straight through), and can see no reason why it should. With the OCS there is a slight (evidently) change.
For lunar and solar viewing there is no comparison to using the bino viewer, but for deepsky etc, I still prefer cyclops.
Gary

[mch62]

Might be easier to move the mirror back instead of cuting new holes and filling old for the focuser and diagonal.
Simple make some 100-130mm brackets to attach to the original mounting points on the cell and tube.
If you slotted the mounting holes in the brackets you could make it adjustable to get the right position.
Once you have the desired length make a shroud to go around the gap from the cell to tube.

Even easier make a truss tube :simple to adjust the truss lengths
:D

Mark

[Mark Elkington]

Thanks all for your replies, quite helpful.

Gary, interesting you still prefer one eye for deepsky - is that due solely to light loss of BVs?

gyro topple, thanks for the first-hand feedback on the Burgess unit - where did you get them from?

Mark - don't I need to move the primary mirror forward? I could shorten the tube at that end, but its a bigger hit on tube balance.

Cheers,
Mark

[gyro topple]

I got mine Burgess BV from http://www.highpointscientific.com/store/viewItem.asp?idProduct=191 - for the same price you have mentioned. They came with 20mm Burgess Wide Angle EPs. I don't recall how much I paid in shipping but everything from North America seems pretty expensive to send anywhere.

1 thing I forgot to mention - The 3 nylon screws that hold the EPs in place SUCK the big one! There is only about 3 turns from tight to falling out and I have already lost one of the screws (works fine with 2 though) I would have much preferred some form of compression ring. The excuse for grease (more like a glue) on the helical focussers is typical of that found on cheap mounts and binoculars.

Adam

[gbeal]

Hi Mark,
yes, possibly the biggest reason is the loss of light. OK it is nice to use both, but if the object is dim I reckon the view is better with the single eyepiece.
Hey great idea from Mark though, this would allow you to move the mirror, and not damage the tube. Even if you made a blanking disk, and used the existing holes, then mounted the mirror cell onto this, thus bringing it forward (I think it needs to go forward as well).
Gary

[Mark Elkington]

Hi Adam,

Thanks for that info. I've read some negative comments elsewhere about Highpoint Sci delivery times, but at least they ship OS.

Interesting that yours came with 20mm WA eps, their now offering 17mm which may be be to back off from the vignetting threshold. Interesting too that your unit quoted 21mm clear aperture; Burgess are now specing them at 18.5mm.

Anyway, at the price who could complain (that's the thread down the hall, BTW :-)

Mark

[Mark Elkington]

Hi Gary,

Some poeple say you lose some image detail and sharpness with BVs, which is always possible with more glass in the light path. Standard Denks are 1/4 wave, Denk IIs are 1/8, so unless 1/8 is just a marketing ploy, I deduce that the Std version introduces at least some distortion. Although the ST review didn't note any different, but it would be subtle and something you'd notice over time.

Re tube mods, the trouble with moving the primay in by 120mm is the mirror cell is attached to the base plate which attaches to the tube. Although...if you used longer collimation screws and locking screws you might be able to gain an inch or two before the whole assembly went wobbly.

I'll report back here with any success (or failure!).

Mark



yes, possibly the biggest reason is the loss of light. OK it is nice to use both, but if the object is dim I reckon the view is better with the single eyepiece.
Hey great idea from Mark though, this would allow you to move the mirror, and not damage the tube. Even if you made a blanking disk, and used the existing holes, then mounted the mirror cell onto this, thus bringing it forward (I think it needs to go forward as well).
Gary