New Dob for mirror refigure job?
Posted 19 November 2012 - 05:36 PM
Posted 19 November 2012 - 06:47 PM
Posted 19 November 2012 - 07:10 PM
Posted 19 November 2012 - 07:44 PM
Thanks Gene, I guess I should have been more specific. I wanted to have something inexpensive to acquire that can be refigured to give premium views without the added cost of a premium dob structure. Wondered if I could get a premium optics view contained in a very basic (and inexpensive) package. I would trade some ergonomics to put more money into the primary/secondary.
The risk your running with buying a cheap mirror and having it refigured is in the anneal. It may not be able to hold an ultra precise figure. Glass type used in a cheap mirror may also be an issue. I would buy the premium optic from the get go, if you can afford it. Zambuto was offering a mirror exchange program for 10" mirrors. You could always try that route. you didn't say what size scope you were interested in.
Posted 19 November 2012 - 07:53 PM
Posted 19 November 2012 - 07:57 PM
Posted 19 November 2012 - 08:15 PM
You don't need to go out and buy one of those telescopes and get the mirror re-figured. A lot of the Chinese dobs are very good right out of the box having good mirrors in them.
I got lucky with my 10" Skywatcher on a trade. I never even tried looking through the telescope before I traded mine for it and got a good mirror, LOL.
But really, most Chinese dobs are good out of the boxes with a few duds here & there.
Posted 19 November 2012 - 08:59 PM
Some considerations if you go beyond just a refigure...you will have the option of a thinner mirror which will cool down faster, but a thinner mirror will affect the balance of the scope. It also will bring the focal plane in and some eyepieces may not focus with the standard focuser. There may not be enough adjustment in the primary to compensate. So you may want to consider going with a slightly longer focal length to make up for a thinner mirror. Alternatively you can do what I did and get a low-profile moonlite focuser. The GSO focuser is pretty decent. If I had known more of what I was doing I may have had Carl increase the focal length by 5mm. Instead I look at it as a good excuse to get a top notch focuser. You also may need to shorten the mirror clips due to the thinner mirror.
See Zhumell 10" mirror replacement. Ignore my typos where I say DSO rather than GSO.
Also keep in mind what others have said about checking out the stock mirror first - you may get a good one. In my case I just wanted to remove all possible doubt and get one of the best mirrors available - especially in light of the the planetary views I was getting.
Regarding a GSO, i.e. Zhumell and others, vs an Orion, I went with the Z because I liked the adjustable clutch bearings and it was a little less expensive.
Posted 19 November 2012 - 09:15 PM
Posted 19 November 2012 - 09:47 PM
I want to know who orders an AP 175 and then contemplates whether to skimp by refiguring a Chinese made mirror.
Someone with no money left over, that's who!
Posted 19 November 2012 - 09:49 PM
Posted 19 November 2012 - 11:12 PM
Posted 20 November 2012 - 12:05 AM
If you're looking for a cheaper option, have you considered one of the "non-branded" makers, such as Dennis at Dobstuff? His kits are reasonably priced and you can then source your own mirror.
Or you can buy used, which is also a good way to save some costs.
Or you can trade that AP175...
Posted 20 November 2012 - 12:43 AM
Posted 20 November 2012 - 05:03 PM
Thanks Stratocaster for your experience and advice. You didn't say....would you take the same route again?
Pretty much. The only thing I might change is to adjust the focal length to compensate for a thinner mirror so I didn't need to get a new focuser. The stock focuser was pretty good.
Also, I offset the cost a bit by selling the parts I no longer needed, i.e. the finder, the stock focuser, and the stock eyepieces.
The whole cost vs benefit equation will depend on the individual. There's a guy in our club with an Orion 10" with the stock mirror and his views are no slouch. But there's no doubt my views are substantially better than those provided by my stock mirror and the added expense was worth it to me.
Depending how much you intend to upgrade the scope over time, you might approach the tipping point of just going for a custom dob.
For example, after I get a new secondary (and probably a spider) I suspect my total out of pocket cost will be around $1800. So it's not quite like starting with a $499 Z10. Consider the cost of a standard Teeter 11" at $1975 (not sure if that includes shipping). Tack on another $950 for a Zambuto upgrade. So for around $2900 you have top flight optics and also excellent construction and materials. $2900 vs $1800? Many might just spend the extra $1100 and go with a custom dob. Then again, many wouldn't. I fall into the latter category.
Posted 21 November 2012 - 10:36 AM
Hmmm..... I was under the impression that this has been recommended/done in the past by others on here. Sounds like I need to rethink this a bit.
I have taken the exact path you contemplate with what began as a stock Orion 12XTi (intelliscope). I've upgraded virtually everything that's possible to upgrade about the optics and mechanicals short of replacing the optical tube and base structure itself or the stock intelliscope DSCs. Below is a complete list of upgrades I've made and their cost and potential benefits. This is of course not done to suggest that you must adopt all of these measures to get worthwhile improvement; it's merely intended to give you a realistic idea of the possibilities.
1) Stock 12" primary mirror refigured by Steve Swayze ($325)
2) Coating by Spectrum ($190 for Max EAL coatings)
3) Shipping mirror for refiguring & Coating (Raleigh, NC => Portland, Or => Deltona, Fl => Raleigh, NC...(apx. $60)
NET COST: $575 RESULT: You do get a premium-quality mirror figure with noticeable improvement in contrast and sharpness of image at higher powers, but the differences between the stock mirror and refigured/recoated mirror are subtle in mediocre seeing/transparency, more obvious on nights with very good seeing and transparency.
SECONDARY MIRROR & SPIDER/HOLDER
1) Replace stock Orion secondary with Protostar Quartz secondary 2.6" minor axis size ($225)
2) Replace stock Orion spider/holder with Protostar Spider/Holder with built-in secondary heater (apx $155)
3) Tool-less finger-adjustable secondary collimation screws ($15)
NET COST AND RESULT: $395 (The tool-less finger-adjustable collimation screws are an ENORMOUS improvement over inset screws for which you need the correct Allen wrench). The secondary heater built-in to the holder uses two of the metal spider vanes to conduct power to it via insulated screws attaching those vanes to your optical tube; it's up to you how to wire power to these screws, e.g. via an RCA jack-wiring to Astro-Zap Dew controller unit to 12V 5VA rechargeable battery attached to base via velcro. You'll also have to drill new repositioned holes for the eight (rather than four on the stock spider) screw attachments holding the spider to the OTA. WORTH IT? Protostar quartz secondaries are as good a figure as it gets and most thermally stable material, but the main practical benefit is removing any risk that the optical improvements from refiguring/recoating the primary don't get degraded by a rough or mediocre secondary. So the actual extent of visible improvement is somewhat speculative depending on the extent (or not) to which your stock secondary was good or mediocre; however, you should read what Mike Lockwood has to say about secondaries at his website (he requests clients send him their secondaries along with primaries for possible refiguring). The spider is mechanically more solidly rigid than the stock spider and secondary collimation is MUCH easier to adjust spot-on, and the secondary heater cuts off at the pass the most vulnerable item to dew in your optical chain, which formerly shut me down much earlier on moist nights than with the current setup. CONCLUSION: The degree of improvement is a bit more speculative from upgrading the secondary and spider, but you do get assurance that your overall upgrade of your optical train is to premium level (the dew heater helps too).
FOCUSER UPGRADE: From stock to Moonlite CR-2 (highly recommended) COST: $165 for single-speed; $100 additional for double-speed option (can be added later) + $49 for needed adapter/riser plates (TOTAL COST: $314). A Moonlite Crayford focuser, especially with the double-speed option is a HUGE improvement over the stock Orion focuser. Buttery-smooth motions, and securely lifts, moves and holds even with heavier eyepieces such as the 31T5 Nagler. The principal at Moonlite, Ron, is very easy, responsive, and incredibly helpful to contact and turnaround for any orders or servicing is very quick. A really good focuser is a tremendous asset in being able to take hassle-free advantage of upgraded optics.
AZIMUTH BEARINGS: Add ebony-star laminate azimuth bearings to underside of base. You can do this either by acquiring a sheet of the proper type of laminate and do the necessary part cutout yourself (not sure about $$) OR purchase the proerly-sized precut bearing yourself from ScopeStuff.com ($79.00 for the correct size for a 12"; includes replacement PTFE pads for the opposite bearing surface as well) and attach it with contact cement.) Ebony-star bearing surfaces are a HUGE improvement over the stock surface, but even with ebony-star, you'll need to periodically wax the surface (Sail-Kote spray marine lubricant works best, DuPont Teflon spray wax a close second, both are superior to Carnuba or Turtle Wax which will gum up quicker and need cleaning/reapplication much sooner than with the first two mentioned; been there, done that with everything).
Replace stock 10x50 finderscope with Stellarvue SV60 (or SV50) finderscope. ($189), rings needed to mount it ($45), total: ($234). The SV60 is a nice, compact refractor in its own right, and you can use ANY 1.25" eyepiece you want in it (though it comes with a 23mm plossl-like EP stock). I've souped mine up with a 24mm Panoptic, yielding a wonderfully panoramic view that is nicely compatible with the depth of most 7.0 to 8.0 magnitude star charts. The downside is that at f/3.75, the fast objective really stresses eyepieces, so going wider than 52 degrees will produce comatic views unless you use better-quality EPs. The other problem is that the foot of the SV60 isn't quite an exact match for the stock dovetail shoe for the original Orion finderscope, and you may need to either replace it or use shims. CONCLUSION: This is a very nice upgrade, but I'd rate this as an optional luxury rather than a necessity, since it contributes nothing optically to your host scope's optical train (except the incidental benefit of making it a bit easier and more enjoyable to navigate the skies).
BEANBAG COUNTERWEIGHT SYSTEM + VELCRO STRIP ALONG SPINE OF OPTICAL TUBE:
A strip of 2" Velcro "wiry half" along the top spine of your optical tube from bottom to just past mid-wayt is MUCH the better way to stow your COL unit when not in your hand, and also gives you a convenient way to rapidly attach/adjust homemade one and two lb beanbag counterweights to counterbalance heavier eyepieces.
COST: Velcro $15 for a 2" wide roll, I used three "wet bags" sold at REI for apx $10 each + whole 1 lb packs of lentils (leaving them in their original plastic bag avoids potential spillage). Wetbags will snugly roll up to fit the size of the beans inside so contents don't flop around in use. The tricky part is getting glue attaching fuzzy-half velcro patch to adhere and cure to the slick wetbag surface.
OVERALL CONCLUSION: The risk is that in spending an additional $1K +/- $500 to upgrade a stock 12XTi, your total costs will come up only $1K to $1.5K shy of the cost of purchasing a used 12.5" Obsession Truss dob (without DSCs) on the used market. No matter how many upgrades you make to a stock Orion 12XTi, you will never quite achieve the mechanical quality of a true premium or semi-premium scope, nor will Orion's magnetic-based Intelliscope functioning be quite so good, reliable, or free of annoying quirks and intermittent bugginess as more premium (and expensive) optical-based systems such as Sky Commander or Argo-Navis. You'll also still have the bulk and weight of a 12" solid-tube structure to deal with, rather than a more compactly transportable truss dob structure. Also, at the end of the day (or night) that Orion XTi base is still merely laminated particle-board, and not as mechanically sound or durable (especially against dampness) as better materials. SO GET YOURSELF AN INEXPENSIVE GROUND CLOTH (PLASTIC TARPS WITH BUILT-IN GROMMETS TO STAKE IT DOWN) IS A MUST-DO CHEAP UPGRADE IF YOU'RE GOING TO USE THE SCOPE ON DIRT OR GRASS, COMPLETELY ASIDE FROM THE OPTICS.
Posted 21 November 2012 - 11:17 AM
Posted 21 November 2012 - 12:45 PM
Posted 21 November 2012 - 01:47 PM
Chris, thank you so much for your detailed write up. It further reinforces my new opinion that just spending a little more gets you a lot more in ergonomics that can't be matched.
One other good intermediate option is available that does give you near-premium ergonomics when all's done, which is to let Dobstuff build you a new strut structure. You can either send them the (upgraded) optics, focuser, etc. from your existing scope to incorporate into the new structure, or you can buy all-new components from them. Many people have upgraded a stock commercial dob in this fashion, often after upgrading their optics by e.g. refiguring the primary mirror.
There's an excellent account of a fellow CN-member, JayinUt, who took the optical components from an Orion XX14i and had Dobstuff build a new structure for him, and he seems to be extremely pleased with the results. Jay is a knowledgeable, experienced, hard-core observer who spends quite a few nights out under dark skies in the Utah desert (I've had the pleasure of observing with him while on a trip to Salt Lake City) and so his recommendations carry a lot of credibility on the matter. You can follow the process of his scope undergoing the Dobstuff upgrade at his excellent astro-observing blog, working from his Nov 10 '12 entry and work backward (he clearly labels which blog dates are about his dobstuff upgrade vs other observing topics if you click on the arrows by the dates to see what the date's blog subject is about).
Posted 21 November 2012 - 03:16 PM