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Space Forum / Amateur Astronomy / November 2004Big richfield refractor
A friend and I have been pushing around the idea of a large richfield refractor. We have been looking but have yet to find a 8+ inch richfield refractor. For cost reasons we are not interested in an APO. Does anybody know of a company that sells big acromats? Tom Wales > A friend and I have been pushing around the idea of a large richfield > refractor. We have been looking but have yet to find a 8+ inch richfield > refractor. For cost reasons we are not interested in an APO. Does anybody > know of a company that sells big acromats? Tom, D&G seems to make only f/10 or slower. Bob Royce (http://www.rfroyce.com/) makes lenses from 6" to 12" aperture in "reasonable focal lengths," and guarantees f/10 or slower to be 1/10th wave. It might be worth asking Bob if he could make what you want. Clear skies, Alan > A friend and I have been pushing around the idea of a large richfield > refractor. We have been looking but have yet to find a 8+ inch richfield > refractor. For cost reasons we are not interested in an APO. Does anybody > know of a company that sells big acromats? > > Tom Wales You might check out: http://www3.sympatico.ca/drapell/sMain.htm Also look at: http://www.jinoscope.com/id1.html They have an 8in F6 refractor for $3800. I think JINOscope will also sell just the lens assembly if you want to build the OTA yourself. >A friend and I have been pushing around the idea of a large richfield >refractor. We have been looking but have yet to find a 8+ inch richfield >refractor. For cost reasons we are not interested in an APO. Does anybody >know of a company that sells big acromats? > >Tom Wales Hi Tom, APM in Germany offers an 8 inch F6 and RR optics in Holland did, but at the moment the company is in a state of limbo. http://www.apm-telescopes.de/englisch/tmb_designed/tmb_designed_8f6_de.htm http://www.so-nl.nl/RR%20Refractors.htm Good luck, Squirrel >A friend and I have been pushing around the idea of a large richfield >refractor. We have been looking but have yet to find a 8+ inch richfield >refractor. For cost reasons we are not interested in an APO. Does anybody >know of a company that sells big acromats? > >Tom Wales Why waste money on an 8" refractor that is only going to be used for rich field work when you can buy a 12" reflector for the same thing that will take power well and cost about the same? >Subject: Re: Big richfield refractor >From: RichA none@none.com >Date: 11/4/2004 7:26 PM Eastern Standard Time >Message-id: <55ilo01o3q93t4bpfrusadtkhs1ho886ln@4ax.com> >Why waste money on an 8" refractor that is only >going to be used for rich field work when you can buy >a 12" reflector for the same thing that will take >power well and cost about the same? *************************** The Newt is the way to go! Any fast 8" Achro is gonna look like a rainbow of colors, unless you only use it for low power deepsky work... Chas P. >> Subject: Re: Big richfield refractor >> From: RichA none@none.com [quoted text clipped - 10 lines] > > Chas P. The OP said he wanted a richfield scope so I would assume he is indeed going to use is for low power deep sky work. >Subject: Re: Big richfield refractor >From: Bill Tschumy bill@otherwiseDELETE.com >Date: 11/4/2004 8:29 PM Eastern Standard Time >Message-id: <KHAid.8425$fC4.2967@newssvr11.news.prodigy.com> >The OP said he wanted a richfield scope so I would assume he is indeed going >to use is for low power deep sky work. ********************************* Bet he would turn it on the planets or moon also, and see lots of vivid color. I had a colorful meeting with a 6" f10 Achro once, yuk never again!!! Chas P. >A friend and I have been pushing around the idea of a large richfield >refractor. We have been looking but have yet to find a 8+ inch richfield >refractor. For cost reasons we are not interested in an APO. Does anybody >know of a company that sells big acromats? I am curious if you have considered the problems associated with large fast achromats. Seems to me something of a losing proposition when compared to a similar focal length Newtonian. jon > >A friend and I have been pushing around the idea of a large richfield >>refractor. We have been looking but have yet to find a 8+ inch richfield [quoted text clipped - 8 lines] > > jon For low power wide field, refractors do have advantages over newts. One is better off axis images. Unlike newts, a proper lens design (Baker doublet, for one ex.) can virtually eliminate coma and astigmatism over a relativly large flat field. Also, a newtonian is limited in how low a magnification you can practically use by the central obstruction starting to cover a significant part of the pupil of your eye. A fast lens is not required for visual work. A normal f/ refractor can be used at low magnifications with the right eyepiece. For my 11" f/16, I made an sorta plossl eyepiece out of surplus cemented achromats that has a 3"+ diameter field lens. It gives me a full one degree field at 50x, with an exit pupil under 7mm. Its actually fun to mix and match various lenses to make your own low power eyepieces. I use a sheet of quadrille paper as a target and vary the spacings and flip different lenses until I can get a focus on the lines throughout the field. This is good enough to give acceptable performance with f/10 or longer objectives. Old binocular objectives are a good source of potential eyepiece lenses. Keep the steeper curves facing each other, and use the shorter focal length lens as the eyelens if the two you're using are different. Jeff Schroeder >A fast lens is not required for visual work. A normal f/ refractor can be >used at low magnifications with the right eyepiece. For my 11" f/16, I made >an sorta plossl eyepiece out of surplus cemented achromats that has a 3"+ >diameter field lens. It gives me a full one degree field at 50x, with an >exit pupil under 7mm. Its nice to be able to build your own eyepieces and scope. That must be one big scope.... Compare that to say a 12.5 inch F4.06 Newtonian, OTA length is 52 inches, and with a Paracorr and 32mm 66 degree WF eyepiece it gives some pretty fine views and has a 1.4 degree FOV at 46X with a 6.9mm exit pupil. Not only that, but it fits in a 1992 Ford Escort Hatchback and it can be setup in less than a minute.... jon Hello, Jon, My experience is consistent with what you report. My 11 inch F4.55 Newtonian, with a 31 mm Nagler, has a field of 1.9 degrees and an exit pupil of 6.8 without the Paracorr, and 1.75 degrees and an exit pupil of 6.0 with it. Central obstruction is less than 20 percent. Works fine for me. I'm not sure why one would want a large achromat for visual work, now that the Paracorr is available for reflectors. I can see the virtues of a small RFT rafractot, say a 4 inch, for wide fied viewing; whether it's an APO or not depends on one's tastes, but I can't see using a large achromat. Too much weight, too much volume, too much color, to suit me. Just my opinion. Bill Meyers >>A fast lens is not required for visual work. A normal f/ refractor can be >>used at low magnifications with the right eyepiece. For my 11" f/16, I made [quoted text clipped - 14 lines] > > jon >Subject: Re: Big richfield refractor >From: "Jeff R. Schroeder" 190@earthlink.net >Date: 11/5/2004 8:23 PM Eastern Standard Time >Message-id: <eIVid.18962$KJ6.10782@newsread1.news.pas.earthlink.net> >For low power wide field, refractors do have advantages over newts. One is >better off axis images. Unlike newts, a proper lens design (Baker doublet, [quoted text clipped - 17 lines] > >Jeff Schroeder ************************** I remember a 11" Refractor back in 1978 in Sky& Tele, i wonder if its the same scope??? Chas P. > >Subject: Re: Big richfield refractor >>From: "Jeff R. Schroeder" 190@earthlink.net [quoted text clipped - 30 lines] > > Chas P. Yep. Same one. Now I usually use it on an alt-az mount on top of my 86 Volvo wagon instead of on its original equatorial. It is a LOT more portable this way, and is used mostly for public and school starparties. Last year I spent many nights at the Griffith Obs. satellite facility in L.A. helping them with the Mars oppo. Jeff >Subject: Re: Big richfield refractor >From: "Jeff R. Schroeder" 190@earthlink.net >Date: 11/7/2004 1:58 AM Eastern Standard Time >Message-id: <rIjjd.7306$O11.3878@newsread3.news.pas.earthlink.net> > Yep. Same one. Now I usually use it on an alt-az mount on top of my 86 >Volvo wagon instead of on its original equatorial. It is a LOT more portable [quoted text clipped - 3 lines] > >Jeff *************************** I remember that scope, it was in the March 78 Sky& Tele i think. I was 14 at that time, with a 60mm scope! Back then any scope bigger than a 60mm was a dream scope for me... Chas P/ > > >Subject: Re: Big richfield refractor > >>From: "Jeff R. Schroeder" 190@earthlink.net [quoted text clipped - 38 lines] > > Jeff Didn't you make that 11" at Citrus College? I moved to Azusa and went to their optics day and night classes during 1972-3 with characters like Ron Royer, Eric Ziering, Steve Padilla, Mike Marcario and Rick Sorenson. Mike and Steve both went on to Mt. Wilson while I continued on in optics and EE in college and ended up doing optics at General Dynamics. Seems like I heard you diamond generated, ground and polished your refractor there, and figured it using distant street lights? How did you arrive at 11"? Did you get a good deal on some blanks from Jim Kent? Mike >> >>For low power wide field, refractors do have advantages over newts. One >> >>is [quoted text clipped - 46 lines] > > Mike Yes, I was in the Citrus classes in 74-75, with at least Ron, Rick and Steve still there. I did the generating, edging and rough grinding there and polished and figured the lenses at home. I got the blanks from Jim Jacobsen of Coulter Optical in partial trade for a big blank I found at the Montrey Park (Garvey ranch park obs.) swapmeet. Both were Schott grade A, and he was nice enough to give me a real deal on them. The flint was originally intended for one of the OAO satellites, but has been molded a little too thin. I'm still in occasional contact with Msgr. Royer, Rick went on to run the Northrup optical shop at Palos Verdes for many years. Jeff > > Didn't you make that 11" at Citrus College? I moved to Azusa and went > > to their optics day and night classes during 1972-3 with characters like [quoted text clipped - 20 lines] > > Jeff Good swap on the blanks. And right about Rick at Northrup. I went up to see him a couple of times at work while on trips for GD to LA. You knew he died of melanoma several years ago? Damn shame, we were good buds. He was a real wack job, funnier than hell. I helped him make his 6" Mak and machined the corrector shell cell for it on his Dad's lathe at their house in Lakewood back in 1973. Haven't visited with Ron or Steve much since Mike Marcario passed away in July '98. Sometimes it still doesn't seem real that Mike and Rick are gone. I know Ron's quite pleased in his new surroundings enjoying the retired life. I think almost everyone in the class helped him work on his astrograph triplet at one time or another. Jim Kent is still around and doing well, we just talked a few months ago. Optico is one of my best suppliers, so I talk with Eric frequently. So there's still a few of us old Citrus optics class guys still hanging on. Good to visit with you. Mike >A friend and I have been pushing around the idea of a large richfield >refractor. We have been looking but have yet to find a 8+ inch richfield >refractor. For cost reasons we are not interested in an APO. Does anybody >know of a company that sells big acromats? For the same money I'll bet my life you get better view through a >8" reflector than you do through a >8" refractor. Tim  SignatureAnyone who qualifies their comments with "just my two cents" is usually over-valuing their contribution. "Tim Auton" and others opined: > For the same money I'll bet my life you get better view through a >8" > reflector than you do through a >8" refractor. Maybe Tom wants a rich field 8+" refractor. T'was a pretty simple question, n'est-ce pas? Howard Lester Hi, to answer some of you. I already have a pretty kick a.s 8" reflector, my friend is a dreamer and has a ST80 which he gets great wide field views with. Soooo he has refractor-itis and wants a larger one. I personally love my dob, it has great wide field performance and when I want (as you already know) I can throw some power on the planets or smaller DSO's and reel em on in. Don't get me wrong, I don't mind refractors, but wide field acros are like you all said, prone to chromic aberration. And the cost of anything over 6" appears to be extremely prohibitive. I think personally it is better to buy a big reflector and spend the big bucks on good eyepieces. :-) Thanks to all who responded. Tom Wales > A friend and I have been pushing around the idea of a large richfield > refractor. We have been looking but have yet to find a 8+ inch richfield > refractor. For cost reasons we are not interested in an APO. Does anybody > know of a company that sells big acromats? > > Tom Wales 8" f/5 Achromat EFL 40.0" Surf Radius Thickness Glass Diam STO 21.352 1.264 BK7 8.3 2 -17.245 0.536 8.3 3 -16.428 0.700 SF2 8.0 4 -68.500 37.767 8.3 Then put a Chromacor on the back end? Mike > > [quoted text clipped - 6 lines] > 4 -68.500 37.767 8.3 > Curious... if I may ask, what were your optimization goals here? > SignatureRick S. http://users.rcn.com/rflrs > >8" f/5 Achromat > >EFL 40.0" [quoted text clipped - 5 lines] > > > Curious... if I may ask, what were your optimization goals here? > -- > > Rick S. For simple achromats I use the following ZEMAX operands rather than canned default operands: Op Code Wvln Hx Hy Px Py Value Weight 1 REAY 0.560 0 0 0 0.4 0 1 2 REAY 0.560 0 0 0 0.5 0 1 3 REAY 0.560 0 0 0 0.6 0 1 4 REAY 0.560 0 0 0 0.7 0 1 5 REAY 0.560 0 0 0 0.8 0 1 6 REAY 0.560 0 0 0 0.9 0 1 7 REAY 0.560 0 0 0 1.0 0 1 8 REAY 0.486 0 0 0 0.85 0 0 9 REAY 0.656 0 0 0 0.85 0 0 10 DIFF 8 9 0 1 11 COMA 0.560 0 1 Very fast compared to default set and gives better results: near-zero spherical aberration at mid-band, pinches C-F together at 0.85 zone, and zero TO coma. That's all you can do in a simple achromat. The 0.85 zone is a judgement call depending on what energy distribution is needed, and can vary from 0.7 to 0.9 or so. Or is this what you were asking? Mike > > [quoted text clipped - 7 lines] >Mike > That's it. Using the GSUM program, I come up with a rough equivalent by assigning both reference frequencies to the e line and tweaking the air space until the 70% zone nulls (along with the 100% zone, OSC and color that are automatic). SignatureRick S. http://users.rcn.com/rflrs > 8" f/5 Achromat > EFL 40.0" [quoted text clipped - 6 lines] > Then put a Chromacor on the back end? > Mike Chromacorr is not designed for good color performance far from optical axis; massive lateral color in a wide field view. A Petzval would be the best bet for a large, effectively fast achromatic system. D Chaffee > Chromacorr is not designed for good color performance far from > optical axis; massive lateral color in a wide field view. A Petzval > would be the best bet for a large, effectively fast achromatic system. > > D Chaffee Yeah, I wasn't sure how Chromacorr worked over field and at low focal ratios, hence the ?. Here's a quick suppertime Petzval I did just for curiosity's sake: 8" f/5 Petzval Wvln: 0.486µm, 0.560µm, 0.656µm Field angles: 0.0º, 0.7º, 1.0º Surf Radius Thickness Glass Diam STO 41.336 1.152 BK7 8.3 2 -19.613 0.282 8.3 3 -19.373 0.700 F2 8.0 4 -63.297 49.816 8.3 5 8.760 0.600 S-FSL5 3.1 6 -10.039 0.218 3.1 7 -10.446 0.500 SF1 2.8 8 -31.994 4.625 3.1 IMG Infinite 1.4 Flat image, axial correction 1/20 wave at 0.56µm, good low-cost glasses, no high $$$ ED, 90%EE at r=25µm at 0º, 66% EE at r=25 µm at 1º, distortion < 0.005% over C-F band. Might make a nice 1016mm f/5 35mm telephoto lens as well, but no ED so UV/IR is out of control and not good for CCD unless spectrally filtered. Mike > > [quoted text clipped - 6 lines] > IMG Infinite 1.4 > Is there any particular reason you went with S-FSL5 instead of (more known to this group and oft maligned herein;-) N-FK5? > SignatureRick S. http://users.rcn.com/rflrs > > 5 8.760 0.600 S-FSL5 3.1 > > 6 -10.039 0.218 3.1 [quoted text clipped - 10 lines] > > http://users.rcn.com/rflrs I don't care, whichever's cheaper. Isn't Ohara's S-FSL5 a great deal cheaper than Schott's N-FK5? A Schott field rep told me once that Ohara can sell ED-type glasses more cheaply than them because Canon buys so much ED glass from Ohara. Mike >> > 5 8.760 0.600 S-FSL5 3.1 >> > 6 -10.039 0.218 3.1 [quoted text clipped - 16 lines] >much ED glass from Ohara. >Mike I wish they'd work on new glasses. One of the latest Canon lenses has 15 elements, 3 of which are ED and they still have horrible chromatic aberration unless you stop them down quite a bit. > >> > 5 8.760 0.600 S-FSL5 3.1 > >> > 6 -10.039 0.218 3.1 [quoted text clipped - 21 lines] > they still have horrible chromatic aberration unless > you stop them down quite a bit. You ain't the only one. We all need a bigger selection of abnormal dispersion glasses with good optical, physical and chemical characteristics that cost less than pure Europium. We need glasses like 880850 and 485230, out in the empty corners of the glass map. We need to get even farther away from the dPgF, dPFe, etc. normal lines. Seven-color crossing lenses over the 0.37-1.01µm band or even wider would be possible, with resulting performance indistinguishable from unobstructed mirrors but with much wider well-corrected fields. I know of special short-run proprietary blends of abnormal dispersion optical glass and liquids being made at certain companies for specific jobs, but they are internally funded and held tightly to the chest. Optical liquids have very unusual dispersion properties, but have inhomogeneity problems in thermal gradients due to higher dn/dT. So, in the non-proprietary world, you just do the best you can by including as many glass catalogs as you can, from Schott, Ohara, Hoya, Sumita, etc. Mike > > [quoted text clipped - 10 lines] >Mike > The problem with Hoya and Sumita is that they use the old Cauchy formula for dispersion instead of the Sellmeier formula. That formula is unsmooth in the first derivative and cannot at all be extrapolated. SignatureRick S. http://users.rcn.com/rflrs > >Seven-color crossing lenses over the 0.37-1.01µm band or even wider > >would be possible, with resulting performance indistinguishable from [quoted text clipped - 19 lines] > > http://users.rcn.com/rflrs Odd - that's not consistent with their current catalogs. I just checked the ZEMAX glass catalogs, and every glass in the Hoya and Sumita catalogs is fit with the Schott dispersion formula. I also checked my downloaded Hoya and Sumita catalogs, and they both use the Schott formula. That formula has continuous smooth first and second derivatives. Maybe you need to download more recent catalogs? Mike > > [quoted text clipped - 8 lines] >Mike > The "Schott" dispersion formula (ZEMAX) is the "Laurent" dispersion formula (OSLO) is the "Cauchy" dispersion formula. All are names for the truncated double-ended series. SignatureRick S. http://users.rcn.com/rflrs >>I don't care, whichever's cheaper. Isn't Ohara's S-FSL5 a great deal cheaper than Schott's N-FK5? A Schott field rep told me once that Ohara can sell ED-type glasses more cheaply than them because Canon buys so much ED glass from Ohara.>> S-FSL5 and N-FK5 are NOT ED glasses. They are both ordinary crowns with characteristics similar to BK7. Roland Christen >>>I don't care, whichever's cheaper. Isn't Ohara's S-FSL5 a great deal >>> [quoted text clipped - 8 lines] >Roland Christen > But N-FK5 really does make a good cemented achromat with F2, even if the color is slightly worse than N-BK7/F2. SignatureRick S. http://users.rcn.com/rflrs >>But N-FK5 really does make a good cemented achromat with F2, even if the color is slightly worse than N-BK7/F2.>> Yes, I agree that you can make an achro with that combination. In any large lens, however, I would not want the curves to be different to eliminate ghost images that are too close to the focused image. Roland christen >>>But N-FK5 really does make a good cemented achromat with F2, even if the >>> [quoted text clipped - 5 lines] >images that are too close to the focused image. > For this case, if I figured correctly, it amounts to only about 0.006" air-space, but is enough to drop the ghost down a little better than 10 magnitudes. Would this be done with just some shim stock spaced around the periphery? >Roland christen > SignatureRick S. http://users.rcn.com/rflrs > >>I don't care, whichever's cheaper. Isn't Ohara's S-FSL5 a great deal > cheaper than Schott's N-FK5? A Schott field rep told me once that Ohara [quoted text clipped - 5 lines] > > Roland Christen I know. The ED comment following did sound like I was connecting them, but I did not intend to. Mike why not just make your own. 12 -18 inches should suffice. try f/5. > A friend and I have been pushing around the idea of a large richfield > refractor. We have been looking but have yet to find a 8+ inch richfield > refractor. For cost reasons we are not interested in an APO. Does anybody > know of a company that sells big acromats? > > Tom Wales > A friend and I have been pushing around the idea of a large richfield > refractor. We have been looking but have yet to find a 8+ inch richfield > refractor. For cost reasons we are not interested in an APO. Does anybody > know of a company that sells big acromats? > > Tom Wales Note that you don't HAVE to have a short focal ratio objective to achieve richest field performance. Using a home-made eyepiece with a focal length of 3" with an f/15 objective gives a 1/4" exit pupil. These are prety easy to build from surplus achromats (see Anchor Surplus and others). For a first approximation get two 6" focal length achromats about 2.5 to 3" diameter (one can be a little smaller) and mount them with the strongest surfaces of each lens facing the other lens and almost touching. Put the smaller one closest to the eye. If you have a lathe it is easy to make a cell to hold the lenses, if not, a kludge made from available tubing (metal, vinyl, cardboard, etc) can be lashed together to do the job. Don't get me wrong, these won't work like they came from Televue, but longer focal ratio objectives make less demands on eyepiece quality. Clif Ashcraft |
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