C-7 Home Page
January 2, 1996
Subject: Selection advice regarding our 4" to 7" aperture Astro-Physics telescopes.
Thank you for considering Company Seven. Based on our years of experience with using telescopes, and that with helping and working with telescope users by our industrial and consumer efforts, we are in a good position to advise others as yourself. In this outline we will try be brief but reasonably comprehensive. If this response does not allow you to form a conclusion, then it will help the reader to better formulate following questions.
The first goal in selecting an instrument is to identify its mission. When selecting a telescope we suggest our customers address three general factors:
If one is willing to sacrifice all portability, and field of view then an argument may be made that the best return on an investment (in terms of strictly photo-visual performance) would be a long focus Newtonian reflector. If comfort of viewing, or the contrast and reliability of another design is preferred, then I would closely examine one of the following telescopes listed on Table 1.
Company Seven inspects and tests every telescope we sell. We are the only firm in the nation that does this and then includes additional informational materials with our computer generated in-house optical report characterizing each telescope. This characterization offers practical advice and predicts field of view on a variety of photo-video, and visual formats. Our experience and studies of published technical reviews employing a scientific approach to comparing similar instruments has led us to conclude that inch per inch, and pound for pound the Carl Zeiss and Astro-Physics Apos offer the most remarkably clear, and high contrast views of any production telescopes. The Astro-Physics models offer views that are similar in terms of detail resolved to the Carl Zeiss, but the Astro-Physics provide greater versatility with a wider selection of custom matched accessories and that are more affordable. Furthermore, Astro-Physics mechanics are better engineered to work with in the field. Therefore, I will concentrate on Astro-Physics telescopes herein.
Astro-Physics currently produces two lens designs; the Christen "EDT" and "EDF". The EDF is the shorter or "faster" of the two; it was initially developed for wide field of view applications. But many consumers regard the extreme portability as a major selection aspect. When used for high magnification observing, an EDF view is similar to that of the longer EDT or the Carl Zeiss "APQ". As an EDF optical tube is so comparatively short, the required mount for it can have less mass, shorter height, and may be more compact than is typical for an EDT refractor. The EDF is now offered in 105mm, 130mm, and 155mm apertures. The EDT in 130 and 180mm. To simplify this letter I will concentrate on the 130mm models for comparisons of an EDT to the EDF:
As I mentioned above, Astro-Physics telescopes are supported by a selection of optional factory and third party photo-visual accessories. For example, Astro-Physics offers a matched photo-visual telecompressor allowing wider photographic fields of view and shorter exposure times, and wider visual fields of view unrivaled in the industry. The 130mm EDF becomes a 585mm f4.5 telescope; with a 55mm Plossl one sees 4.7 degrees of sky at only about 11X! This is the absolute widest field capability of the larger high resolution, one man portable telescopes.
On either telescope the conventionally accepted nominal exit pupil of 7mm would be derived when operating at 18.6X; this requires about a 36mm ocular on the EDT, or 32mm on the EDF telescope. Any lower may be considered an inefficient use of light gathering power on faint, or extended objects. While the strengths of the EDF are obvious, do not overlook the focal length advantage of the EDT high magnification photo-visual work (such as on Planets, or double stars):
Only a Questar 7 (Maksutov-Cassegrain) can offer about the most clear, and high contrast view of any production Catadioptric system rivaling an apochromat refractor. On an evening with constant temperature the view through a Questar 7 will compare to a good 5" or 6" apochromat in terms of details shown, and similar to a 6" in terms of apparent brightness (assuming similar magnifications are employed between the two telescopes). The Astro-Physics 155mm EDT and EDF models offer views that are similar or better in terms of detail resolved, and they are notably brighter and provide greater versatility than the Questar 7. These differences are predictable; the refractor has the most efficient light throughput of these designs. Also the refractor focal length is far shorter than the Questar 7 and is one half or less costly.
If one compares low-magnification wide field viewing ability, then the Astro-Physics Apos are the best instruments of these. The 155mm f7 EDF telescope in particular has an effective focal length of about 1085mm making this a very bright and versatile platform.
Note the data in these tables derive from actual tests of instruments selected at random, and from manufacturer data. The data in Table 5 assumes the instrument at "Prime Focus" with no optional Positive telecompressor optics in place to reduce effective focal length.
The Astro-Physics lenses are computer optimized "Super ED" (ultra low dispersion) glass triplet apochromats corrected for residual spherical aberration and coma. Christen optics are hand matched to guarantee zone-free accuracy of a very high order for the entire system. The "Starfire EDT" and "EDF" lenses exhibit no sphero-chromatism or secondary color over the entire visual spectrum; color correction visually and photographically appear perfect. With no air spaces between the lens elements, and multi-coated outer surfaces the light transmission is guaranteed at up to 97% over the visible spectrum. The "EDT" and "EDF" optics are corrected into the I.R. and U.V. (C to G). The use of premium homogeneous cut block glass insures that high power views are sharp and clean when atmospherics permit.
Published reviews and controlled tests have demonstrated the Astro-Physics optics to be superior in most optical and mechanical criteria to all others but the Carl Zeiss "APQ" Apos. However, the current production Zeiss models are beyond the Astro-Physics price league entirely, and the accessory selection is complicated and limited by todays standards. Some Apo refractors such as the premium Japanese instruments were considered to be similar or superior optically until eclipsed by the introduction of the "EDF" and "EDT" designs by Astro-Physics in 1990.
Even if the competing premium Apos were equal or superior in optical performance we still believe the Astro-Physics is a better value. Consider the cost and availability of a comprehensive system of matched accessories; a photographic (only) telecompressor for a Takahashi 5" may cost $2000 while the Astro-Physics cost $300 and gives better images photographically and can be used visually too!* The availability of Japanese (and to a degree Zeiss) accessories is poor; some of these appear to exist only in a price list. The telescope prices themselves are also beyond reason for most; a Nikon 4" Apochromat (at f12 it should be an Apo) model with mount or another popular 5" premium Japanese telescope with mount is as much or more than a complete Astro-Physics 180mm (7.1") f9 "Starfire EDT" with mount!
* It has been pointed out to us by a customer that there is some question about the accuracy of this statement. Somebody on a forum in 1998 was so harsh as to accuse this of being a lie; anyone on the forum could have called to discuss this with us and we might have headed that off by adding this following information. This information derives from our experience with a customer in Nevada who also owned the Takahashi 5 inch triplet Apo telescope. His results when shooting film with this, while good, were not as good as he achieved on similar or the same objects shot subsequently with a similarly similarly equipped Astro-Physics 13cm telescope. And he was happy that he could use a 2.7" Telecompressor/Field Flattener not only for photography but also for viewing. We do recommend Takahashi, it is a fine telescope and we do advise prospective customers who can not wait for an AP or find a Zeiss APQ to buy the Takahashi; we refer people to John Stiles at OGS to buy these telescopes. But we do cringe at the costs even of some Takahashi accessories however well made - after all how much can a 6x 30 Finder be worth?
And in fairness we point out that these prices of the Tak set up include more than the lens, to set up the AP similarly would add about $100 to $150 in hardware. Sorry if anyone felt misinformed.
And in fairness we point out that these prices of the Tak set up include more than the lens, to set up the AP similarly would add about $100 to $150 in hardware. Sorry if anyone felt misinformed.
Examples of how Astro-Physics compares: at the June 1989 "Texas Star Party" a photo taken with an older Astro-Physics 5"f8 Starfire design (90 min. with hyper sensitized 6415 film) showed stars 2.5 magnitudes fainter than those taken with a Takahashi 12"f3.6 "Epsilon" reflector telescope (exposure on the same hypered film for 100 minutes)! A 7"f9 Starfire visually showed a team of observers including a professional astronomer from Lowell Observatory (who specializes in variable star observing) stars as faint as 15.6 even though the sky was not completely darkened! Shortly afterwards a squall developed and viewing concluded.
At the 1990 Texas Star Party, a 7"f9 Starfire (one of four at the event) showed better quality images of the planets, and even the brighter deep sky objects than nearby telescopes of larger aperture. On the Ring Nebula, a nearby 14" Schmidt-Cassegrain resolved it as a very bright ring, surrounded by three bright "semi-stars", against a bluish grey-black background. The 7"f9 "Starfire" at similar magnification showed a dimmer but more detailed Ring structure, nicely framed by three intense diamonds against a black velour background.
At the July 1991 total solar eclipse, many of the best and most realistic photographs of the eclipse were produced on Astro-Physics telescopes. A photo taken by Roland Christen with a 13cm (5.1") "Starfire EDT" on our Pentax 6x7 camera in Mexico was judged so realistic that it was selected to grace the cover of the October 1991 (fiftieth anniversary) issue of "Sky & Telescope" magazine! Numerous examples of Astro-Physics art works appear in many publications.
The following is the text from a letter from Martin Cohen in response to a forum discussion regarding best return for money spent on apochromatic refractors:
"Not that I dispute the generally great optical and mechanical qualities of Takahashi, Pentax, Nikon, etc. If Astro-Physics did not exist, then I'd be proud to own and sell these lines (correction - after AP then Carl Zeiss would have had to not exist too!). But I do not believe they are the best systems, not the best value in Apo technology. But lets get down to it:
I do not put much weight onto unscientific or non technical comparisons. There is too much latitude for personal opinion or variable conditions, even on accessories. And who can tell what shade of a color a planet should appear? (aside from avoiding obvious false colors).
Company Seven inspects and tests every telescope we sell. We then include additional informational materials with our in-house report characterizing each telescope. This document offers practical advice and predicts brightness, field of view on a variety of photo-video, and visual formats. It has helped us to build our base of practical experience.
Our experience and studies of published technical reviews employing a scientific approach to comparing similar instruments has led us to conclude that inch per inch, and pound for pound the Carl Zeiss and Astro-Physics Apos offer the most clear, high contrast views of any production refractor telescope. The Astro-Physics models offer views that are similar in terms of detail resolved to the Carl Zeiss, but the Astro-Physics provide greater versatility with a wider selection of telescopes and matched accessories and that are more affordable. Furthermore, Astro-Physics mechanics are better engineered to work in the field. I have owned many premium telescopes, but I use my AP's the most.
Lets look at technical and market issues:
AP telescopes are supported by a comprehensive selection of optional factory and third party photo-visual accessories (from AP made telecentric lenses for use only with DayStar Hydrogen-Alpha solar filters to SBIG, TeleVue, etc.). For example, AP offers a wonderful matched 2" photo-visual Barlow. And their 0.75X photo-visual telecompressor allows wide photographic fields of view and shorter exposure times and wide visual fields of view unrivaled in the APO industry. The 130mm f6 EDF becomes a 585mm f4.5 telescope; with a 55mm Plossl one sees 4.7 degrees of sky at only about 11X! This is the absolute widest field capability of the larger high resolution, one man portable telescopes. No other firm even offer a base 13cm model as fast as f6 anywhere in this class.
As an example of how Astro-Physcis telescopes have been well regarded historically: the November 1989 issue of –Sky & Telescope”¾magazine on page 522 published data on limiting stellar magnitudes as reported by numerous observers using 6 inch aperture telescopes. The reported magnitude ranged from 12.5 to 15.6; the highest data points being turned in were by Brian Skiff a professional astronomer who used his personal older Astro-Physics 6”f8 Standard Christen Triplet model telescope! Mr. Skiff reported 15.6 magnitude data points at 300X, and several data points at above 15 magnitude at 200X.
Published reviews and controlled tests have described the AP optics to be superior in most optical and mechanical criteria to all others but the Carl Zeiss "APQ" Apos. However, the now discontinued Zeiss APQ models were beyond the Astro-Physics price league entirely, and the accessory selection was complicated and limited by todays standards. Some Apo refractors such as the premium Japanese instruments were considered to be similar or superior optically until eclipsed by the introduction of the "EDF" and EDT" designs by Astro-Physics in 1990.
Even if those competing premium Apos were equal or superior in optical performance I still believe the AP is a better value. Consider the cost and availability of a comprehensive system of matched accessories; price a photographic (only) telecompressor for a Takahashi 5" then the Astro-Physics at $300 (which gives better images photographically and can be used visually too!) The availability of some Japanese (and to a degree Zeiss) accessories is not good; some of these appear to exist only in a price list. The telescope prices themselves are also beyond reason for the performance; a Nikon 4" Apochromat with mount or another popular 5" premium Japanese telescope with mount cost as much or more than a complete Astro-Physics 180mm (7.1") f9 "Starfire EDT" with mount! One paid less for an AP 206mm EDF triplet APO than one would for a Takahashi 6" FC Doublet (which is damn near impossible to find in the U.S. or Japan)!
In August of 1992 at the Takine observatory in Fukushima, Japan, and at the "Starlight Festival" at Ishikawa a comparative test of several popular 12.5 to 13cm telescopes including the Takahashi was performed; these included bench resolution, star test, and observing on the night sky. A Japanese magazine "Gekkon Tenmon" ("Sky Watcher") reported the results in their October 1992 issue. The Astro-Physics 13cm EDT was rated superior in optical figure and optical performance. The simulated double star resolution "results were Astro-Physics had the highest resolving power”. The variance is in part attributable to the fact that the Takahashi was a 125mm aperture versus the Astro-Physics 130mm; the difference in resolution was proportional to the aperture. They went on to state "in fact, it was their SFX 130EDT, highly regarded in the U.S. that showed the unquestioned best views in our comparative viewing test”; and –the tested scope showed star images that were about symmetrical inside and outside focus; a telltale sign of good optics.” "The star images were sharp and clear cut on globular clusters and loose clusters. Saturn looked clear and high-contrast. The telescope consistently delivers "hard" images at high power". While the comments about the Takahashi were though provoking relating to their optical quality control and figuring: "inside and outside of focus; the outer ring of the image outside of focus exhibited slight blurs; it looked like a little on the under corrected side”. They went on to state "but now that competitors are on the market, it would be positioned as a middle of the way telescope, without anything to differentiate it".
A translation into English of a report published July 1993 from Germany also confirmed that in March 1993 critical, controlled testing of 10cm models using a "Siemens Star" and artificial star concluded the AP 105 and the Carl Zeiss 105 APQ were judged equal to each other, and superior to those others tested (which included the Pentax SD, HF, and UF models). Their conclusion "No discernible quality difference between the Traveler and the APQ on either the artificial star or the Siemens test...both optics belong to the very best that is presently available in the fast refractor market. Whether the considerably higher price of the APQ will be accepted by the market, time can only tell."
And finally look at resale value. Just try to find an AP 130 or 105 EDT (or any used AP) for less than retail used. I sell many used scopes, but none other holds its value or actually appreciates so well. And see what one pays for a used 130 made by other firms. According to a letter to me on an astronomy forum from another person:
Mechanically, the Astro-Physics telescopes feature full precisely machined construction of virgin aluminum, phosphor bronze, or stainless steel. There is no chrome plate to peel, or cheap metal to corrode, and no plastic to crack or wear. The internal light baffling is the as well designed and executed we have seen on any current or previous production telescope; the Astro-Physics draw tube is also well baffled. Optical tube assemblies are fitted with a reversible, slide on and positive locking lens shade/dew shield (a particularly attractive feature to those who travel, or have limited travel volume), and is fully flocked internally. The main tubes are finished in a durable white epoxy. Lens cells for the 5.1" EDT and larger models are "push-pull" designs which allow realignment of the lens cell to the tube in the field if ever necessary. The focusers are smooth and accommodate a variety of accessories and payloads. The standard Astro-Physics focuser is a 2.7" diameter, rack and pinion model of 2.5" inner diameter draw tube, with a 2.7" x 2.5" extension, 2.5" to 2" and 2" and 1-1/4" reducers. There are no marring set-screws in this well integrated system. All adapters and reducers (even the optional matched 2" 2X telenegative lens) feature non-marring positive locking mechanisms that maintain a sure and aligned grip.
The "EDF" series is the fastest photo-visual design of the Astro-Physics line. While these are excellent as multiple use portable telescopes, they excel at astrographic applications. The standard 155mm and 206mm "EDF" telescopes feature 4" diameter focusers with matched 4" field flattening lens (put in place only for photography) to accommodate a focal plane up to 105mm in diameter. This can accept as large as 4"x5" cameras for very wide field photography: 5.25 degrees on the 155mm EDF with star images of 15 microns at the edges in the C to F spectral region! For those with less emphasis on medium format photography, or for those seeking a lighter and less expensive version, the two "EDF" telescopes are available in a lightweight configuration with the 2.7" focuser in place of the 4" components.
The mounts designed by Astro-Physics for the 130mm to 155mm instruments are the Model 600E and 900 series mounts. A 155mm "EDF" on a 600E with a wood field tripod is easily transported, one man manageable, yet rigid and economical from a performance point of view. Although the 600E mount will handle up to the 6"f9 for visual use and photography in a wind free environment, the 900 mount would add rigidity to improve astrophotgraphic efforts at wind prone sites.
The mounts designed for the 155mm to 180mm instruments are the Model 900 and 1200 series A 155mm or 180mm "EDF" telescope on a 900 mount with a portable pier is easily transported, one man manageable, yet astonishingly rigid. The 900 mount is the minimum we suggest to manage the 155mm telescopes if astrophotography from windy sites is anticipated. Although the 900 mount will handle up to the 180mm EDT for visual use and photography in a wind free environment, the 1200 mount would add rigidity to improve astrophotgraphic efforts at wind prone sites.
These mounts incorporate lessons learned over some years. They are particularly well suited for integration with advanced systems such as our CCD tracking and imaging systems. One could operate from a rechargeable 12 volt D.C. deep-cycle Marine or Recreational vehicle battery, or with a transformer from overseas currents. There are plans to offer retrofit able computer control electronics for the 900 and 1200 mount series in the future as the available technology improves in reliability and accuracy.
For current availability of these telescopes please contact us when you are close to selecting a particular model. Company Seven fills orders for Astro-Physics telescopes chronologically from our back-orders or from stock; this is a very popular instrument line and we have generally not been able to maintain it in stock, though we do maintain most models on display.
Keep in mind that there may be optional accessories required with any of the telescopes discussed herein, if not desirable for your particular applications. You may already own some compatible accessories; indeed many buyers "graduated" into this quality of telescope and may already own preferred compatible accessories. We will remain available to provide counsel for your needs.
To accept an order we would in keeping with factory policy require a fifty (50%) percent down payment. This payment will go through Company Seven to the Astro-Physics factory. The down payment will guarantee you a chronological position on the waiting list, and the current listed pricing on any components which might later go up in price. We ask orders include mailing and shipping address, and telephone numbers of the user for our records. The final payment is forwarded to Company Seven prior to delivery. International orders are shipped by air common carriers including Federal Express, and United Parcel Service. Freight charges and any import or customs fees are borne by the customer.
The most convenient form of payment for our customers is cash, check (in U.S. Dollars), or with a VISA or Mastercharge credit card. We will bill the credit card for product only as it is accepted up at our showroom or shipped. Company Seven will also accept payment by International Money Order, Wire Transfer or Western Union (inquire for routing instructions).
Astro-Physics is a comprehensive and well integrated telescope system, supported by the only firm in North America that has as much experience with many of the best products in the industry. I encourage you to take full advantage of our experience and resources, and if at all possible to visit our showroom to see these instruments. The showroom is open Monday to Friday 11:00 am to 6:00 pm, and Saturday 10:00 am to 5:00 pm Eastern (Washington D.C.) Time. Company Seven is formally closed Sundays, U.S. Federal Holidays and from 25 December to 1 January inclusive for the holiday break. If you have any further questions about specifications, applications or procurement of an instrument feel free to contact or visit with us.
Martin C. Cohen
Contents Copyright 1994-2000 Company Seven - All Rights Reserved