ObservatoryScopes Versus Competing Instruments

ObservatoryScope Versus the Competition . . .

Presented below is a product comparison chart of ObservatoryScope's telescopes versus competing telescopes. The comparison chart compares 20" aperture telescopes since this is the smallest size manufactured by ObservatoryScope plus all of our competitors. Note, however, that the performance specifications shown below for our instruments apply up to and including the largest aperture size which we manufacture for each instrument classification (Folded Newtonian or Ritchey-Chrétien).

It is our intention to keep this comparison chart as accurate and unbiased as possible. We encourage our competitors to submit any corrections and specifications for any incomplete/unknown data fields so that this comparison chart may be updated accordingly. The product comparison chart does NOT include any price/performance comparisons or overall telescope performance summaries because there are too many variables involved in order to draw meaningful conclusions based on the specifications shown below.

Note that some data fields for a competing manufacturer's instrument may be listed as "unknown" because the relevant specification is not published on the manufacture's web site. This does not imply that the manufacturer does not have a specification for the "unknown" data field or that the specification is substandard in any way. It simply means that the relevant specification is not currently published on the manufacturer's web site.

Telescope Type:		Newtonian Derivative	Ritchey-Chrétien
Manufacturer/ Instrument Model:		ObservatoryScope 20" Folded Newtonian	DFM CCT-20 Cassegrain	ObservatoryScope 20" Ritchey-Chrétien	OGS RC20-140FN Ritchey-Chrétien	OMI Nighthawk 20" Ritchey-Chrétien
Price:	Unit Price:	$97,500.00	Not published	$130,000.00	$142,000.00	Not published
Price:	Complete System?	Yes	Yes	Yes	Yes	Yes
Applications:	CCD Imaging:	Yes	Yes	Yes	Yes	Yes
	CCD Photometry:	Yes	Yes	Yes	Yes	Yes
	Aperture Photometry:	No	Yes	Yes	Yes	Yes
	Spectros- copy:	No	Yes	Yes	Yes	Yes
	Visual:	No	Yes	Yes	Yes	Yes
Optical Tube Assembly:	Construction Materials:	Steel	Steel	Steel	Aluminum	Aluminum with carbon trusses
	Geometric F/ratio(s):	F/5; F/5.7 with Paracorr Moderately wide field Good immunity to sky glow	F/8.1 or user specified Narrow or moderately wide fields Excellent immunity to sky glow	F/8 or user specified Narrow or moderately wide fields Excellent immunity to sky glow	F/8.1 or user specified Narrow or moderately wide fields Excellent immunity to sky glow	F/8 or user specified Narrow or moderately wide fields Excellent immunity to sky glow
	Center Box at DEC Axis¹:	Yes	Yes	Yes	Yes	Yes
	Tube Design:	Four point open truss; closed tube optional Symmetrical design Rapid cooling with open tube Fans for closed tube cooling	Closed tube or optional four point open truss Symmetrical design Rapid cooling with open tube Fans for closed tube cooling	Four point open truss; closed tube optional Symmetrical design Rapid cooling with open tube Fans for closed tube cooling	Closed tube or optional four point open truss Symmetrical design Rapid cooling with open tube Fans for closed tube cooling	Three point open truss Asymmetrical design with potential for asymmetrical flexures Rapid cooling with open tube
	Baffling²:	Primary Secondary Focal plane	Primary Secondary Focal plane	Primary Secondary Focal plane	Primary Secondary Focal plane	Primary Secondary Focal plane
Mount:	Construction Materials:	Steel	Steel	Steel	Aluminum	Aluminum
	Mount Type:	Fork mount Symmetrical design	Fork mount Symmetrical design	Fork mount Symmetrical design	Fork mount Symmetrical design	Fork mount Symmetrical design
	R.A.-DEC Perpendicu- larity:	±2 arcseconds	Not specified	±2 arcseconds	Not specified	Not specified
	Track Through Meridian:	Yes	Yes	Yes	Yes	Yes
	Track Through Zenith:	Yes	Yes	Yes	Yes	Yes
	Slew & Image Immediately³:	Yes	Yes	Yes	Yes	Yes
	Base Design:	Large trapezoidal pyramid Excellent vibration dampening	Large triangular pyramid Excellent vibration dampening	Large trapezoidal pyramid Excellent vibration dampening	Small oblique cuboid Moderate vibration dampening	Small oblique cuboid Moderate vibration dampening
Drive System Mechanics:	RA:	Hybrid band-worm drive (= to 40", 2520 tooth worm gear) Immune to damage from contaminants	30" friction drive coupled to reducer Can be damaged by contaminants	Hybrid band-worm drive (= to 40", 2520 tooth worm gear) Immune to damage from contaminants	18" worm gear, 570 teeth Adequate size for aperture	Friction drive coupled to reducer Can be damaged by contaminants
Drive System Mechanics:	DEC:	Hybrid band-worm drive (= to 40", 2520 tooth worm gear) Immune to damage from contaminants	28" friction drive coupled to reducer Can be damaged by contaminants	Hybrid band-worm drive (= to 40", 2520 tooth worm gear) Immune to damage from contaminants	15" 360 tooth worm gear Adequate size for aperture	Friction drive coupled to reducer Can be damaged by contaminants
Telescope Pointing and Tracking:	Temperature Compensated Design⁴:	Yes Very stable pointing with delta T	Not specified	Yes Very stable pointing with delta T	No Pointing drift with delta T	Not specified
	Tracking Precision⁵:	<1 arcsecond MAX over 300 seconds; <10 arcseconds MAX over 1 hour Observatory class performance	<1 arcsecond MAX over 300 seconds; <10 arcseconds MAX over 1 hour Observatory class performance	<1 arcsecond MAX over 300 seconds; <10 arcseconds MAX over 1 hour Observatory class performance	<1 arcsecond RMS (after modeling) Duration not specified Very good performance	<0.0008 arcsecond / second RMS Observatory class performance
	Raw Pointing Precision⁶:	<15 arcseconds MAX to 30° from zenith; <40 arcseconds MAX to 60° from zenith Observatory class performance	<30 arcseconds RMS Zenith distance not specified; assumed to be entire sky Observatory class performance	<20 arcseconds MAX to 30° from zenith; <45 arcseconds MAX to 60° from zenith Observatory class performance	Not specified	<30 arcseconds MAX Zenith distance not specified; assumed to be entire sky Observatory class performance
	Calibrated Pointing Precision⁶:	<5 arcseconds MAX to 15° from zenith; <12 arcseconds MAX to 60° from zenith Observatory class performance	<20 arcseconds MAX to 60° from zenith Observatory class performance	<5 arcseconds MAX to 15° from zenith; <15 arcseconds MAX to 60° from zenith Observatory class performance	<30 arcseconds RMS (after modeling) Zenith distance not specified Good performance	<5 arcseconds RMS Zenith distance not specified Observatory class performance
	Pointing Repeata- bility⁷:	<3 arcsecond MAX for an offset move of 5 degrees; <6 arcseconds MAX for an offset move of 30 degrees Observatory class performance	<10 arcseconds RMS Offset move not specified Observatory class performance	<3 arcseconds MAX for an offset move of 5 degrees; <9 arcseconds MAX for an offset move of 30 degrees Observatory class performance	Not specified	Not specified
	DEC Backlash:	<1.5 arcseconds MAX Observatory class performance	"Zero backlash" Observatory class performance	<1.5 arcseconds MAX Observatory class performance	Not specified	"Zero-lash" Observatory class performance

¹ Center Box at DEC Axis: This is an important feature that helps to minimize tube flexure, especially if the tube is mounted on a German equatorial mount. Note that a center box may not be necessary for lightweight tubes composed of extremely strong composite materials. The need for a center box is strictly an engineering issue based on the structural design, the materials used and the weights and torques of the optical components and attached instrumentation.

For fork mounted telescopes, the presence of a center box helps to preserve coincidence of the DEC axes stub shafts on each side of the OTA and maintain the critical alignment of the DEC drive components.

² Baffling: Specific baffling techniques are not examined nor compared. Systems are merely examined for presence of appropriate baffles and/or light shields at strategic locations throughout the optical path. Closed tube systems obviously feature inherent baffling (the tube) for the primary.

³ Slew & Image Immediately: A fork mounted telescope should be capable of slewing and imaging immediately (within a reasonable settling time of up to 5 seconds for vibration and stresses due to flexure) if adequate attention has been paid to the design of the drive system's mechanics, overall mount stability and system harmonics versus vibration.

⁴ Temperature Compensated Design: Indicates whether the drive system features design means to compensate for differences in coefficients of expansion of the drive system's components, relative either to each other or to the structure of the instrument at the point(s) where the drive system is attached. Failure to factor such compensations into the design of the drive system (assuming it is necessary to do so) may result in slow drifts (offsets) in pointing accuracy as the temperature either rises or drops.

The lack of temperature compensated design may or may not be a problem, depending on the magnitude of the resulting pointing errors and the software used by the manufacturer to implement the calibrated pointing model. TPOINT is used by many manufacturers to implement a calibrated pointing model. The current version of TPOINT does not include the ability to correct for any lack of mechanical temperature compensation with the drive system. The proprietary control system software used by some manufacturers may include this ability (if it is necessary due to the magnitude of the resulting pointing errors), thereby alleviating the need to incorporate physical temperature compensation means within the drive system design.

⁵ Tracking Precision: The tracking precision specification should include a specified duration to be meaningful. A specification of maximum (MAX) error is preferable to a specification of root mean square (RMS) error. Drive systems with a MAX error should repeatedly perform within the MAX specification. A drive system with a RMS error specification could exhibit sudden errors significantly greater than the RMS value over a very short duration, yet still possess the stated RMS error.

⁶ Pointing Precision: The pointing precision specification should include a zenith distance specification to be meaningful. A specification of maximum (MAX) error is preferable to a specification of root mean square (RMS) error.
The telescope's Raw Pointing Precision is measured after applying pointing corrections for precession and nutation, the atmospheric effect of refraction at the current temperature and atmospheric pressure, and the relativistic effects of stellar aberration. No other corrections should be applied when measuring the raw pointing precision of the telescope. The telescope's raw pointing precision will then be limited by the following factors:

1.	Polar Error (altitude and azimuth polar alignment error)
2.	R.A.-DEC Non-perpendicularity (right ascension axis not being perpendicular to the declination axis)
3.	Optic Tilt (non-perpendicularity of the optical axis relative to the declination axis)
4.	Tube Flexure (inherent flexures within the telescope's overall closed or open tube structure)
5.	Mount Flexure (inherent flexures within the remainder of the telescope's mount, excluding tube flexure)
6.	Optic Flexure (inherent flexures within the support mechanisms for the telescope's optics, the attachment point for the focal plane instrumentation, and the inherent flexures within the focal plane instrumentation)
7.	Drive Errors (all decentering, periodic and/or misalignment errors of any gears, worm to gear engagements, friction disks, drive shafts and support bearings employed throughout the telescope's drive train on each axis)
8.	Drive Ratio Errors (manufacturer's error in calculating/measuring the exact drive ratio of a friction drive or a band drive system)
9.	Drive Backlash Error (any inherent "slop" within the telescope drive train, particularly affecting the DEC axis since it is assumed that the RA axis will "catch up" within a few seconds)
10.	Ambient Temperature Variation (pointing errors resulting from the differences in the coefficients of expansion of the various materials used within the telescope's drive train if either mechanical or software based temperature compensation is not implemented by the telescope's manufacturer, plus any new optic flexures which occur within the associated optical support mechanisms due to changes in temperature)
11.	Grit Errors (any errors caused by grit or other contaminants within the telescope's drive train)
12.	Drive Train Wear (any errors resulting from physical wear or deformations over time, or errors due to damage by grit or other contaminants to the elements within the telescope's drive train)
13.	Hysteresis (any remaining yet recurring and completely unpredictable errors which effect the telescope's pointing accuracy)

The telescope's Calibrated Pointing Precision is measured after a full pointing model (flexure map) has been created and implemented to compensate for all of the above errors. Obviously, the telescope's Calibrated Pointing Precision will be limited by the telescope's inherent hysteresis, any lack of mechanical compensations due to temperature errors, and any lack of support within TPOINT (or a similar software based pointing corrector) for any required higher order terms or formulas which could properly correct for the above errors.

⁷ Pointing Repeatability: The pointing repeatability specification should include a specified offset move to be meaningful. A specification of maximum (MAX) error is preferable to a specification of root mean square (RMS) error.

NOTES:

1.	A telescope that is capable of performing large offset moves and still return almost exactly to its original starting coordinates obviously possesses a properly supported optical train and tube assembly, and is relatively free from hysteresis.
2.	Cassegrain and Ritchey-Chrétien optical systems employ a secondary which, by nature of the optical design, amplifies any inherent but extremely slight collimation errors that might occur during the repeatability test. Optics can be supported fairly rigidly, but only to the point where more rigid support would stress the optical elements and distort the optical surfaces. If the optics are supported too rigidly, the result would be degraded image quality at the focal plane. Allowances must be made for this fact and for the amplifying effect of the secondary, which is usually between 3 to 5 times the primary, depending on the specific optical design and F/ratio. Values of up to 10 to 15 arcseconds pointing repeatability for Cassegrain and Ritchey-Chrétien optical systems, particularly if a secondary mirror focusing mechanism is employed and/or the specified offset move is several degrees, is quite good by any standard.
3.	A telescope which is inherently designed to have very low hysteresis should have excellent pointing repeatability. If such a telescope exhibits poor pointing repeatability, then the problem can usually be traced to a loose or improperly tensioned component(s) on the telescope. This is usually quite easy to remedy once the source(s) of the problem is identified.