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Thread: Mount Tracking Error / Mount Charateristics

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    Default Mount Tracking Error / Mount Charateristics



    Since this question frequently arises, I'm going to do a short-segment series on telescope mount error, similar to the Newtonian Back-Focus Distance series.

    I'm going to start with a simple set of "rule of thumb" approximations.

    Embedded in this post is a table, presented for the time being in image format that contains the details.

    For context associated with the tabular data:

    - Low-end, consumer-grade/commodity mounts are representative of dual-arm fork alt-az, and their wedge adapted counterparts found on Celestron and Meade SCT's, as well as GEM mounts, including the CG-5/EQ-5, LXD and similar GOTO/motor-driven mounts.

    - mid-range consumer-grade/commodity mounts are representative of GEM mounts, including the Synta EQ5P/Orion Sirius, Synta EQ6P/Orion Atlas/Celestron CGEM

    - high-end consumer-grade/commodity mounts include the Losmandy GM-11, Celestron CGE, and CGE Pro

    At the high-end consumer/commodity boundary with the low-end professional mount class there begins to be considerable overlap. Similarly, the low-end and mid-range professional grade mounts to some extent overlap.

    - the Paramount ME is representative of the low-end professional class mount, along with some of the Losmandy and AstroPhysics mounts. This relates more to mount capacity, less to the mount's precision and tracking error spec

    - mid-range and high-end, along with observatory class mounts are manufactured on a more or less custom basis or in very limited quantities. Some are "one off", particularly in the observatory class mounts. Manufacturers include Astrophysics, RCOS, Dream, and Losmandy, among others. Prices for these mounts exceed $20,000 USD, and range to well over $1,000,000 USD for observatory/scientific grade mounts.

    The tabular material is presented with relative pricing in normalized format, as opposed to absolute dollar values.

    The tracking error is measured in arc-seconds, exposure durations are approximated for the range of arc-second error, and expressed in seconds.

    Here is the table:
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    With the initial segment for context, we can examine a few questions, pertaining mainly to how much tracking precision is required, or how much tracking error is tolerable.

    All other factors being equal, the mount tracking error tolerance is a function of:

    - the focal length of the telescope in use for the imaging session
    - the characteristics of the camera/imager in use
    - the declination of the target object in the sky

    The first is easy to consider - the longer the focal length of the imaging telescope, the less the error tolerance in tracking (or stated differently, the higher the tracking precision is required).

    It is possible to make compromises for poor tracking precision by reducing the focal length of the primary imaging optic, either via use of a shorter focal length telescope, or by use of a focal reducer. This comes at the cost of reduced image scale, as well as larger field of view. It is also possible to use corrective guiding, within a certain envelope to reduce the impact of tracking error.

    Declination issues from from two sources; incorrect or poor polar alignment, and atmospheric seeing. In both cases, the higher the object is above the celestial horizon, the less the impact tracking error will have on the image. One of these root cause factors is easy to minimize or eliminate - develop the skills and precision in polar alignment sufficient to eliminate PA as an error source. Drift alignment is the best answer, and although it can seem intimidating and complicated, it is relatively straightforward to accomplish, and should be considered a necessary skill for an imager. Atmospheric seeing is correctible, but at a cost - active, or adaptive optics and guiding can be utilized to eliminate or minimize this source of error.

    I'll deal with camera/imager related factors at this point, because they're among the more complex and variable in the collection of issues.

    There are two types of camera category to first consider; "cooled", and "uncooled". These categories apply to an imager, be it a DSLR camera, CCD, or video imager. Within the "cooled" camera category, there are three common types; "convective/conductive", "thermoelectrically cooled, or TEC", and "liquid cooled". In the convective/conductive subcategory, the camera uses heat sinks and or cooling fans to provide cooling. In the TEC case, the camera uses a Peltier thermoelectric cooling system, some of which can also use auxiliary liquid cooling and/or convective/conductive components including fans. The liquid cooled category typically use cryogenic cooling systems, including dewar reservoirs, active cryogenics that may including compressors, vacuum pumps, radiators and other heat exchangers. The latter subcategory is typical of scientific and research cameras, atypical of anything most amateur astronomers would use.

    Why is this important?

    Ultimately, the camera/imager forms one variable in the "ceiling" on the length of a single exposure duration. The most significant factor on this ceiling is noise, and in turn, the dominant contributor to this figure of merit is thermal noise. Thermal noise arises from the ambient temperature at which the camera is used, combined with thermal energy generated by the camera's electronics. There are certainly other sources of noise, including read amplifier noise, and so on, but the most significant source is thermal noise. Every camera/CCD imager has an inherent thermal noise floor, and every degree ambient temperature increase, or decrease add to, or subtract from the inherent noise floor.

    Uncooled cameras have the shortest duration exposure thresholds, because past some point, the thermal noise gain overwhelms the signal gain in that exposure/frame.

    Cooled cameras have longer exposure duration thresholds, because the cooling system controls the buildup of thermal energy, thus allowing the imager's exposure duration to be increased without being dominated by thermal noise.

    Aside from thermal noise floor and ceiling thresholds, there are other camera-related factors that limit total maximum single exposure duration. Some are mechanical, for example certain types of shutter systems have limited "open" duration due to energy, latching conditions, charging systems, etc. used to operate the shutter. Some have limited well depth - that is to say the CCD/CMOS sensor converts photons from the imaging spectrum into electron equivalent. These electrons fill "wells" in each pixel of the sensor. When the electron well fills to capacity, a condition is created where the excess energy may bleed to adjacent pixels, or into other supporting circuitry. This condition is known as "blooming". Most CCD imagers have anti-blooming functions, but these only work within limits.

    Another factor is pixel size. More pixels is generally better, but small pixels are inferior to large pixels. More large pixels in a sensor is better than more small pixels in sensor. This is an electrical issue, and relates back to the sensor's noise figures of merit. Large pixels are also better from an error tolerance perspective as it relates to the effect of tracking error on the image.

    All this goes to say that one needs to understand thoroughly the nature of the imaging camera. It will have a certain quantum efficiency, which relates to the minimum exposure duration necessary to image a given object. The camera will have a certain maximum tolerable exposure duration. One needs to control for the mount tracking error within the minimum and maximum exposure duration envelope of the camera.

    It is unnecessary, for example to attempt to extract tracking precision from the mount significantly beyond what is possible with the imaging camera. If one's camera has a maximum possible single frame exposure duration of four minutes, it isn't necessary or cost effective to have a mount that can deliver accurate, unguided tracking of an hour.

    In summary for this segment:

    - telescope focal length is one variable impacted by the mount's tracking performance/error
    - the choice of camera influences the required level of mount tracking precision/error tolerance
    - accurate polar alignment is required to minimize the effect of mount tracking error
    - the mount tracking precision/error value must map into the envelope of exposure duration (minimum and maximum) required by the camera/imager
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    Austin Thanks for your very insightful post. I was wondering if you can help me with my current problem. I have a Nexstar 8SE scope which is mounted on a Celestron Advanced GT mount. Initially after I got the mount I used it to take some very nice photos of M42 and M31 and noticed perfect tracking (exposure time was 30seconds and star trails were nonexistant). Recently however I cant get it to eliminate star trails at all even at 10 second exposures (15 to 20 second exposures are really bad). I polar align and use the 2 star alignment with 2 or 3 additional calib stars. When I ask the scope to find something, it finds it dead on without issue. It just doesnt seem to track very well for astrophotos which is something I was hoping for with this better mount that I bought. I am not sure what is going on. I have done a factory reset a few times without any improvement. Please help if you can.

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    A few conceptual comments:

    - for AP, precise alignment affects both pointing accuracy and tracking accuracy

    - pointing accuracy (GOTO) and tracking accuracy are largely unrelated. It is possible for a mount to have excellent pointing accuracy and terrible tracking precision, and vice versa

    - tracking accuracy is much more difficult to achieve, to improve, and to diagnose/measure

    A handful of things come to mind based on the information you've provided.

    A Factory Reset on the Celestron/Synta mount series will cause various parameters stored in the hand controller to be reset/zeroed. This includes the cone error value, RA index value, and in mounts that support PEC, the correction table (does not apply to the AGT), as well as backlash settings.

    Unless you calibrate the mount after one (or more) Factory Resets, the mount will operate without them, but may (will likely) suffer from pointing errors related to the lack of cone and RA index error. To a lesser extend you may have tracking problems that come about after initial mount alignment and slew to target - in this case due to the loss of the backlash settings. If you haven't done so, you need to run the Calibrate action from the Utilities menu on your hand controller. This needs to be done every time the OTA is removed from the mount, because it affects the cone error in the mount (related to OTA and mount coaxial alignment).

    I'm going to infer that you are not auto-guiding, or using some external utility like Phd Guider, and instead relying on the mount itself for tracking.

    In this case, precise polar alignment is the key to helping eliminate trailing errors in the image. If you are not drift aligning, you'll almost certainly see trailing errors in exposures longer than about 10 seconds. These will worsen depending on the altitude and azimuth of your target, and change direction based on the nature of the underlying misalignment and target (direction the OTA is pointing). The drift alignment procedure is described in your AGT manual, and other forms of it exist in various internet documents, so I won't cover it here.

    If you have achieved good polar alignment, and have drift aligned the mount, the remainder of your tracking error will come from PEC (not easy to deal with on your AGT mount) and other forms of mechanically induced error. The most common are balance errors.

    A poorly or otherwise marginally balanced OTA on the mount will create significant problems with tracking for long exposure AP. The mount drives, including motors and gearing end up fighting the balance-induced problems, which combine with periodic error, any polar alignment error, and so on. Paying careful attention to balancing the OTA and connected components on the mount will give you big dividends in terms of improving tracking precision.

    Beyond this, you drive into mechanical tuning of the mount - this involves taking precise measurements and a systematic approach to diagnosing what to look for (first), followed by partial or complete disassembly of the mount, cleaning, polishing, perhaps replacing some components, lubrication and reassembly. There are descriptions of this procedure for the AGT on various internet sources, as well as at least one service organization that will do it for fee (expensive relative to the cost of this mount).

    Before going down this road, check the simple things:

    - balance the OTA and any necessary components on the mount - this includes the finder, camera, etc.

    - calibrate the mount after initial alignment

    - pay careful attention to polar, and drift alignment

    It won't pay to do anything else until you're sure that these three issues have been thoroughly addressed.

    The initial "good" tracking performance may just have been a happy accident. The AGT mount is in all honesty a lower-end mount. It takes a lot of effort (some of it on a repeated basis) to get excellent high precision tracking out of it for long exposure AP.

    There are also other factors to consider, like temperature, wind conditions, level conditions for the tripod/mount, all of which become variables that affect tracking.

    The last thing that comes to mind for this mount is the external DEC axis drive interconnect cable. More than once I've seen the RJ-style connectors on this cable either oxidize, or become loose over time due to mechanical relaxation on the plastic spring tab (on the connector), or the contact wires inside the female end. The mount will still point accurately in many cases, but it won't track when DEC correction is required. This usually surfaces for someone who is auto-guiding, and DEC corrections don't get made by the mount itself, the mount only tracks in RA unless some external forces acts on it - i.e. a human providing DEC correction using either an OAG or finder-based manual guiding, or via an ST-4 correction on the guide port. The connector pins can be cleaned with either a pencil eraser (male ends), and the spring tab can be restored by (gently, very gently) bending it out slightly and away from the connector body.
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    Thank you for the great information Can I ask you, what you would consider a great mount for someone that wants to do astrophotography? Something compatible with a Nexstar 8SE?

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    At price points affordable to the vast majority of us, the class of mounts OEM'd by Synta are generally the best starting point.

    These include the CGEM (Celestron), Atlas/Sirius EQ (Orion), and HEQ5/6 (Skywatcher).

    This mount platform has reasonable capacity for most refractors up to probably 6", medium SCT's up to 11", and Newtonians up to probably 8". The larger, heavier OTA's will still require careful balancing, but a working load up to about 40 lbs. (maybe 45 lbs. with careful tuning) is reasonable on this class of mount.

    This particular Synta mount in its many commercial forms is well-understood, fielded in a large enough volume to be well-supported by both the amateur community as well as the distribution companies (Orion, Celestron/Skywatcher in order of quality of support provided), and at roughly $1100 as a street price average in the U.S., it won't break the bank.

    Out of the box, in the hands of a patient and skilled user it provides good tracking accuracy, good to excellent pointing accuracy, and is reasonably reliable over a range of operating conditions most of us are willing to observe in (temperature/humidity).

    With a little bit to a lot of additional effort (tuning and hyper-tuning), this mount is capable of highly accurate tracking for the extreme end of long-exposure AP, and will offer excellent performance for short to moderate exposure AP. In the latter case, it will track accurately enough to extract the longest possible exposure in low to moderate light pollution zones.

    In terms of procuring this mount, they become available on the used market through two paths - very serious AP/imagers who are stepping up to Paramount or AstroPhysics mounts (the next level of performance), or by those who've given up on AP due to the rigors and costs and have become disenchanted. Via either path, these mounts are usually in very good shape, either having been tuned and cared for by a serious imager, or are still almost in "new in box condition", having received very little field use.

    New, they are easy to obtain via either Orion or Celestron/Skwatcher, and priced competitively. The choice of distribution/branding is largely personal, with Orion probably offering better support and service at a slightly higher average price.

    These Syna mounts are all based on the same mechanical platform and components - by brand they differ in terms of electronics, firmware/software (user interface and slight feature/function), and some electro-mechanical differences (steppers vs. servo, encoders, other miscellany). These mounts also differ over model year and serial number versions - the newer vintages are "better" than the older with respect to new-in-box operation. Things like internal lubricant choice from the OEM have improved over time (Google "Synta Grease" or search for this term here on the forums).

    That's mostly my objective opinion, based on experience with various mounts over the years - this isn't to say that other OEM's don't make good mounts, as they certainly do. But for price/performance these days, the Synta based mounts are a good starting point for a mount under $1500....
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    Austin Thanks so much. One last question ( I promise its the last one for a while). What is your opinion on the new HD scopes from Celestron? The SCTs I mean. For example if I want to do imaging as well as star gazing, is it really worth getting an 11 inch HD vs a standard 11 inch SCT Celestron scope?

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    Quote Originally Posted by Summersweet Lane View Post
    Austin Thanks so much. One last question ( I promise its the last one for a while). What is your opinion on the new HD scopes from Celestron? The SCTs I mean. For example if I want to do imaging as well as star gazing, is it really worth getting an 11 inch HD vs a standard 11 inch SCT Celestron scope?
    I freely admit my (positive) bias:

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    Default Re: Mount Tracking Error / Mount Charateristics

    thx for the info.


    regards

 

 

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