I purchased the NGT-18 to observe Gamma Ray Burst transients which brighten and then diminish in magnitude quickly. It is probable that the transients may dim to 22nd magnitude before I can get them recorded.
To capture transients of this depth will likely require integration times of about 23 minutes unfiltered or 38 minutes filtered.
To achieve integration times of this length will require a very precise polar alignment.
Using the polar error formula of TXF/M=23636.23, we can compute the polar alignment error radius as follows:
X=23636.23M/TF where M=25 microns, T=38*60 seconds, and F= 2057.4 mm.
The error radius in arc-minutes, X is therefore 0.126 arc-minutes.
This error be reduced either in azamuth or in altitude.
The azamuth adjustment is made as follows: A equalangular triangular shaped piece of metal has a leg length of 31 inches. This frame allows an azamuth correction of about 1.25 degrees by turning a screw having 15 threads per inch. Therefore one quarter of a complete turn of th screw through 90 degrees will move the azamuth axis by 1/(4*15) inch over a length of 31 inches. The angular displacement in arc-minutes is therefore 60 *atan((1/60)/31)=1.85 arc minutes. Following this logic, a 1/8 turn will move the bar through 0.95 arc-minutes.
The second adjustment needed to reduce pole alignment error is the alt adjustment. This adjustment is made in one of two ways.
The most reasonable adjustment is made using the alt adjustment on the telescope itself. A second adjustment method is more subtle, and cannot move the telescope through large angular settings.
The altitude adjustment on the telescope is made using the turn knob for altitude. This adjustment is made against a circular metal sling shaped as two circular arcs. The diameter of a circle which would fit the circular sling of the telescope can be computed by knowing that 15 degrees of the circle have a radius of 5 inches. This gives a circumference of 24*5 or 120 inches. This leaves a circle having a radius of about 19 inches. The sling is moved across this arc using a screw having 10 threads per inch.
A 90 degree adjustment using the adjustment screw corresponds to an angle correction of 4.5 arc-minutes per quarter turn, or 2.25 arc-minutes per eighth turn. Note that this is substantially worse than the adjustment size for azamuth.
A finer adjustment can be made by turning the leveling screw at the apex end of the azamuth bar. This is the side pointing to the south. This has the effect of raising or lowering the apex of azamuth bar thereby adjusting altitude. The amount of the adjustment is computed as follows:
The azamuth bar is a triangle having equal length sides of 31 inches. The radius of the circle through which the altitude is raised is the length of the line drawn from the southern apex through the center of the opposite side. This length is 26.85 inches which can be computed by taking the square root of the absolute difference of the squares of the other two sides (31 and 31/2 respectively). The screw adjustment at the apex has 12 threads per inch, therefore, computing the angle in a similar way to the other calculations, the 1/8th turn of the apex adjustment screw produces a 1.33 arc second adjustment, which is a good bit better than the 2.25 arc-minutre adjustment possible using the altitude screw on the telescope.
The NGT-18 therefore provides a reasonable expectation for adjustment in azamuth of 0.925 arc minutes, and 1.33 in altitude. taking the square root of the sum of the squares of these two values gives a total expected adjustment size of 1.62 arc-minutes of granularity using 1/8 inch turns.
Applying the equation T=23636.23M/FX
where X=1.62, F=2052, and M =25 microns, T computes to be 291 seconds, or about 5 minutes.
8 accumulations each with a 5 minute integration time will produce the desired result.
