Hi Denis,

I think I know what's happening here.

Guide now draws comets from three different datasets. (It uses
the MPC list so it can get the very latest comets; a list by Patrick
Rocher at the IMCCE to get most comets since 1995; and an "historical"
list going back to the earliest recorded apparitions of Halley's
Comet.) Imagine the first dataset used has magnitude parameters
indicating that the object is mag 14, and the second puts it at mag
13, and the third at mag 12.

In that case, if your limiting magnitude is 14 or greater,
Guide will find the object in the first dataset and draw it. It
keeps track of what's already been drawn, so when it comes across
the object the second and third time, it's ignored.

Suppose your limiting magnitude is 13.5. When the object is found
in the first dataset, it's half a magnitude too faint and is skipped.
Then it's found in the second dataset at mag 13, and it's drawn.

And... you see where I'm going here... at limiting mag = 12.5,
it's drawn from the third and last dataset.

I don't have a good answer to this off the top of my head. Let me
get back to you on this one...

-- Bill

On 09/19/2013 07:23 PM, Denis wrote:
> When comets are set in "Data Shown.." on "Auto", decrease the stars
> magnitude to the point where a comet is about to disappear, its position
> changes about a magnitude before the event. After the disappearance,
> increasing the magnitude reverses the process, when the comet reappears
> it will move when magnitude is further increase by one.
>
> If one creates a track as the comet appears and then decreases the field
> which increases magnitude the comet changes position and the track
> become invalid.
>
> We tried on 3 different computer systems and replicated the error with
> more than one comets. Guide version is from 17 June 2013.
>
> This only append when a comet is about to disappear or reappear.
>
> Hope this is clear enough explanation to find a fix.
>
> Denis
>
>
> On 14/08/2013 8:24 AM, Bill Gray wrote:
>> Hi Pierre,
>>
>> There's a standard magnitude formula, using H (absolute magnitude, the
>> magnitude the asteroid would have if it were one AU from the sun as seen
>> from the sun itself) and G (the 'slope parameter', which roughly describes
>> the magnitude as a function of phase angle).
>>
>> The absolute magnitude H essentially combines the diameter and albedo to
>> say "this is how much light the object reflects". It's what you actually
>> observe when you point a telescope at the object: you can't tell if it's
>> a really big object covered in black paint, or a small object covered in
>> mirrors. (You can put a lower limit on the object's diameter based on the
>> assumption that it reflects 100% of the incoming light. It might be
>> twice as big if the albedo is really 25%, or five times as big if the
>> albedo is actually 4%. Or a truly massive alien artifact cleverly disguised
>> as a small asteroid, covered in black paint.)
>>
>> The IAU standard magnitude formula is :
>>
>> mag = H + 5 log(r * delta) - 2.5 log((1-G)phi1 + G * phi2)
>>
>> phi1 = exp[ -3.33( tan( beta / 2) ^ 0.63)]
>> phi2 = exp[ -1.87( tan( beta / 2) ^ 1.22)]
>>
>> ...in which logarithms are to base 10, r = the distance between the
>> asteroid and the sun, delta = distance of the asteroid from the observer,
>> and beta = phase angle.
>>
>> If an asteroid has been observed for a while over a wide range of phase
>> angles, then the slope parameter G can usually be determined. But for most
>> objects, a default value of 0.15 is assumed. I don't know what that value
>> corresponds to in a physical sense.
>>
>> -- Bill
>>
>>
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