Imaging

1° calibration star
STF 1927 ......
STF 1864 ......
STF 1879 ......
STF 1884 ......
etc ...
Pictures taken with the 8" at 6m25 focal distance. 		First of all we capture a calibration star (rom the calibration list published by the french Commission des étoiles doubles or from the WDS calibration list)

Then we capture the stars at the program of the night ...

The session finally ends by a new calibration star.
This second calibration star permits to confirm among other things that there was not accidental change of the position of the camera.

New advantage of the webcam in relation to the other systems : its speed.
We can capture as many frames as we wish.
100 to 150 pictures are sufficient to get a lot of measurable frames.
A well led session permits to capture from 6 to 10 stars/hour
We don't need no more to spend a whole night outside to acquire only some measurements !

Reductions

Well, we have our images, it's now necessary to transform all that material in good old astrometric values !

From the calibration stars we deduct the calibration constants,
that is to say the exact orientation of the picture in relation to the sky
as well as its scale (the sampling)
Calibration Stars

353°3 - 16 ''06
Shift of the picture in angle = +0°92
Sampling = = 0''18498/pixel

It is now time to measure the pictures acquired during the evening
Here are the values measured for our short session
STF 1864 110°57 - 5''55
STF 1879 085°88 - 1''55
STF 1884 055°59 - 2''11
etc ...

 

Astronomical softwares possess functions calculating the stars photocenters.

Most of the time the pictures are not stacked but measured one by one and the final measurement is the average of every individual measurement.
It is obvious that measuring them by hand is an unattractive work and an easy way to do mistakes.
So, I prefered to develop my own software to do most of the work automatically :
- Sort the images
- Calculation of the calibration constants
- Automatic reduction of a set of frames
- Log generation, these logs can be exported to databases or spreadsheets for further use

This software is free and distributed on demand (look at the download page)

What does that give?

A same couple will be observed two or three times at close epochs.
The average of these observations will provide the final measurement.

STF1785 is an orbital binary. That is to say that the two stars evolve around the same center of mass. This system is followed regularly since its discovery. We saw it browsing a complete orbit.

The picture taken with hardly more of 3 meters of focal distance is strongly enlarged and stacked on a drawing of the orbit with the different measuring points that permitted to calculate it.

STF1785 ephemerid for the date of observation gives: 174°7 - 3"29 Hei1988d (Gr 2).
Its grade 2 permits to grant him a good confidence.
Let's compare to our measurement done on three nights as describes previously.
The measured value is: 174°82 3"31 (3n).
The difference between the observation and ephemerids is called o-c in the jargon (observation minus calculus). Here this difference is weak o-c = +0°12 / +0"02.

We know as well a very few number of stars. Hundreds have orbits sometimes calculated with very weak degrees of confidence , the calculators need measurements points !

STF1719
1991.25 359°00 / 7''01 (H)
2002.29 358°66 / 6''94
STF1721
1991.47 357°50 / 6''26 (T)
2002.29 358°78 / 6''32
STT261
1991.61 339°90 / 2''44 (T)
2002.29 339°21 / 2''47
STF1757
Eph 2002 : 127°3 / 1"98 Hei1988d (Gr 4)
2002.29 129°21 / 1"95
o-c : +1°91 / -0"03
STF2021
Eph 2002 : 354°2 / 4"097 Hop1964b (Gr 4)
2002.40 354°85 / 4"12 o-c : +0°65 / +0.02

A few other results obtained with the same instrumentation.
They are compared to Hipparcos measurements (HIP), Tycho (TYC) or to the orbital ephemerid.

So no more micrometers?

No, no don't think that!
There are many things that the webcam cannot do,,
first of all you need a computer.
Then the webcams remains limited to the couples with a small magnitude difference.
After several hundreds visits of double stars, all used webcams shows severe limitations as soon as the difference of magnitude passes beyond 2,5 to 3 mag.
The micrometer is equally better with tight stars.
On the other hand, as soon as the separation reaches about the double of the resolution power, I bet that the measures are systematically at least as good as those obtained with the micrometer (on a same instrument of course!)

Ok, ok, what are the options ?

Webcams commercial versions
 m <= 9
Diff mg < 2.5
Webcams Deep Sky mod
 m>9
Diff mg < 2.5
Webcams with B&W sensor
 m >> 10
Diff mg < 2.5 / 3

Webcams in commercial version, that is to say without electronic modification can reach the 9th mag at F15.

Beyond it becomes necessary to proceed with some modifications to reach longer exposure times.
These modifications don't bring necessarily the hoped gain because some color sensors are too much noisy.

The very best way is to replace the color sensor by a b&w one far more sensitive and less noisy :
- noise coming very slowly, with these sensors you benefit of slightly better dynamics and therefore more difference of magnitude.
- sensitivity being better, you reach very easily m>10 with a high focal ratio (F/30) with very acceptable exposure times (about 1 second).

A lot of couples to the surroundings of m=10/11 are disregarded. Isn't that the source of pretty programs ?

My webcam is commercially suited without modification, could I do some valuable work either ?
Under our latitudes, there is about 1500 couples with a magnitude difference < 2 and with the separation accessible to an 8" telescope.
At a rate of 100 stars per year, that makes a fifteen years program !!!
Surely sufficient to absorb the 50$ insvestment !

Conclusion

in spite of some limitations the webcam proves very pleasant to use.
Its major advantages are:
- simplicity of use
- impersonal instrument
- speed allowing many observations in one night
- comfortable reductions with the help of computer software
- precise measurent with an unbeatable quality/cost ratio

Double Stars are waiting for you!!!