SSP3 photometer sensitivity

This may or may not be helpful to you.

I am well aware that some positions inside the reticle ring are more sensitive than others on my SSP3.  Pretty consistently, I get the most counts from a given star if I put it at position X.  

To keep this from becoming a potenital systematic error in the data analysis, I always place all stasr on point X to take the reading.  And keep an eye on the counts during repeated observations to detect if it has wandered off that point.

In my experience it is a fact of life with this kind of instrument that otherwise is very repeatable and dependable.

Hi Tom,

What is your focal length?  With a standard photometr, a fabry lens is included so that the photomultiplier sees the pupil of the telescope rather than a focused star on only part of its sensor.  With the SSP-3, you are dealing with a single silicon photodiode, with a physical size of 0.5, 0.75, 1.0, 2.0mm, depending on the unit (the 1mm detector is standard).  Optec quotes a less than 1% variation over a 0.05mm star image on the focal plane, or basically uniform at the 1/20th of the sensor level.  the reticle is supposedly equal to the size of the full sensor.  Therefore, you shouldn't see huge variations in received flux, as long as all of the star's profile is falling on the sensor.  The closer to the center of the reticle ring you are, the more this assumption is valid.  If you put the star on the outside edge of the ring, then part of the star image can easily be falling off of the sensor.

One way of improving the photometry would be to defocus slightly.  make the star image ~0.5mm in size, so that more of the silicon photodiode is involved in the detection provess.

Arne

Hi Tom,

If we assume that your SSP-3 has the 1.0mm sensor, then it covers a 100 arcsec diameter circle.  A focused star with, say, 4arcsec fwhm, would then occupy 1/25th of the sensor diameter.  Any other star that happens to be within 50arcsec of the star you are measuring will also fall on the sensor, assuming the target is centered.

My recommendation:  put the star at the center of the reticle ring.  If you are really interested, then raster-scan a star across the 100arcsec field, and map out the photometer sensitivity.  I'll bet that you find a nice flat plateau where the readings are essentially identical, and that plateau should be pretty close to being centered where the reticle ring is.

Arne

Tom,

I had a similar problem a few years back. You can adjust the position of the reticle yourself via the allen screws accessible from the outside of the photometer but it is a tedious,  time-consuming, fiddly job! If I remember correctly I spent a night cursing and swearing while adjusting! To do this adjustment you need a very stable mount that is accurately set to the relevant celestial pole and tracks accurately. (I had a permanent setup of a shed with a roll-off roof and an EQ6 equatorial head mounted on a steel pier that was concreted in the ground.)

You then find a star with a suitably strong signal (counts). Scan back and forth in one direction until you find the maximum signal then slacken and tighten the opposing screws to line up the reticle with the star. You then do the same for the orthogonal direction. Sounds simple but I warn you it was not. I am satisfied that it is now sufficiently aligned as indicated by my results in a recently published JAAVSO article on I-band measurements of semi-regular red variables.

I would offer to come over and help you but suspect you are a long way from Tasmania! All the best.

Terry

I'm not sure what the "problem" is. It is not surprising that the counts fall off when the star is positioned at the edge of the reticle circle. The instrument is designed to operate correctly when the star is centered in the reticle. I used an SSP-3 for many years, with portable telescopes that did not have perfect drives or highly precise polar alignment, so there was always some minor drift during three 10-sec counts. But I never had a problem, even when the star was halfway to the reticle edge when I flipped up the viewing mirror; the photometry was always sky-limited, usually in the +/- 0.003-0.005 range

Tom,

Did you get my email in reply to yours? Have you positioned the star across the illuminated reticle ring in two orthogonal directions noting when the counts drop off. If reticle is properly adjusted the drop off in counts should be the same for any position near the edge of the reticle ring. If this is all okay then you may have a spot of different sensitivity on the detector although Optec claims (in the manual) to visually inspect each detector to ensure free of surface defects.

Terry