# Solar Orbiter -- Payload Working Group

## Action Completion Form

***[Action ID Number:]{.underline}*** 5.1 (UVC)

***[Responsible Working Group Member:]{.underline}*** Silvano Fineschi

***[Date of Completion:]{.underline}*** 11 May 2003

***[Action:]{.underline}*** UVC Pointing

***[Report:]{.underline}***

The Solar Orbiter spacecraft is expected to have the capability of
offset pointing from the nominal sun-center direction with an angular
range of about ±2° (that is, less than ±1 R at 0.2 A.U.). This
capability will allow the pointing towards the solar poles, during the
out-of-ecliptic encounters, of the high spatial resolution
remote-sensing instruments with limited field-of-views. The impact of
the S/C offset pointing on the coronagraphic observations is discussed
in the following.

In order to compensate for the S/C offset pointing, the following
alternatives for the coronagraph may be considered:

1.  Fixed pointing with over-occulting;

2.  Adjustable pointing with optimised occultation;

3.  Fixed pointing with optimised occultation and no observations during
    offset.

In the following, the pros and cons of these options will be discussed
to help the Solar Orbiter Project and any proposing UVC team to maximize
the scientific return of the coronagraph.

**1. UVC with fixed pointing and over-occulting**

In this alternative, the UVC would have the same fixed co-alignment of
all the other remote sensing instruments. The S/C's offset pointing
would be compensated by over-occulting the solar disk by the expected
maximum angle of offset. This angle would be somewhat less than 1 R, at
0.2 A.U. In this way, the FOVs of the on-disk, high spatial resolution
imagers could be pointed at the solar poles.

[Pros]{.underline}

By requiring no pointing mechanism, this is obviously the simplest
alternative.

[Cons]{.underline}

Assuming that the over-occultation is sized for the closest approach
during the nominal mission, that is, 0.2 A.U., then the inner edge of
the UVC's FOV would be about 2.3 R (that is, 1.3 R + 1 R
over-occulting). During the extended mission, the perihelion distance at
0.32 A.U. would further limit the inner FOV to about 4 R.

Figure 1-Right gives an idea of how the UVC over-occultation from 2.3 R
up to 4 R would severely limit the observations of the inner corona
between 1.7 to 2.5 R. This is the region where UVCS/SOHO measurements
indicate that the acceleration of the solar wind already takes places.
Note that the in the visible-light images, the diffraction due to the
vignetting of the external occulter would further limit the inner
coronal FOV, compared to that in the UV/EUV images (cf. Fig.2).

The observations during S/C offsets would only partially alleviate this
limitation. Even if the S/C offset were to bring one side of the inner
corona within the UVC's FOV, still, the [eccentric occultation would
introduce unbalanced stray-light and diffraction patterns]{.underline}
that would be difficult to characterize and that would reduce the
quality of the images.

![](media/image1.png){width="2.899212598425197in"
height="2.899212598425197in"}![](media/image2.png){width="2.95in"
height="2.9374015748031495in"}

**Figure 1.** [Left:]{.underline} UVC field-of-view with adjustable
pointing and optimized occulting. Occulter's inner edge is 1.3 R at 0.2
A.U. (nominal mission), and 2 R at 0.3 A.U. (extended mission).
[Right:]{.underline} UVC f.o.v. with fixed pointing and ±1 R
over-occulting. Occulter's inner edge is 2.3 R at 0.2 A.U., and about 4
R at 0.32 A.U.

![](media/image3.png){width="4.622916666666667in" height="3.675in"}

**Figure 2.** Optical performances of UVC. The diffraction due to the
vignetting of the external occulter (upper-left panel) would limit the
inner FOV more in the visible light images (upper-right panel) than in
the UV/EUV ones (lower panels).

**2. UVC with adjustable pointing and optimised occulting**

If the UVC were given the possibility of adjustable pointing, then the
occulting could be optimized for the closest encounter.

[Pros]{.underline}

[In this case, the inner edge of the UVC field-of-view could range
between 1.3 R at 0.2 A.U. (nominal mission), and 2 R at 0.3 A.U.
(extended mission).]{.underline} Figure 1-Left shows the advantage of
such FOV inner edge in imaging the inner corona.

[Cons]{.underline}

The adjustable pointing would require an additional mechanism not
included in the current UVC strawman configuration. This may have an
impact in complexity, and in the mass budget.

However, the S/C may be expected to offset point along one direction
(i.e., solar north-south). Therefore, the UVC adjustable pointing may be
achieved with a very simple mechanism with only one degree-of-freedom.

As an example of a possible pointing mechanism for the UVC, the
UVCS/SOHO's two degree-of-freedom (i.e., pitch and yaw) pointing is
briefly described below.

[Pointing Mechanism *a la* UVCS/SOHO]{.underline}

The UVCS pointing mechanism allows an offset pitch and yaw pointing of
the telescope assembly over ±1° range (Ref. 1). The telescope possesses
a ring that rests upon two crowned and hardened front support rollers at
symmetric positions about the telescope centerline (cf. Fig. 3).

The rollers are supported by roller bearings mounted by metallic stands
that are driven inward and outward by translation stages, providing a
bi-directional motion of the telescope assembly.

![](media/image4.png){width="6.0in" height="3.9583333333333335in"}

**Figure 3.** The UVCS/SOHO's pitch and yaw pointing mechanism consists
of two translation stages that by moving inward and outward provide
bi-directional motion.

Retention springs integral within the roller bearing subassemblies
restrain the telescope roll/pointing ring onto and maintain constant
contact between the ring and the drive rollers. Offset pointing
positions within ±16 arcminutes of Sun-center are measured by the Sun
sensor, while pointing positions outside of this central zone are
computed based on position telemetry of the two translation stages. The
fine-pointing Sun sensor, which consists of four redundant photodiodes
located at positions about the Sun-center reference line, provides a
Sun-center determination to within ±4 arc-seconds by measuring relative
photodiode signal intensities. This fine pointing control can be
utilized following roll maneuvers to eliminate any Sun-center
misalignment caused by the telescope roll mechanism. The translation
stage drive capacity allows offset pointing at a baseline rate of
approximately one arc-minute per minute, but slower and faster pointing
rates are available for implementation through the flight software. [The
power consumption of the pointing mechanism is about 6 W.]{.underline}

**3. UVC with fixed pointing, optimised occultation, and no offset
observations**

This alternative would call for not observing with the UVC during S/C
offset. The occultation would be optimised for the closest encounter's
distance,

*[Pros]{.underline}*

This would combine the pros of alternatives #1 and #2.

*[Cons]{.underline}*

The S/C offsets would take place during the out-of-ecliptic periods of
the encounters. Therefore, by not observing during those periods, UVC
would lose one of the major advantages of the Solar Orbiter platform,
that is, the out-of-ecliptic viewpoint.

***[Conclusion:]{.underline}***

The S/C offset for pointing the remote-sensing instruments with limited
field-of-views during the out-of-ecliptic periods does not represent a
showstopper for the UVC observations. Any proposing UVC team should
consider [the adoption of a simple pointing mechanism to compensate for
the S/C offset. This would be preferable to the alternative of a UVC
fixed-pointing approach]{.underline}. This last approach would have a
negative impact on the UVC scientific return because [it would require
over-occulting, or limiting the UVC operations during the most
interesting periods of the encounters, that is, during the
out-of-ecliptic passages.]{.underline}

The UVCS/SOHO experience has proven that a simple, reliable pointing
mechanism can be used for coronagraphic observations. The average power
consumption of UVC (\< 30 W) would not be affected. The 6 watts required
for the pointing mechanism are consistent with operating a stepper motor
in the envisaged Configuration Mode of the UVC power budget.

***[References:]{.underline}***

1\. Kohl, J. L., et al., 1995, "The Ultraviolet Coronagraph Spectrometer
for the Solar and Heliospheric Observatory," Solar Phys., **162**,
313-356.