Shack-Hartmann

Shack-Hartmann testing of the 40inch

What is the SH test
What is required in order to perform the SH test on the 40 inch
The bare bones of the procedure
More details
Results

What is the SH test

This test is a method, alternative to interferometry, for testing the optical quality of optical systems such as telescopes. In the Shack-Hartmann test, an improved version of the Hartmann test, light from the optical system being tested comes to a focus and is made parallel by the collimator, which also images the pupil on a "lenslet array". The displacements (shown by red arrows) due to the aberrations from the optical system, with respect to the ideal wavefront (shown by black arrow) are used to compute the aberrations (in terms of Zernike polynomials) and the wavefront.

The mathematical expersions of the first 7 Zernike terms ( Tilt, Coma, Triangular Coma, Defocus, Spherical aberration, Astigmatism, Quadratic astigmatism) are given here. To see examples of how the abberations described by Zernike terms look like go to the Spot-Optics collection .

What is required to do the SH test on the 40 inch

Sensoft datataking and reduction software and hardware key
The Sensoft CD contains the software that controls the motion of a stepper motor inside Puntino and also does the data taking by controlling the ST-8 CCD as well as the data analysis. This motor moves in and out of the optical axis a calibrated LED which we use to take reference image frames.
SBIG ST-8 adapter
The version of puntino we have requires only one ST-8, it is not supplied with a finder CCD. This ST-8 should be mounted on the flange of puntino with a T adapter, especially made for the 40inch.
Cables
The chasis of puntino has an outlet for an RS232 cable. This is a special cable that connects puntino to the laptop. The stepper motor is controlled with this one, essentially putting a reference LED on the optical axis when a calibration is required. No further alignment of the puntino optics is needed. The ST-8 cable that connects to the hardware key which must be installed on the laptop parallel port is needed.
12V DC power supply
Any 12V power supply will do. Just make sure that the polarity is correct: inner possitive, outer negative, however, we have one specilly bought for Puntino.
Mounting puntino on the 40 inch.
According to Rajiv if we suspect that the focal place is inside the tail piece, it is better to dismount it and mount Puntino. However, if it is tedious, then we should mount Puntino without dismounting the tailpiece. When we do the SH analysis, then if we find spherical aberration, we will have to shift the focal plane in or out till the spherical aberration gets to zero. The Sensoft software gives indications for moving the focal plane though the distance it gives by which we would have to move will not be exact as there is a corrector infront of the mirrors. Charles has already made the adapter to the tailpiece.
Experimenting with a Meade 8.
I talked to Rajiv about using one of our Meades here to test Puntino down here before going to the 40inch. Rajiv thought it is a good idea though Puntino is custom made to focus with the 40inch, but it is possible to trick the software and get some familiarily with it. If you have any comments on this i would like to hear about it please

The bare bones of the procedure for the 40inch

Mount the SBIG CCD using the T-mount adapter (supplied), removing the T-mount from Puntino by loosening the two (hidden) Allen screws, one at the top, the other at the bottom of the flange (see picture later on).
Connect Puntino to the serial port of the computer using the supplied cable.
Connect the 12V power supply (inner polarity: positive).
In the WFS section, move the stepper position to Reference by clicking on the radio button. The reference light comes on automatically. Choose the SBIG CCD from the drop down box (same WFS page). Connect the CCD to the PC using the correct cable (the cable that was supplied by you to us was not working so make sure that you use a new one).
Turn on the cooling. Set the exposure time to 1sec, and take the reference frame (by clicking on Exp on the toolbar). It should be like the one included in this document. Use binning for the initial adjustments.
Align the spots along the rows of the CCD.
Take a good frame (about 200 seconds), with dark subtraction (specified on 2nd page of WFS).
Choose the directory on 1st page of WFS for saving the data.
Using Windows, rename it to mir.fit.
Take another reference frame and call it ref.fit.
Now go to SH property page, select the directory (specified in WFS) where you have saved the frames, select the ref and mir frames. Choose the Zernikes and do the analysis.

HOW THE TEST IS DONE

Mount Puntino on the telescope, either on-axis or off-axis.
Under remote control from the PC, take the reference calibration frame, using the inbuilt light source.
Point the telescope to a star (about 6-7th magnitude) using the star catalog in Sensoft. Take a 30 second exposure (to integrate out external seeing).
Do the Analysis using Sensoft. The results are ready in about 10 seconds.
Take multiple SH frames for reducing noise due to air-effects. Run Analysis. Sensoft automatically computes the average values.
Align the telescope using the indications given by Sensoft.
Find the correct focal plane using the indications given by Sensoft.
Correct support problems using the plots of the wavefront/mirror surface.
Look at the image obtained with CCD#2 for estimating seeing, or for having a look at the out-of-focus image of the pupil.

STEPPER MOTOR: ADJUSTMENT AND TROUBLESHOOTING

The stepper motor of Puntino has two positions at the entrance of the instrument:

In the first position (Mirror), a 145mm hole defines the entrance aperture for finding the star

In the second position (Reference), a LED (with a pinhole) is the source for taking the reference SH image.

Each step of the motor is 0.021mm.

The stepper motor is controlled from the Stepper property sheet under WFS. The serial port to which the instrument is connected can be selected here.

Two values for the stepper motor position need to be specified: absolute position for the Reference and Mirror positions.

The absolute value of the stepper motor position for the Reference is near, but not exactly, zero. To get the correct position, insert a value of zero in the box for the Reference (see figure below). Go to the Exposure property sheet, and click the Reference radio button. The reference light is switched on automatically. Take the reference SH frame. Check if it is centered on the CCD, the direction of decentering depending on the way the CCD has been mounted). If it is not, insert a value of, say 2 steps, and in the Exposure property sheet, click the radio-button for taking the reference SH frame (this will move the stepper motor), and check the centering again. Repeat the procedure until the centering is OK (within a few pixels). Normally the value should not be more than 10 steps.

The tolerance for the centering is not extremely critical, and should be about 1-2 pixels.

To the step value found for the Reference position, add 620 steps.

The speed of the stepper motor:

The default value is 300. It can be increased up to 600 if a faster speed is desired.

The motor status commands can be used to move the motor. If there are any problems, the user will get a message: \346OK\306 or \346Not OK!\3 06, along with a code. In case of problems, please email us.

The light on or light off command can be used to switch the reference light on or off.

TRIAL RUN AND TROUBLESHOOTING

It is recommended that a trial run be made initially. Once the correct values have been obtained for the threshold and ellipticity, in general there is no need to change them, unless the air conditions change appreciably.

Getting the correct exposure time for the reference and mirror frames

Choose an exposure time of 1 second, and obtain a reference frame.Ensure that no optical element in the setu p is cutting or vignetting the pupil.

Make sure that the CCD rows and columns are aligned along the spots within a tolerance of one pixel. This is done by displaying the reference frame, and using the cursor to note the (x,y) positions of the extreme spots along the same row or column.

Optimize the exposure time, ensuring that the pixels in the spots are not saturated. This will entail taking the reference frame (after the dark frame has been subtracted) and finding its centroids, using the option 'Centroid reference frame only' in utilities. If desired, the graphs of S/N ratio can be examined . Use automatic threshold.

Repeat the above steps and optimize the exposure time for the mirror frame.

Use the mirror frame to ensure that the WFS is approximately near the correct focus of the telescope (or optical instrument) being tested. This can be ascertained after the combination of the mirror frame and reference frame, by noting the distribution of spots. This of course presumes that there are no large aberrations in the telescope optics (like spherical aberration). In that case, rely on the position of the nominal focus. The adjustment does not need to be precise, as defocus is another aberration that is measured by Sensoft.

Analyzing the frames and a check on the output

Uncheck the boxes (if any) in utilities. Then do the analysis.

Check the following output parameters:

Examine the plots the reference and mirror centroids, to see if there are any obvious missing spots (threshold too high), or gross irregularities in their distribution over the pupil (threshold too low). This means that automatic thresholding has found a value that was near the correct one, but needs further tweaking. Adjust the threshold accordingly, and find the centroids again (without automatic thresholding). Use this value of the threshold for subsequent frames.

Look for any obvious problems with the combined frame by examining its plot. If necessary, increase the threshold, and/or decrease the ellipticity cut-off.

The normalized outer radius of the pupil should be between about 0.99 and 1.02 (click Details button at top of the output sheet). If not, check the diameter and focal length of the aperture stop (the focal ratio) specified in the Optics setup. Else, the WFS is far from the correct focus.

Check the rmsx and rmsy values (the root mean squared values of the residuals), which should not differ by more than about 20-30%. Check for possible causes: bad combination of mirror and reference frames? Air-turbulence effects? Telescope tracking?

Check the total value of rms. For very good seeing, its value is about 0.1''. In bad seeing, it can go up to ~0.7''. Beyond this value, it is usually due to other problems (such as problems with the combined frame).

If everything\306s OK, the results can now be used.

How the reference frame should look like. Note that in this frame the spots are not aligned along the row. This should be done by rotating the CCD.

Results

Kiriaki Xilouris