The Amazing HyperStar

A Guide to Optimize Performance
John Hayes,Ph.D.

The Starizona HyperStar™ adapter provides high quality, wide field imaging capability for Celestron C6, C8, C9.25, C11, and C14 telescopes. In addition, the HyperStar acts as a focal reducer to increase the optical speed to about F/2.0. Table 1 shows the first order parameters for each system. The HyperStar can produce spectacular results and is arguably a modern day replacement for the original Schmidt camera. With good image quality, high optical speed and large aperture, it is an ideal fit for modern solid-state CCD and CMOS sensors. The amazing thing is just how quickly a F/2 system can produce a high quality, wide field image! (see figure1.)

In spite of its advantages, the HyperStar system is not quite as easy to use as a modern day apochromatic astrograph and many users have trouble achieving good results. The problem generally isn’t with the optical system; it’s often the result of not knowing how to properly align and use the system. Although Starizona provides user instructions for aligning and using the HyperStar system, the manual is fairly basic. This guide is intended to provide additional information to get the best performance out of the HypeStar System. Although we will discuss Ceslestron telescopes and mostly DLSR cameras, the discussion is relevant to Meade HyperStar systems and CCD cameras as well.

It’s also important to recognize that nothing is perfect and the HyperStar is no exception. So, we’ll also take a brief look at what works well and what doesn’t along with a few cautionary notes.

Understanding HyperStar Alignment

Right up front it is important to understand that if the HyperStar system is properly aligned, it will produce tight, round star images over a very wide field.  If you see comatic star images, the HyperStar is out of alignment. Comatic stars look roughly triangular…and that’s bad. Star images that are not round should immediately raise suspicion that the optical alignment needs alignment!

Schmidt-­‐Cassegrain telescopes (SCTs) incorporate a thin aspheric corrector plate to correct the spherical aberration inherent in a Cassegrain configuration incorporating all spherical components. The Hyperstar adapter takes advantage of the fact that the corrector plate compensates for some of the spherical aberration in the system at the primary focus. The Hyperstar adapter still has to correct for a significant amount of spherical aberration as well as for other field aberrations to achieve good performance; however, it is the strong spherical aberration correction that causes the field to become highly comatic if the adapter is not wellaligned.

The trick to optical alignment is to get everything “on-axis.” The SCT corrector plate has a “weak” optical axis, which means that it can work well even if it is not perfectly aligned. The spherical  primary  mirror  has  no  axis  and  any  line  that  passes  through  the  radius  of curvature (RoC) can be “the” axis. The HyperStar adapter, on the other hand, has multiple elements and it has a very strong optical axis.   In order for the system to work well, the optical axis of the HyperStar must be aligned to pass through the RoC of the primary mirror, which will define a single optical axis for the whole system. Figure 2 illustrates the proper alignment of the HyperStar adapter relative to the corrector-primary mirror combination.

When you unpack a new HyperStar adapter, it is unlikely to be well aligned to your telescope and you will not achieve the best possible optical performance until you do some alignment.

Figure 2.  A) The Hyperstar is shown misaligned so that its axis does not pass through  the primary RoC. In this case, the HyperStar is shown with pure decenter but in general, misalignment is cause by decenter plus tilt. B) The HypterStar is tilted to make its axis pass through the RoC of the primary. This redefines the optical axis of the system. The corrector is no longer perfectly perpendicular to the axis but the angular error is so tiny that it won’t matter. Here, the distances and tilts are shown greatly exaggerated for the purpose of illustration.  In reality the distances and tilts are very small.

Aligning the HyperStar System

In order to optimize the performance of the HyperStar, it must be properly aligned. This is an easy process that generally only has to be done one time.

1)     Pre-­‐aligning your telescope is OPTIONAL. Please read through this section to understand what you are doing before deciding whether or not you want to make this  adjustment.

Before using the HyperStar, it is helpful to center the corrector plate/secondary mount assembly on the optical axis of the telescope, which is defined to be along the center axis of the baffle tube. At the factory, Celestron optimizes the relative alignment of the corrector/secondary assembly to the primary to maximize image quality. In some telescopes, Celestron applies zonal correction to the secondary mirror to minimize the total system wavefront errors, which requires that the

secondary be carefully aligned in both clock angle and position to minimize wavefront errors. Realigning your corrector MAY impact the performance of your telescope, so do so AT YOUR OWN RISK.

Having said that, the correction applied to the secondary mirror is generally fairly small and slowly varying so moving or rotating the corrector/secondary assembly by a small amount is unlikely to seriously affect the optical performance, but that may depend on your particular telescope. The optical configuration also means that the wavefront shears at about ¾ of the distance that the corrector is translated, which reduces the alignment sensitivity a bit. This is a controversial subject, but I have personally centered three C14 corrector plates with no immediately obvious effects on image quality. In an Edge system, centering the secondary, on the baffle tube insures that the optical axis of the combined primary-­‐secondary system is well aligned with the components in the baffle tube. It also minimizes the amount of tilt that has to be used with the Hyperstar to align it with the telescope.

You can check the centration of your corrector/secondary assembly and if it is way off, here is how to center it.

a.       Remove the secondary and stand 6-­‐12 feet in front of the telescope. Sight down the optical axis and carefully look at the centration of all of the circular parts.  Some of what you are looking at will be physical components and some will be images of components formed by the primary mirror.  Take your time and you’ll quickly discover that you can easily see if all of the circular parts do not appear to be perfectly centered. Pay particular attention to the inner edge of the secondary mounting hole relative to the central baffle tube and outer edge of the primary mirror. You want that edge centered with respect to primary even if the outer edges formed by the corrector mounting parts and tube may not appear perfectly centered (although they typically come out very closely centered as well.)

b.       If things are not centered, you will need to center the corrector plate on the optical axis. Start by loosening the screws holding the corrector plate retainer ring.   Loosen all of them by 2-­‐3   turns.

c.        There are four nylon tipped setscrews around the perimeter of the corrector plate that are used to define the axial position of the plate. On some telescopes, Celestron included locking setscrews on top of the centering screws so check that you can turn the actual setscrews that touch the corrector plate. Determine which way you want to move the corrector and loosen the setscrew(s) in that direction.  Make sure that the corrector plate is loose in its cell and grab the plate through the secondary hole to gently slide the corrector toward the loose screws. Some plates (particularly those that have been in the cell for a long time) tend to stick to the cork (or paper) shims in the cell and may require that you pull the plate free from the shims before you can move it. Rule #1: Be Gentle! Once you move it to where you want it, gently tighten the nylon tipped screws on the other side of the plate to hold it in position. Be very careful to only just touch the screw tips to the plate to hold it in position. You do not want to actually tighten these screws! Over tightening the centering screws can damage or even break the plate— so remember rule #1. Rule #2: DO NOT attempt to translate the corrector plate using the screws! The plate may be stuck to the fixed side of the

mounting cell and it is possible to chip the plate by try to move it with a setscrew. Rule 3: ALWAYS move the plate gently by hand and then secure the position with the screws. This is not a difficult adjustment, but it is bad news if you break or damage the plate so remember rule #1.

d.       Once you have the corrector in it’s new position, sight down the axis to see how well everything is centered and repeat the process starting from step b) until you have everything as well centered as you can make it.

e.       With everything aligned, gently retighten the screws that hold the corrector-­‐ retaining ring. These screws should not be over-­‐tightened! The correct  torque is achieved by using a small screwdriver with only moderate finger friction against the screwdriver shaft to tighten the screws.  You’ve pretty   much got is right if the plate simply cannot rattle in the cell.  Very little  clamping  force  is required.

f.         NOTE:   You will have to realign the secondary after performing this    procedure. The small amount of alignment required to center the plate (typically +/-­‐ 1 mm) will normally have little significant effect on the optical performance of the telescope—so long as you properly realign the secondary mirror. Figure 3 shows how a fairly well-­‐centered corrector plate will appear looking back down the optical     axis.

Figure 3. A C14 with the secondary removed after centering the corrector plate. It’s hard to get it perfectly centered but with care, it is possible to “eyeball” the center of the corrector to within about 1mm.

2)      Aligning the HyperStar to the telescope (this is the important part.)

a.       Before you mount the HyperStar, you should check that it is set for a nominal “zero angle tilt” and that there will be sufficient adjustment range. An easy way to do this is to use shims to set a nominal “zero position.” Select shims in the thickness range of 0.030” – 0.040”. Loosen the tilt adjustment screws, insert three identical shims and re-­‐secure the adjustment screws to create a uniform gap determined by the shim thickness. (See Figure 4.) When you are done, make sure that all of the push-­‐pull tilt adjustments are secured and remove the shims. The HyperStar is now nominally set for “zero” tilt.

Figure 4. Shim the HyperStar using three identical shims with a thickness in the range of 0.030-0.040” to set it for zero tilt before aligning the HyperStar to your telescope.

b.       Mount the HyperStar and place a circular mask precisely in the middle of the telescope aperture. The mask should be roughly half the diameter of the corrector plate and it should completely hide the camera in the pupil. Thin, foam core art-­‐board is easy to cut and makes a good mask.  Cutting the   center hole so that the mask pushes securely onto the tapered housing of the HyperStar works well to make sure that it is precisely centered.

c.        Carefully center the image of a bright star and defocus the image so that at 10x, the image is moderately bright and sharp as seen on the back of the camera (BOC) LCD live-­‐view display. (You may have to adjust the exposure or ISO value to achieve the best brightness.)

d.       Adjust the push-­‐pull tilt screws on the HyperStar to center the shadow of the mask on the star image.        If the corrector plate is fairly well centered, the shadow of the mask should be fairly well centered right from the beginning. Your goal is to carefully fine-­‐tune the circular shadow and to make sure that it stays centered as you move through focus. If your corrector plate is well centered on the optical axis, you should not have to turn the adjustment screws by more than about one turn. If you are turning the screws by more than that, there is probably something wrong! I measured my HyperStar and found that it was tilted by only about 0.012” after alignment, which is roughly 1/3 of a turn on the adjustment screws.

Once you have the mask shadow centered, firmly lock the alignment screws and you are done. I like to use a screwdriver to lock the screws a bit tighter than finger tight. Figures 5 and 6 show how a defocused star image will appear before and after alignment. If you are careful about mounting and removing the HyperStar adapter, it will hold alignment quite well. Once the HyperStar is properly aligned, it will produce tight, round stars from corner to corner over the whole field. You will immediately notice if the system goes out of alignment because you will see comatic star images—even in the middle of the field. It is a good idea to very carefully check image quality on the first image before any long imaging session.

Figure 5. A defocused HyperStar star image with a large alignment mask showing how the system looks when it is out of alignment.   The goal is to align the HyperStar tilt so that the alignment mask shadow is well centered through focus.

Figure 6. A defocused HyperStar star image with a large alignment mask showing what a well-aligned system looks like. The mask shadow remains centered through focus. Note: This image is very close to focus and the irregular Fresnel diffraction pattern is due to air turbulence.

Producing Pinpoint Star Images

Once the HyperStar is aligned, there are still a few things that you can do to improve image quality. The two most important items are to achieve good focus and to control diffraction effects.

Achieving Accurate Focus

The total depth of focus for a monochromatic diffraction limited optical system is given by:

Df = 4.88 l (F/#)2

The HyperStar systems are not diffraction limited and are designed to produce a spot size roughly 2.5 times the size of the Airy disk. Therefore, the total depth of focus for the HyperStar will be about 27 microns (0.001”.)   The bad news is that’s a pretty tight tolerance; but, the good news is that it isn’t all that hard to achieve. With a DLSR, it is a very hit or miss proposition to try to focus simply by making the stars on the BOC LCD display look as small as possible. A much more precise and reliable method is to use a Bahtinov mask for focusing. The Bahtinov mask is a clever diffractive optical element that forms crossed linear diffraction patterns that shift position through focus. When the patterns cross exactly in the center, the system is perfectly focused. Figure 7 shows a well-focused Bahtinov diffraction pattern.

Figure 7.  A well focused star images with a Bahtinov Mask.

A problem with the HyperStar system is finding a way to mount the focusing mask. One common method is to mount a dew shield on the telescope and then to hang the mask behind the camera on the dew shield. This works well with a rigid dew shield but it buries the camera inside the dew shield where it’s difficult to access. This just won’t work at all if you are using the BOC LCD for focusing. Another approach is to make a split mask to allow the mask to be mounted on the front of the telescope in front of the camera. Figure 8 show an example of a plastic laser cut split mask on a C14.

A camera with an articulated focusing screen makes it easy to set the focus. A small mechanic’s inspection mirror on a wand is useful for seeing the screen on the back of a camera that doesn’t have an articulating screen. Even a cell phone screen (with the display turned off) makes a useable mirror for checking focus. On larger scopes such as the C14 it’s impossible to reach the focus knobs while looking at the camera screen and motorized focusing can greatly help to make focusing less of a hassle. Some cameras have remote WiFi display capability and if they allow a magnified view of the screen this is another great way to go. The only problem with WiFi capability is that is drains battery power very quickly so it’s best to switch it off after focusing.

Because of the short depth of focus, it is very import to be careful while focusing. First, be sure to approach focus so that you have to turn the focus knob counter-clockwise to achieve final focus. This insures that the focusing nut that drives the mirror is pushing the mirror upward toward the secondary to reduce the chance of slow mechanical drift. A very useful accessory is to replace the standard Celestron focus knob with a dual speed Feather Touch focuser. The fine control definitely helps to more precisely set the focus. If you pass the correct focus position, rack the focus outward by turning the knob clockwise and start all over again.

When you are done focusing, gently tighten the mirror locks to reduce the possibility of the mirror moving during the exposure.   In my experience, these locks are not perfect, but they do help. Carefully focus the camera, lock everything down and then do a single quick       exposure to check focus before starting a long run. If the temperature is changing, the focus will drift and should be re-­‐checked  every 15-­‐30    minutes.

Automated focusing systems can also be used to set and periodically check focus. You’ll need software that will communicate with your particular camera model and a motorized computer controlled focusing motor to use closed-loop, automated focusing. You’ll also have to deal with wires crossing into the pupil but that’s manageable (see below.)

Figure 8. A hinged, split Bahtinov mask made through an online laser cutting service. The mask can be quickly mounted on the telescope for checking focus without removing the camera.

Controlling Diffraction

1)     If you use a DSLR and your camera is larger than the HyperStar adapter it will protrude into the entrance pupil and create diffraction effects. (This is pretty much of a non-­‐problem if you use a CCD camera with a circular form factor.) These diffraction effects can be greatly reduced by making an elliptical mask that fits over the camera to completely “hide” it within the pupil. The mask can fit in front of or behind the camera.  If you want to avoid any diffraction spikes, it is also best to avoid having wires cross into the pupil, which requires the use of either a wireless intervalometer or a wired intervalometer that is rubber-­‐banded to the camera so   that nothing protrudes beyond the edges of the camera mask (as shown in figure 9.) If you must have wires, you can avoid diffraction spikes by making sure that there are no straight sections of wire in the pupil. Another approach is to just create four equally spaced wires through the pupil the way a secondary spider might and accept the diffraction spikes as a part of the image.

Figure 9. An elliptical mask hides the camera and intervalometer edges within the pupil.  This helps to reduce diffraction effects so that star images are round.

Field Illumination

Field illumination in the HyperStar system, particularly with large sensor DSLR cameras, is extremely non-­‐uniform. The high optical speed of the system coupled with the fact that the exit pupil is located quite far from the camera causes a large amount of vingetting when using a typical DSLR that has the image sensor recessed deep within a DSLR mirror box. Figure 10 illustrates the situation.  The edges of the mirror box cause significant vignetting of the edge rays at even modest field angles. All of this is coupled to the normal cos4q radiometric intensity roll off with field angle. Obviously a smaller sensor or a sensor that is not deeply recessed in a narrow mechanical housing (such as a CCD camera) will not experience such severe vingetting.

This means that HyperStar images must be very accurately calibrated with flat data. It is normally not sufficient to simply use a gradient removal tool to flatten the field. Flats must be taken before the camera is rotated or moved and should be recorded with the system properly focused. Figure 12 shows a contour plot for a master HyperStar flat from a C14/Canon 6D combination made with a large electroluminescent panel (shown in figure 11.) Obviously the shape of the camera mirror box heavily influences the form of the intensity contours.  Flat acquisition methods are beyond the scope of this guide so the reader should research this topic elsewhere if unfamiliar with how to take flats. Because of intensity fall-­‐off, it is important to take very high quality flats to avoid artifacts and to achieve good flat field correction.

Figure 10. The (roughly) F/2 beam from the HyperStar makes it more difficult to evenly illuminate a large DSLR sensor deeply recessed within a tight mirror box.   The edges of the mirror box will introduce significant vingetting at the edges of the field. A CCD camera that has a shorter flange to focus distance in a wider mount will not suffer as much from this particular issue.

Figure 11. A 22" x 20” homemade electo-­‐luminescent (EL) panel for producing flat frames with a C14. This kind of flat box works quite well to calibrate HyperStar images

Figure 12. A contour plot of the flat field illumination pattern with a  C14  HyperStar system coupled to a Canon 6D with a full size sensor (36mm x 24mm.) The pattern shows the strong rectangular vingetting pattern caused by the camera mirror box.

Selecting a Sensor

The diameter of a diffraction limited Airy pattern is 2.44lF/#. As we’ve said, the HyperStar is designed to produce a spot roughly 2.5x the Airy diameter so the spot size will be around 6.7 microns under ideal conditions. Depending on the size of the telescope and the conditions, seeing effects will typically add 1-­‐4 microns to that diameter so the typical seeing limited spot size will be in the range of 8-­‐11 microns—if everything is perfectly set up. Therefore, any camera with pixel size in the range of about 4-­‐7 microns will work quite well with the HyperStar system.

The HyperStar can be used for narrowband imaging; however, the high optical speed and the location of the camera in the entrance pupil present some challenges for narrowband operation. Because of the high optical speed, stray reflections from filters will be large. The large beam angles also create problems for narrowband filters (see the Limitations section below.) In addition, the filters have to be changed manually because filter wheels are just too large. The bottom line is that although narrowband imaging is possible, it’s hard and a high quality apochromatic refractor is probably better suited to narrowband imaging. The HyperStar is better suited to one shot color (OSC) cameras—whether DSLR or CCD.

Figure 13. Thin film filters in a fast beam can form large stray reflections that are amplified when the image is stacked and stretched. The diagram on the left shows the strays as dotted lines. The image on the right shows what the strays look like in an actual HyperStar image. The each stray is a shadowgram of the exit pupil. Since the strays change shape over the field they can be difficult to remove in post processing.

HyperStarLimitations

Nothing is perfect and the HyperStar is no exception. Here is a list of issues to consider.

Stray reflections. On it’s own, the HyperStar is free of strays; however, if you add any filters to the system, it is possible to introduce significant strays around bright stars. Figure 13 shows how these strays are formed, why they are so large, and what they look like. Of course, this is the exact same problem that will occur with any other fast optical system when the filters are used in a converging beam. The size of strays can be reduced by using very thin filters and by placing them as close as possible to the focal plane.  However, placing the filters close to the sensor will increase the brightness of the strays. Another strategy is to move the filters so far from the focal plane that the intensity of the strays becomes negligible. The problem with that strategy is that it requires very large filters. In the HyperStar adapter, the filter position is fixed so you get what you get. In my experience, it is probably best to use the HyperStar system without any filters. Regardless, it is very important to keep any filters you use as clean as possible and to take flats for each filter (before it is moved.)

Limitations on filter wheels. The HyperStar is mounted in the entrance pupil of the system so it is difficult to use any accessory that is physically large and that includes most (all?) filter wheels. That means that filters must be manually exchanged. Starizona offers an accessory filter drawer to make the process easier.

Narrowband, thin film filter considerations. Narrowband, thin film filters are designed to work at a zero-­‐degree angle of incidence (i.e. normal to the surface.) As the angle of the illumination beam increases, the center frequency of the filter will change. Since the HyperStar operates with such a fast optical beam over such a large field, the bandpass of thin-­‐film filters will be broadened and the center frequency will shift over the field. This is a challenge for narrowband imaging. Fortunately Baader offers a set of filters specifically designed for the HyperStar system. These filters have better performance at high focal ratios though they do not provide an extremely narrow pass-­‐band.

Accessibility to the camera. DSLR cameras that do not have an articulated LCD display screen are somewhat difficult to use with the HyperStar since the camera sits suspended from the corrector plate. On a C14, that may place the camera out of reach. You may need a stepladder to access the camera or look at the screen. If you need a dew-­‐shield it gets even worse! You’ll have to pull the shield every time you need get at the camera. It’s doable but it’s not convenient. Using WiFi to read the screen and control the camera remotely is a real advantage with a dew-­‐shield. It’s workable, but sometimes it’s a bit awkward!

Power management. Remember that unless you are really careful, running wires through the pupil will create diffraction effects. Using batteries for everything simplifies this problem. Just make sure that they are charged before starting a long run!

Diffraction from the baffle tube. At large field angles, the inner portion of the optical beam will strike the SCT baffle tube. That can create bright, single-­‐line diffraction patterns (along with shadows inside of strays reflections) from bright stars near the edges of the field.

Fortunately, these effects aren’t super-objectionable.

Guiding. There really isn’t a good way to guide through the telescope with an OAG or ONAG system with the HyperStar system. A guide-­-scope is about the only way to effectively solve this problem. If everything is firmly mounted, a guide-­-scope can work well with the short exposures needed for the HyperStar. The biggest issue with a well-­-implemented guide-­- scope system is often due to primary mirror movement so be sure to use the primary mirror locks. They will not completely solve the problem but they will definitely improve the performance. Just remember that the angular guiding tolerance for any diffraction-­- limited system at F/2 is the same as at F/10. Of course, the HyperStar is not quite diffraction limited so the big advantage is the shorter exposure times. In general, any system that can limit rms guiding errors to within about one Airy disk radius will perform extremely well with the HyperStar system. With typical seeing conditions, even guiding to within about +/-­- one Airy disk diameter (rms) will work just fine.

So, How Well Does It Work…in the Real World?

With good guiding and careful focus, the HyperStar produces tight, round stars over a large field. With my C14, I typically see minimum FWHM star images in the range of 2 pixels in a stacked image, which translates to 12.8 microns or 3.9 arc-­‐sec with a Canon 6D, which has 6.4 micron pixels. Figure 14 shows a contour map of FWHM for a typical stacked HyperStar image. Keep in mind that the stacked image integrates focus variations, seeing effects, and any other errors that contribute to less than perfect performance; but, it’s what counts in terms of final image quality.

Figure 14.  A contour map showing FWHM of a stacked HyperStar image.

With care, the HyperStar system provides excellent image quality over a very large field. The low focal ratio and large aperture made possible by the HyperStar allows for very short exposure times, which reduces the need for exceptional long-­-term guiding performance. It also means that superb images can be taken using relatively short total exposure time so that more objects can be imaged in a single session. Figure 15 compares the image quality of a HyperStar image to an image taken with the F/7 Edge focal reducer. The HyperStar is an extremely valuable accessory for anyone with a SCT who wants to do large aperture, wide field imaging.

Figure 15. Comparison of HyperStar detail taken at F/1.9 with the same object taken at F/7 using a Celestron C14 Edge. (A) The original 2 x 3 degree HyperStar field. (B) A close up of M20 from the HyperStar image rotated to roughly align with © a high-­‐resolution image of M20 taken with an Edge F/7 reducer on the C14. The Hyperstar is not diffraction limited so the ultimate resolution is not as good as imaging done at the prime focus—or even with the F/7 reducer.  Still, it is quite good given the scale of the full field.