Basing your deep-sky imaging on the Hyperstar lens assembly from Starizona [http://www. starizona.com/] is sufficiently differentiating that I feel the subject deserves its own Chapter. Figure 1.1 shows the Hyperstar lens assembly for an 11" Celestron Nexstar GPS scope. This is an earlier model that does not incorporate collimation screws as part of the Hyperstar lens. The Hyperstar lens replaces the secondary mirror in a Schmidt-Cassegrain type reflector and turns the SCT into a Schmidt Camera http://www.schursastrophotography.com/schmidt.html. Although this lens assembly was initially designed for Celestron telescopes only, recently Starizona have started producing Hyperstars for some of the Meade range of SCTs. The Hyperstar is basically an x1 field corrector, in other words it does not reduce at all, but it does flatten the curved field that results from just using the primary mirror alone to form an image. Let's be frank here, this was an amazingly bold move for Celestron to take, and I am highly impressed that the company tried to get this advanced optical engineering into the hands of the amateur imager. Turning an f#10 SCT into an f#1.85 Schmidt Camera is inspired thinking, and it creates an immensely powerful imaging tool. But there is a problem, and it could well be the reason that Celestron themselves no longer offer the Fastar option*. An optical system operating at f#1.85 may be an extremely fast system, allowing objects to be imaged in a very short period of time, but it is also a very unforgiving system in terms of any misalignment of the optical elements.
* The Starizona "Hyperstar" was originally the Celestron "Fastar". Celestron never made a "Fastar" for the 11" GPS scope; this is why I bought my Hyperstar directly from Starizona in the States.
To give you some idea of how precise some of the requirements are, using the Hyperstar at f#1.85 on the Nexstar 11 GPS scope with the SXV-H9C colour CCD, your depth of focus (critical focus zone) is 7.53 microns, that is 7.53 x 10-6m. The diameter of a human hair by comparison is something like 80 microns! I think you can probably see where the trouble is going to be found when trying to use the Hyperstar for high quality imaging.
As stated above, the Hyperstar replaces the secondary mirror in an SCT; you remove the secondary mirror completely and place the Hyperstar in the secondary mirror cell. This cell actually has something close to 1mm of clearance between it and the corrector plate, so the positioning of the cell in the X-Y plane (the plane of the corrector plate) is only good to about 1mm in each perpendicular direction. Now it doesn't matter that the position of the secondary mirror is only good to about a millimetre because the secondary mirror comes with those collimation screws mentioned earlier, so the optical system can be precisely collimated. The original "Fastar" and the earlier "Hyperstar" assemblies did not have collimation screws, so it was a hit and miss affair where your lens assembly sat within the corrector plate. Now recall that your depth of focus is only 7.5 microns, and your possible error in the X-Y position of your Hyperstar is 1,000 microns - and you begin to see a major problem looming. You are very unlikely to have a collimated system when you fix your Hyperstar in the "randomly positioned" secondary cell. The outcome of this is terrible star shapes across the whole field of view, and extremely bad coma at the field edges. I do not know if the "collimation screws" on the later Hyperstar models are able to properly collimate the system, as I have had to work with the earlier model that had no adjustment, so I had to find another solution to this problem.
How did I know there was a solution to this problem? This is one example of such an unbelievable stroke of luck that I should probably play the lottery on a regular basis! When I used the Hyperstar for the very first time, indeed the star shapes were terrible, and the coma was completely unacceptable at the field edges, although the nebulae themselves seemed to come out very well. I then took off the Hyperstar as I didn't like the star shapes and did some f#6.3 imaging at the eyepiece end. After the speed of the Hyperstar I quickly became frustrated at the slowness of f#6.3 imaging and the fact that dust doughnuts now became a problem (something else I didn't have to worry about with the Hyperstar) - so I replaced the Hyperstar. Now here's where the unbelievable luck comes in - I must have, at odds of millions to one, put the Hyperstar back in smack bang on the "sweet spot"!
I obtained good round stars across the whole 1 degree by three-quarters of a degree field of view, and there was no coma to be seen. At the time I thought nothing more of this and simply assumed that I had done something wrong the first time I fitted the Hyperstar.
Fast-forward four months. I am trying to get M81 and M82 into the single Hyperstar frame, but I need to use the diagonal, and this means rotating the Hyperstar assembly by 45 degrees. No bother, I reach into the end of the 11 GPS and give the assembly a twist, the cell can be rotated within the corrector plate reasonably easily. Absolute disaster! I am back to terrible shaped stars and unbelievable coma - what had I done? Well even for me I fairly quickly understood what had happened. By some miracle I had put the Hyperstar right on the collimation point for the system, and in rotating the assembly I had shifted it off this position and lost collimation completely. I felt physically sick for around two days. However, after the panic had started to subside and logic started to kick in again something became apparent. At least there was a "sweet spot" position for the Hyperstar in the corrector plate where everything is perfectly collimated -I had an existence proof. Now all I needed to do was position the Hyperstar once again in the "sweet spot" and I'd be off imaging again in no time. It wasn't quite so easy of course. My first attempt was to use the usual technique for collimating an SCT, defocus a star, check the star shape on the monitor, and then manually move the Hyperstar by sliding the assembly around the corrector plate until the star shape looked good. Within a couple of minutes it became clear that even the smallest movements I could make manually were far too coarse to be able to collimate the system. Once again the odds against me putting the Hyperstar right on the "sweet spot" that second time hit home. O.K. so I cannot manually move the Hyperstar around, I am going to have to build some sort of precision cell-shifter.
My solution to the problem of precision-shifting the secondary cell within the corrector plate can be seen in Figure 1.5. Taking this route you can appreciate took a great deal of faith that what I was about to do would work! It meant taking a drill to my beloved Nexstar 11 GPS that I had only used for imaging for 4 months now. I could call it a day and give up the imaging and go back to visual observation (the thought did cross my mind) and I wouldn't have to take a drill to the scope. Figure 1.5 shows you that I was foolish enough to believe my idea would work, and I did take a drill to the scope. I drilled four 8mm clearance holes in the end ring of the 11 GPS to take four 8mm diameter threaded rods. I used Araldite to stick four 8mm nuts at right angles to the corrector plate to accept the ends of the 8mm threaded rods. The nut threads were drilled out for this purpose. I then threaded nuts to the inside of the scope end ring, onto the threaded rod, and I would screw up against these to physically push the secondary cell, and the Hyperstar lens, around on the corrector plate.
Again I went back to the "classical" method of collimating an SCT by defocusing a star and then moving the Hyperstar with the push rods to try and get a nice symmetrical star shape. After a little experimenting I began to get a "feel" for how I should move the Hyperstar to get collimation, and after a couple of nights, unbelievably, my Hyperstar setup was once again collimated!!! Hooray!!! So I quickly went back to imaging and found a further problem. There were now strange rectangular boxes of very fine lines around very bright stars looking like some sort of strange diffraction pattern. It was some sort of strange diffraction pattern; it was the threads of the push rods! Some black insulating tape wrapped around the push rods eradicated the diffraction pattern, and I was back in business.
Four push rods across the corrector plate will of course produce diffraction spikes around bright stars, that's O.K. I like diffraction spikes anyway. However, this was nothing new to me as a Hyperstar imager as I had to get four cables from the back of the SXV-H9C out to the scope, the computer and the autoguider anyway, so I was used to diffraction spikes. It just meant that now I needed to tape the cables down to the push rods to make sure I only got one set of spikes.
There are further fine-tuning details to attend to in order to create fine Hyperstar images. The system is fast, which is very good, but this also means that sky glow quickly becomes a problem; there was also a problem with "star bloat" due to imaging the infrared. The sky glow problem could be significantly reduced, and the infrared bloating eradicated, by using an IDAS light pollution (LP) filter. This beautiful filter cuts out the main sources of light pollution (Sodium and Mercury lines) as well as attenuating the infrared wavelengths that cause bloating http://www.sciencecenter.net/hutech/idas/lps.htm. There is another added bonus of using this filter, it doesn't upset colour balance like many other LP filters do so there is less image processing for you to do to get the colours looking right.
Another "fine-tune" I carried out that I believe had some positive effect, was to very carefully tighten up the corrector plate retaining screws by about half a turn each. Why would I want to do this? Remember, the depth of focus is a tiny 7.5 microns, and you have a weighty Hyperstar plus CCD cantilevered off this corrector plate. I don't think it will take much to get 7.5 microns of movement as you pan around the sky, and I wouldn't be surprised if the plate itself did not deform to the order of a few microns.
Finally, there's that 7.5-micron depth of focus! You are shifting that huge 11" diameter mirror up and down a shaft for focus, and you have to control its movement to just a few microns. You are not going to do that with the as-provided manual focuser! Remove the Celestron manual focus knob and fit a "Feath-erTouch" focuser http://www.starizona.com/search.cfm?Category=0&Product=1 &Keyword=microfocuser for very fine focus control. One further addition; if your fingers are as clumpy as mine you will still have trouble getting good focus. Just touching the focuser shifts focus and it becomes difficult trying to see the FWHM focus numbers on the monitor whilst fiddling around with a focus knob. The solution to this problem is to fit an electric focuser to the FeatherTouch such as those supplied by JMI http://www.jimsmobile.com/ also see Figure 1.2. Now you're finally done!
Solving all these rather difficult technical problems makes Hyperstar imaging more of a Black Art than a true science.
But what if you persevere until you get your Hyperstar system tuned and finely collimated - what then? Well, personally, I believe you have one of the finest amateur imaging systems available on the planet. You have ultra-fast imaging, a wide field of view, and a big aperture scope - all the ingredients to have a pretty amazing time imaging deep-sky objects.
As discussed earlier, my Nexstar 11 GPS sits on a modified Celestron wedge, and a Celestron 80mm wide field refractor is used with the SXV autoguider CCD for guiding the system. Since you are imaging at f#1.85 and you are autoguiding at f#5 using the lightweight Celestron refractor, it is quite easy to achieve extremely tight autoguider control. I had no difficulty in maintaining 0.2 pixels RMS error in X and Y all through an imaging session, and very often the error would be 0.1 pixels or less RMS! You will find this level of accuracy has both plus and minus points. On the plus side, your stars certainly come out round! On the minus side, you will start to see CCD noise feeding through into the darker areas of your image as some rows may have a slightly higher gain than others and this becomes very apparent when you have such very high guiding accuracy.
However, even this problem is very easily resolved in Maxim DL, you activate the "dither" function in the "Sequence" menu and you make the scope move a little after each sub-exposure. In this way you can eradicate the CCD noise. If you don't want to have the scope move after each sub-exposure, you can simply physically move the scope a little bit after say an hour's imaging, reset the autoguider and start up the sequencer again.
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