Category: Technical intro

After sharing my last entry with the mailing list for our local club, several members were kind enough to take a look at the write up I provided.  Consensus among those members was that I am not so much looking at a tracking problem as I am a differential flexure problem between the guide scope and the main scope.  They suggested that the pattern of the trailing pixels across the frame, largely in a consistent direction, couldn’t really be explained by tracking problems or polar alignment issues, but rather by a slight but steady flexing problem.  The wisdom on this, both from club members and from other reading on the interweb, is that “It Doesn’t Take Much!”.

It is not to say that periodic error and mount performance might not be a contributor to my elongated stars.  Obviously, periodic error (and especially non-periodic error) improvements are sure to at least make tracking easier and more accurate.  But the hypothesis that was suggested, is that poor tracking would not explain the hot pixel pattern shown in the image of my last post.One straightforward solution to reducing differential flexure is to use an off axis guider.  In short, this device uses a small prism mirror to pick off a small portion of the incoming light through the main scope, rather than using a guide scope.  While this setup can me a bit more difficult to get set up than a separate guiding scope, it inherently removes potential flexure between the two scopes and the connecting mounting hardware.  This is especially recommended on larger scopes and scopes where the mirrors are not firmly attached, and so, can shift as the mount moves to differing orientations.  I found a used one and will give it a try once I have the correct combination of spacers to use.In the meantime, though, I decided to try an experiment with what I had on hand.  Rather than use my ED80 on by ADM side-by-side setup, I setup my 102mm imaging refractor directly on the top rail of my AT8RC.  I wasn’t sure, but wanted to see if this arrangement was more rigid.  Also my 102 has a more rigid focuser, and the AT8RC sits directly on top of the mount  rather than slightly off-set.  I really haven’t heard of anyone having issues with the rigidity of the ADM side-by-side setup, but I decided to try this arrangement, anyhow.The image above is the result of that effort.  The tracking isn’t perfect, but the trailing hot pixels are much more tightly grouped together.  The image itself is hazy and soft because of the conditions, but the star shape is improved somewhat.  Note that the image duration for the subs in this image are the same as the subs in the image from my last post (12 minutes).  I am encouraged that with an off-axis guider, results will be even better.  And further, I located a higher quality stepper motor gear to replace the existing gear,  This represents the last identified anomaly from the periodic error investigation and resultant tune-up that I performed.Note that the conditions were terrible the night I ran this experiment.  I couldn’t spot Polaris until after 10PM though the haze and clouds.  It was so humid that condensation formed on the glass of my cooled CCD, and after it dried, it left behind a number of spots.  Barely any of the stars of Sagittarius were visible.So a tip of the hat to folks at AAAP, especially Nate, Fred, and Frank, who took and look and helped me out.

Starting in the spring with the acquisition of my AT8RC, I was worried about whether my my past polar alignment and tracking have been adequate to support this new, much longer focal length.  I got hung up on polar alignment accuracy, initially, but after starting to image, it appears that either my PE is too bad to guide out.  This picture of the Eagle Nebula (12 minute subs, lightly processed to see what is happening) shows the trailing of concern.  To my eye, the main issue with the image is the overall star elongation, more or less oriented in the same direction over the entire image.  I would expect that errors caused by polar alignment would show up more as rotational problems than this type of consistent streaking. Note: As always, click on the image three times to see the full size image. First click goes to image gallery, second click expands to full size of browser window, and third to full size of image.

So after some thinking, some forum searches, and some fruitless imaging nights, I decided to try to analyze my periodic error and attempt to logically determine the source of my difficulties.

To undertake the Periodic Error measurement, I decided to use CCD-Ware’s tool, PemPro.  It includes modules to assist in both polar alignment and periodic error measurement.   All of the measurements were taken through the AT8RC with a Meade DSI camera, resulting in an image scale of around 1.2 arc-sec/pixel.  The load was not necessarily consistent.  In some cases, the load included a guide scope and camera to simulate my fullest load, in others, it only included the 8″ AT8RC.

The first set of data shows the state of the mount before any work was done.

Great huh? That is what I thought at the time. But I came to understand, perhaps, a little about this data and what it means. The initial chart simply shows the periodic error as measured by the distance the mount travels away from its expected position. The seven cycles are adjusted for any drift caused by less than ideal polar alignment. This data is captured by taking images of a star while the mount is tracking (but not guided) and charting how the star moves compared to it’s expected position.

It can be difficult to understand exactly what the data is telling you in the initial form, and that is where the second chart comes in. The second chart is a frequency analysis, which essentially breaks down the overall error into errors that are contributed by individual frequencies. The fundamental frequency for my mount is just under 8 minutes (which equates to one revolution of the worm gear). Other periods of interest are also noted in the graph.

Finally, the third chart displays two curves. The first is a fitted curve to the periodic error data measured in the first chart as can best be fitted when combining all the cycles (that is, what do you get when you combine all those cycles into one curve). The second curve is a calculated curve that reflects the best effort to correct, by “programming out”, the periodic error with Periodic Error Correction. The idea behind periodic error correction is to modify the standard tracking of the mount by anticipating repeatable errors and sending guiding commands to the mount before those errors occur. Inherently, this periodic error correction is limited to errors that are integer multiples of the worm period.

So Ray Gralek, the author of PemPro, was nice enough to take a look at my data and offer an opinion. In his view, the contribution of error by the stepper motor gear is rather high. And unfortunately, because the stepper motor gear period is not an even integer multiple of the worm period, there is not an opportunity to address this error with Periodic Error Correction (PEC). So the only real way to address this error is to look into any correctable mechanical causes.

As I was deciding what to do next, I did happen to notice that there was a discernible “clunk” in my mount in Right Ascension, and I began to wonder, hopefully, whether this might actually be the cause of my stepper motor gear error. There are articles on line describing methods for adjusting the mesh, and especially there is one I liked in the files section of the EQ6 Yahoo Group. I followed these instructions and combined with an overall adjustment to the tightness of both the RA and DEC axes, I headed out with renewed confidence to take some beautifully tracked pictures.

Not so fast, astronomy breath…

Another round of attempted image capture (no PEC applied), another set of images that may or may not have been any better. SO… I took another round of data, showing the mount after the mesh adjustment. Note this data was taken with a fully loaded mount.

So now for some interpretation… To me, qualitatively, the data may have calmed down a little bit, but while it is a relatively smaller magnitude, there is still a relatively large contribution from the stepper motor gear. Further, the rather large data spikes are still evident in the raw data. Perhaps these represent excursions that are just too large and rapid to be able to adequately guide out.

So, I was sure that if I took the mount apart, I would almost certainly find something that was lodged in a stepper tooth and if it were cleaned out, thinks would be miraculously improved.

And so, it seemed to me, while already taking the mount apart, it would make sense to also perform a “hypertune”… the primary components of this are a general cleaning and adjusting and also the replacement of the questionable quality bearings which support the worm gears. So I ordered a kit, and performed the hypertune.

This time, since the moon was living large, I went and captured more PE data rather than directly trying to image. The results of the post-hypertune analysis follow:

Hmmmm.  So qualitatively, I might try to talk myself into thinking that the data is perhaps a bit smoother, and that the non-worm period errors are perhaps tamped down a bit… but overall, the PE is still about the same.  NOT the home run I was looking for. The pessimistic side of me wonders if I have made much of a dent in the problem, or have even helped to make the errors more guide-able.  I am hoping, but not convinced.

For completeness, to date, here are a few cycles of data (it was getting late) with PEC applied:

Conclusion

So what does this all mean? Well, I am not yet sure. I can’t help but feel like I haven’t really addressed the issue. Granted, there is still substantial periodic error contribution from the worm gear, but if it is repeatable, it can be removed with correction. Based on an article I found, I have placed an order for replacement stepper gears (and transfer gears, too). Based on that data point of ONE, i hope that the stainless stepper gears are higher quality and of a better tolerance. Anyhow, they are not very expensive, so it seems worth a try. Beyond that, I am considering whether I should just stick with the shorter focal length scopes and this particular mount.

Regardless of how this phase turns out, it has presented a vehicle to much better understand my mount, Periodic Error, and PHD Guiding. The images I have taken with a shorter focal length scope while busy trying to sort out these tracking issues show tighter stars and improved tracking over images I have been able to capture in the past.

As April is mere hours from becoming May, I am surprised and disappointed that I still haven’t been able to get out for a real night of astronomy.  Setting up this blog was exciting in February and March in anticipation of getting out, but at the end of April, it is the tiniest bit sad…

However, here we go anyway.

On an EQ-G, initial polar alignment of the mount can be achieved through the polar alignment scope and EQMOD’s polar alignment utility.  Through a simple series of steps, EQMOD will end up placing an alignment circle in the appropriate position.  The operator then uses the mount alt and azimuth controls to move Polaris into the small alignment circle, and polar alignment should be achieved to the inherent level of accuracy allowed by this method.  There are some nice pictures of the results in my previous post.  This is the approach I have used prior to this year, but I don’t really know whether it is good enough. 

From reading various yahoo groups, I suspect the majority of contributors would think that polar scope alignment method is not good enough for long exposures, and particularly, not good enough for long exposures at 1625mm focal length.   Additionally, a disadvantage of using the EQMOD utility is that the alignment must be completed without having the gear fully loaded on the mount, or the gear may collide into the tripod/pier.  Further, some suggest that fully loading the mount may induce some differential flexure and affect polar alignment.  Also, this approach means that the gear is always being set up after dark, after Polaris becomes visible in the polar scope.  The budding photographer may, at times, prefer to get set up before dark.

As an alternative to the polar scope alignment method (which is certainly more than adequate for visual observing), there are also software tools available that can make use of the main telescope to assist in polar alignment.  Here is one nice reference on polar alignment software.  For the most part, these polar alignment software tools fall into two main categories.  The first category relies on evaluating the difference between actual star positions and expected star positions to calculate any polar alignment corrections that are needed.  The number of iterations required for an adequate result is based on the accuracy of the initial rough polar alignment.  The second software category is a software-assisted drift alignment – a method that relies on observing the path a star deviates from its expected path over time, then calculating needed alignment corrections.

One software tool, Alignmaster, is a member of the first category.  I have only briefly tried to use this so I have yet to form a solid opinion.  Very first impressions are:

• The “best” star pairs it wants you to use are often quite low to the horizon and hard to locate
• It does not attempt to use any installed cameras, so these must be driven by other software (or you could use an illuminated reticule and do it visually)
• If it works and is accurate, it is likely to be substantially faster than software assisted, drift alignment software
• Some people in the “groups” say it works great

There are number of examples of the category 2 software, with prices ranging from “free” to $150+.  In several cases the polar alignment function is just one of a suite of programs included in the package.  While my instincts are to purchase the most expensive package first, maybe, for once, I will start instead with a free one.   One example of this program is EQ Align.  Although it sounds like it might be a member of the EQMOD family of software, it is not.   Again, based on no actual experience to date:

• This software can fully integrate with both the mount and the camera
• There are comments about earlier versions being buggy, but also those bugs have been resolved and the developers are very responsive
• The program was developed in Spanish but is largely translated and seems fully usable to a non-Spanish speaker
• The drift method is generally acknowledged to be the most accurate alignment method, but does take some time, because as the name implies, it depends on tracking how far a star drifts over time

In summary, the initial strategy I plan to follow is to switch my polar alignment method from the use of the polar scope to the use of a software assisted method described above.  Initially, it will be necessary to master the techniques in these methods and become proficient enough to be able to form an opinion as to which method is “good enough”.  Obviously the polar scope can still be used to approximate polar alignment by estimating the position of Polaris through the scope.  But the step of actually rotating the polar scope to align the correct hour-angle actually be done, and a visual approximate will instead need to be employed.

Hopefully, I will be able to report some initial results on these approaches soon… and also to comment on first light on my now  6-week old AT8RC.