Dust Prevention

Image result for haboob in phoenix

My observatory is in the Arizona desert. As such we get a tremendous amount of dust in the air.

  1. I’m sure there is a large amount of dust suspended in the air in general.
  2. We get occasional high winds which pick up a lot of dust as they blow across the desert.
  3. We get occasional dust storms which are amazingly dense dust – you can’t see 3 feet in front of your face. Apparently “dust storm” is politically incorrect, so the media has taken to calling them “haboobs”. The image above shows a storm coming across the desert. Notice the mountains in the background; that wall of dust is BIG.

I spent 4 days cleaning the observatory. The dust is tough to get up; it is thick and deposits like mud (probably electrostatically charged). You can’t use a Swiffer or dust cloth – at best that just smears the dust around. You have to use rags and water to pick it up.

I have implemented some anti-dust strategies which I hope will help:

  • I made a cloth skirt which goes around the outside of the dome. The skirt is about 6 inches long, hanging off the rotating dome and dragging on the roof. I’m hoping this will provide another barrier to dust coming in that way. The cloth has two ropes sewed into the edges. The top rope allows the cloth to pull tightly against the plastic band holding it against the dome, making a tight seal. The lower rope weights the bottom edge of the skirt to make sure to drapes down onto the roof.
  • I plan to install some weatherproof sealing against the edge of the shutter. The upper shutter in particular does not seal tightly when closed.
  • I put cloth drapes over the computer table and various equipment cabinets. I also put a drape around the base of the mount. When I need something I just lift the drape (to get at a tool chest, for example). I put Velcro on the computer drape to hold it on the monitor – it kept sliding off to the back.  Now, every so often I will just need to wash the drapes.
  • My electronic cabinets were designed for upstate New York, where I encountered serious insect invasion issues. The cabinets have open front and back doors for air flow, with two layers of screen keeping out bugs. I added a cloth layer to the doors to hopefully keep out dust while still allowing air flow.
  • I changed the scope Park position back to pointing the scope downward. I will need to change this in the summer; the heat causes grease on the Celestron Edge baffling to drip onto the corrector plate. However, the rest of the year I hope that less dust will settle on the telescope glass if it is pointing down.

Weird Diffraction Spikes Part 2

The saga comes to a close…

Celestron Help

After talking to the Celestron Rep (Will) it sounds like I missed something when inserting the secondary. There is a small set screw on the inside of the secondary which needs to fit into the slot on the secondary housing (the tube passing through the corrector plate).

Will also made a couple of other points.

  1. the slot in the secondary housing, the mark on the secondary, and the set screw should all point at 1 o’clock. That is how his system is set up.
  2. The secondary housing is not glued, it is just tight. Apparently it is very easy to bind the threads, the tube can flex. He described a couple of ways to try and tighten it so the housing is stable.

I retried inserting the secondary, making sure the set screw is positioned correctly. It seems to fit better.

Voila! Now I can focus correctly. I then worked on collimating, and was able to make that work.

I then tried the collimation tool in CCDInspector; I’ve owned this tool for awhile but never tried this piece. It was a little confusing at first. There are various colors which I anticipated were trying to tell me stuff, like which direction to turn the screw – apparently not.

One very helpful aspect – I connected Maxim to the ACP Telescope hub and put in the correct image scale (0.39″/px). This enables the function in the right click menu on an image to re-position the scope to point to a particular spot. This makes it much easier to recenter the scope after each adjustment.

After I got the hang of it, I was able to get the collimation to less than 5″, perhaps under 3. At that point the seeing and clouds were preventing any improvement. The collimation is clearly better than what I used to get using my manual method. A focusing run shows very symmetric patterns on the stars.

Starizona Help

Now that I am correctly inserting the secondary, I still need to get the secondary housing firmly attached to the corrector plate. Before starting with Will’s suggestions I called Dean at Starizona to get his thoughts. He had some other techniques I could try to tighten the housing.

In the end, I decided to bring the OTA to Starizona rather than risking messing up the housing. I’m glad I did. Dean was very helpful and knowledgeable. It was great just seeing him do some of the various tasks – there are a lot of things one hears about, and that I have done, but don’t really know how to do exactly.

Dean pulled the corrector and managed to get the two parts of the housing separated. He then replaced the paper (almost invisible) gaskets with a rubber type, then re-installed the housing in the plate.

He then cleaned the corrector, doing a much better job than I had. Very interesting watching his technique.

Now came a possibly controversial part. There are several threads out there about whether the corrector should be moved. In particular, a long series of posts on Cloudy Nights which concludes that the plate should not be moved from the factory setting, and should not be physically centered (the factory setting is the correct optical centered position). Dean disagrees – after putting the corrector back in, he adjusted the position to get the secondary/housing/corrector centered by looking down the OTA from 6-12 feet. He says he has convinced Celestron they are aligning their correctors incorrectly? They came out and visited to see how he does it…

Also, Dean positions the secondary and housing to point at 3 o’clock instead of 1 o’clock.

Anyway, he did his alignment which included very slightly shifting the corrector. He also collimated using an artificial star. He clearly has a very experienced eye.

Mystery Solved

The source of the trails on the corrector was identified by Dean. Apparently when it gets hot the grease on the primary baffle can bleed some of the more volatile components. Dean showed me the trail running down the primary baffle; clearly the same fluid on the corrector plate and under the edges of the plate.

I have (cleverly, I thought) defined the scope’s park position to have the OTA pointing down. The idea was to keep some of the Arizona desert dust from collecting on the corrector plate. This was a bad idea, since it allows the grease to drip onto the corrector.

If the OTA is pointed up, the grease will run down into the back of the OTA. Apparently this is not a problem.

Subsequent Collimation

After I got the OTA remounted, rewired, and balanced I was curious to see how Dean’s collimation was, as per the CCDInspector tool. It was pretty good, about 14″. Certainly better than what I would get (about 50-70″). I then played with it and managed to get it to 1.5″. At that point the direction line is flitting around quite a bit, but generally less than 1″. Wow!

Now I have centered the focus position near 3500 on the Optec, locked the mirror down, and started re-running VCurves. I’m eager to see how the overall performance of the system might be affected.

Weird Diffraction Spikes

For a little while I have been getting strange diffraction spikes from the Edge 11/STF8300M system, as seen in the sub below. The brighter stars have 2 diffraction spikes, when they should not have any (no support vanes to cause them). The two spikes are at a 10-20 degree angle from each other.TestRedFilter15Min

The crop below shows is zoomed in a bit to show the spikes better:


I have been working on my Dome driver for the Arduino controller; I thought these were due to the OTA not pointing accurately out the dome’s slit. However, when I got around to checking, the OTA was in fact pointing precisely out the slit.


Big trails seen on the left. Two trails are at an angle, leading to the observed diffraction spikes.

So, I went out and started to look at possible causes. Right off the bat, when I looked at the corrector plate, there were 2 “trails” on the inside of the corrector plate! It looks like a snail crawled down the plate. The trails are at an angle, matching the angle of the diffraction spikes.

SnailWallsIn addition, there are a couple of trails on the inside of the tube, along with a blotchy area. These look like liquid has somehow run down the tube and dried.

There are also a bunch of small drop “splatters” on the inside of the tube. These are hard to see in the image because the corrector is also fairly dirty on the outside.







So, time for a fun new project – removing the corrector plate and cleaning it. I have seen lots of comments about dangerous this is…

Removing the Corrector Plate

Cloudy Nights had a very good description about removing the plate, although no pictures so I don’t necessarily know what they are referring to.

Step 1. Carrier Box

IMG_0632Cut down a cardboard box, taped in a couple of pieces of foam. This will hold the corrector plate when I get it out.









Step 2. Remove the secondary.

Position the scope pointing slightly down and unscrew the secondary.

Am I supposed to remove the holder that holds the secondary?  I don’t see how to do it. After I got everything out I tried again with no success. It seems like it should unscrew into two pieces, but maybe  they are glued together or something. In the end I do my cleaning with the holder remaining in the corrector.

Problem? The secondary holder is loose in the corrector. It spins easily, and moves laterally a millimeter or so in each direction. Is this right?

Step 3. Position the scope with counterweights up, scope pointing up.


Step 4. Remove the retainer ring.


The two pieces of tape are cut, allowing me to reposition the retainer ring. This shot also shows the problematic inside streak on the corrector plate.

I don’t know if the positioning of the ring is important. I assume not, but will proceed as if it is:) I put two pieces of blue tape on the ring and tube wall, then cut the tape. The tape pieces will allow me to re-position the ring correctly. Removed 8 screws.

It is tricky getting the ring out. There are two pins sticking out for holding the lens cap. I had to squeeze the ring a bit to get by the pins.

Step 5. Remove Corrector plate.

Marked the plate position with two more pieces of blue tape. Couldn’t find the holding/adjustment nylon pins discussed on CN.

Pulled on the plate (using the plastic secondary holder tube) and it popped out. OK, now I see the nylon adjustment pins I was supposed to loosen.



Liquid pooled under the corrector plate at upper right of photo.

Danger, Will Robinson! OK, this is weird. There is liquid in two places on the corrector plate edges, and on the corresponding support ring. The liquid is clear but kind of oily/viscous. I have no idea what this is, but I’m pretty sure it is the same stuff that left the trails on the corrector and the tube.










Liquid pooled on edge of corrector plate, connected to trail across plate

Remote Possibility: Several months ago (4-6?) I cleaned the corrector plate. I used a solution of isopropyl alcohol and water, applied with cotton balls. I suppose it is possible that some of the solution slipped past the corrector. However, I can’t imagine a) there was enough solution to run like this, and b) how could iso and water not evaporate in the Arizona over multiple months?


Step 6. Clean inside of tube?

Tried to clean the specks on the interior of the tube. I’m afraid of messing up the tube, so in the end I left the inside alone.

Step 7. Cover opening with Saran Wrap.

IMG_0641To prevent dust blowing into the tube while I work on the corrector, I covered the OTA opening with Saran Wrap. The wrap doesn’t stick to the OTA like I expected, so more blue tape to hold the wrap in place.







Cleaning the Corrector

I followed the steps laid out by Clay Sherrod at Arkansas Sky Observatory. Assembled all the solutions, filtered, mixed… Got some rubber gloves to handle the corrector so I don’t get fingerprints on it; oops, the gloves themselves leave prints. I end up handling it by the secondary holder that I could never get out.

The tracks on the corrector plate are mostly removed. If I breath on the plate I can still see them, but otherwise they are gone.

Cleaned the outside face of the corrector as well. There are very small spots which I cannot get off. Tried a variety of things with little effect. Sometimes I could get one, but not often. They are hard to see anyway. Oh Well.


Put everything back together, no problems. When putting the plate back in, I loosened two of the nylon alignment pins precisely 1/4 turn. This allowed the plate to pop back in, after which I tightened back the 1/4 turn.


Hurray – more problems. First, the secondary still rotates easily and moves laterally. I had hoped that putting the secondary back in would tighten things down somehow. Did it do this before and I never noticed?

Second, I cannot get it to collimate. The stars are huge donuts. Adjusting the screws as usual ends up with one of the screws being completely tightened. Of course that one needs to be tightened more.

I think I have not assembled the secondary correctly? I will need to talk to Celestron and see if they can help.

Update AstroMC, FocusMax

Updated AstroMC/B6 from 2.3.1 to 2.4.1/B8. I am hoping this will eliminate the recent spate of ACP disconnects because “dome is already slewing”. This began when I installed the replacement dome controller, which is the new format in the gray box. We’ll see.

Also updated FocusMax to When I ran the new version it attached to Maxim (already running) and immediately clobbered the STF8300 USB port. Maxim did not see them in the camera list.

Tried repowering 8300, reseating USB cable, no luck. Finally rebooted Thor, and everything seems to work again.

Hallelujah, Unguided Tak Images

I am kind of afraid to log this, it may jinx the system…

The Tak is successfully running 15 minute exposures unguided!

I can’t really say why it is now working. I tightened down the clamshell holding the Tak a bit, otherwise nothing unusual. After completing the insulation and drywall project, I ran a new Tpoint model and tried things.

First I wanted to guide using the Edge 11; this has worked in the past. However, it wasn’t working – the guiding seems to be working well, the guide star stays in the tracking box, reasonably small guide errors. BUT, there seems to be a bit of flexure between the Tak and the Edge? Stars are slightly trailed in 10 minute exposures. ProTrack is off so it doesn’t interfere with the guiding.

Just for fun, I tried a 10 minute exposure with ProTrack. Say What? It actually worked! Even 15 minute exposures are working well. Longer than that is not necessary, my camera theoretically works efficiently with 10-15 minute exposures.

It has been running great for about a week now. I am nervous about switching systems between the Tak and the Edge. I have a separate model for the Edge, but maybe switching the systems will mess things up again. We will see.

 Tak Model

The Tak model has 206 points (some are thrown out, of course). The final Sky RMS = 12.7 arcseconds; excellent, but no different than values I had previously.

Tak 206 point Scatterplot

Tak 206 point Scatterplot. Click for larger image

Tak SuperModel terms

Tak SuperModel terms


Tak SuperModel graphs. Click for larger image


Tak Orthographic errors. Click for larger image.






Critical Focus Zone

Original Critical Focus Zone Calculation:

CFZ = 1.6 * λ * f^2

λ = 500 nm (green)    f = 10 for Edge11

New Critical Focus Zone

Traditional Definition

CFZ = 4.88 * λ * f^2


Goldman NCFZ

NCFZ = .00225 * θ * sqrt(τ) A f^2

θ = seeing (3 arc seconds)

τ = focus tolerance 15% visual, 5% AP?

A = Aperture



CFZ1 =  1.6 (500) (10)^2 = 80 micron

2 micron/step per spec

CFZ1 = 80 micron (1 step/2 micron) = 40 step

CFZ2 = 122 steps

NCFZ = .00225 (3) sqrt(5) (279.4 mm) (10)^2   =  420 micron (1 st/2 micron) = 210 steps

NCFZ is 5 times bigger than CFZ1, twice CFZ2



CFZ1 = 1.6 (500) (4.5)^2 = 16 micron

6000 steps / 2.1 cm * (100cm/m) * (1/10^-6)  .28 step/micron    or 3.5 microns per step

16 micron (.28 step/micron) = 4.57 step

CFZ2 = 13.9 step


NCFZ = .00225 (3) sqrt(5) (90 mm) (4.5)^2   = 27.4 micron

* (.28 step/1 micron) = 7.67 steps

NCFZ is 1.7 times CFZ; still very small


Celestron OAG failure

I spent quite a bit of time trying to get the Celestron OAG to work with the Tak Sky90. In the end, I cannot get it to work – I can’t get the guide camera to come to focus.

I am using the Starfish guide camera. It attaches to the nice helical focuser, but is too far out to come to focus. It needs to get closer by 10-20mm.

The main camera focuses fine. I have tried various adaptor spacings with no luck. I think without the helical focuser it might get there, but then of course I have no way to adjust focus.

I have subsequently played around with using the Edge11 as my guide camera. It looks like I can get the Edge to see out of the slit everywhere except small regions near the horizon at the North and South, so this may work fine.

Unfortunately, when I tried guiding for 30 minutes, I seem to be getting a small amount of drift. It looks like flexure somehow? I will have to play with tightening up everything in hopes of getting that to work.The guiding within Maxim looks fine – the graph is nice and steady. It looks like Maxim is guiding correctly, but flexure (or something) is causing the Tak to drift.

HAlpha 30 minute exposure

HAlpha 30 minute exposure


Another thought – if I use the Edge as the guider, maybe I can set up Maxim to focus the guider (Optec) independently of FocusMax focusing the imager (Sky90/EasyFocus). Would not be able to automatically focus in ACP (well, maybe with some coding) but at least maybe I could easily focus manually. It is awkward to load the Optec focuser into FocusMax, switch Maxim to the other camera, focus guider, switch everything back.

Drift problem continues

Finally got a chance to test the full system,  after the refractor only system worked so well. I’m not out of the woods yet.First, after all the weight has been put back on the mount, the polar alignment has shifted again. This time it wants 5 ticks in Az and 9 ticks in Alt. So, it appears that the original PA wasn’t wrong – the PA changes with weighting down the mount. Don’t know why this is, Bisque seems quiet about this.

Second,  I played with the idea of randomizing the  order of points when doing the TPoint model. AAG mapper does the points in order, and they are quantized in Alt values. I exported the points into Excel and a) randomized the order, and b) varied the Alt so the points have more variety.

In 50 point runs, random points give the same polar alignment results compared to ordered points. However, you get longer slews on average, and much more large errors on the orthographic view. As a result, you get a bigger circle RMS value for the model. For example, ordered 50 point runs gave a supermodel with 20 arcseconds radius while randomized runs gave a radius of 28 arcseconds.  When I ran the big model (374 points) I got a radius of 20 arcseconds instead of the earlier (November) 9.9 radius.

The points have a general NS line rather than being symmetric. Another odd feature, the dD versus n graph shows a linear trend upwards. PWallace notes the abnormalities, but doesn’t have a theory as to cause.

Also, the error buckets are in 20 as bins rather than 5, another reflection of the increased errors from randomizing the points. Note that the effect of randomization would be more pronounced in the bigger model, since the typical slews would be very short in the ordered case. Random slews jump all over the sky, and result in bigger errors. I suspect the randomized model is a better characterization of the scope since one often makes large slews to targets.

test images showed slight Dec drift in 10 minute exposures with ProTrack running. In the East and South ProTrack can almost eliminate the drift now, but not quite. The drift seems to be more pronounced in the West for some reason.

Drift Problem Solved?

I seem to have found the source of my drift problem, although it is perhaps slightly early to tell for sure. Unfortunately, the answer raises a number of other, potentially scary, questions.

The Story

I removed the Edge 11 and mounted the Sky90 refractor. Got everything balanced, etc, ran my tests. Still drifting, same as before.

At this point I still had the 350 point TPoint model in TSX, created with the original complete Edge + refractor setup. It was still helpful in pointing, but I certainly didn’t expect it to be useful in ProTrack. I decided to rerun a model since the 350 point model certainly no longer applied to the simple refractor setup, and I was guessing that might be the next thing Daniel would ask for. I ran a 50 point model to start, and WTF? the polar alignment is WAY off, 11 tics in azimuth and 5 tics in altitude (11 arcminutes and 5 arcminutes, I think each tic is 1 arcminute). The earlier responders were right about the alignment; I had ignored that issue since I “knew” it was OK, I had just aligned 2 months earlier.

I ran several more 50 point models while I got the alignment adjusted, then ran a 128 point model for now. Alignment is “Good enough” in both axis; I will do a more complete model when I get back to the desired configuration. The drift is completely gone in all directions (East, West, and South targets). 10 minute exposures are great even without PEC or ProTrack. Hallelujah!


So, this raises questions. The prior polar alignment was done November 12, with 7 or 8 50 point runs followed by the 350 point run all run the same night. Somehow, in two months the alignment shifted that much.

1) I have occasionally bumped into the OTA or counterweight shaft while working in the small area in the dome. Would this be likely to cause an 11 arcminute shift?

2) Would typical maintenance tasks such as greasing the gears, adjusting camera positions, or rebalancing be likely to cause this?

3) Can the polar alignment be impacted by the weight of the payload/counterweights? The original system was about 50-60 pounds of OTA and cameras; I needed both counterweights and the extension shaft to get it balanced. The refractor-only setup has maybe 10 pounds of payload, only needs 1 counterweight up near the mount, no shaft extension. Perhaps the heavier system can make the pier “bend” somehow?

4) Scary – can a mount settle or shift this much over 2 months? Might it still be shifting?

ImageThe concrete base is 18 inches square on top, about 28 inches square at the bottom of the hole. The hole is 42 inches deep. The base and pier are isolated from the building. The pier itself is 8 feet long, 8 inch diameter steel pipe filled with sand. I am located in Arizona, with desert “caliche” sand/clay in a flat yard. The cement was poured in January 2012, good polar alignment was done 9 months later onNovember 12, then it is bad two months later in January 2013. Is this normal for settling? Might I hope it is done, or should I expect continuing shifting?

4) Other than running a new TPoint model, is there an easy way to verify polar alignment precisely? I expect to want to verify alignment every week or two, at least until I am confident it is stable.



My next task is to remount the Edge and verify it is working correctly. I will run new TPoint models there to see if alignment shifts. Assuming things are under control there, I will remount the whole system and recheck.