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.

Electroluminescent Flats


I have been using Sky Flats in ACP for some time. They work well, but have a couple of procedural limitations:

  • I can’t always be in a position where I can kick them off at a reasonable time. I forget, then it is too late/dark to start them. I suppose I could a) start early in the day, but I don’t like the idea of the observatory sitting open all day, or b) play with the #waituntil command, although I don’t think it actually affects taking the flats.
  • There isn’t enough time to get all my filters in. I do 25 frames for each of 6 filters (R,G,B,Ha,Oiii,Sii). By the time it gets dark enough to begin taking the frames, there isn’t enough time to do all the filters. So, I have to manually edit the flat plan file for the flats I plan to use that night. Of course, I tend to mess that up, or later want to change my mind..

So, I would like to do panel flats. I have played around several times with a flat panel screen mounted on the dome. It works OK, but has problems: 1) the small dome means the panel is not perpendicular to the OTA, 2) I don’t have a way to adjust the light levels for the various filters. For example, the HAlpha filter needs a brighter light than the RGB. I have played quite a bit with different lamps illuminating the screen, but it has never worked well.

I recently ran across a post somewhere (This site is similar to the site I found) that talked about using an electroluminescent panel from I have looked at EL panels before, but they are pretty expensive. This panel was not as much, cost about $200.

To mount my panel I simply cut a hole in the cardboard shipping material. The cardboard folds nicely around the panel to protect it. Velcro strips hold the panel to the shelf.

IMG_0623I built a “shelf” to hold my panel. The shelf is adjustable so I can position it about an inch from the OTA, fairly perpendicular.The shelf swings back and forth on the long rod, and slides up and down.




The shelf angle is adjusted with the 1/2 inch PVC arm which slides up and down a wood rod which fits inside the pipe.IMG_0624

The shelf stays in position all the time; as the OTA swings around it does not hit the panel. When I take flats, the OTA swings into position. As it does, it misses the panel by about 1/4 inch. The back end of the scope (camera) also misses the panel.

The panel includes a little inverter box with a knob that varies the brightness of the panel. Originally I intended to replace the knob with a computer controlled potentiometer to adjust the intensity. However, it turns out that all the filters work when the intensity is all the way up. The only exception is the Luminous filter. If I want to do Luminous flats I need to manually turn the knob down to the minimum setting.

Note that HAlpha and Sii flats require long exposures (50-70 seconds). The panel is not white, but heavy in the blue and green spectrum.

In operation the panel power supply wall wart is plugged into my DigiLogger power supply. The ACP AutoFlatConfig file allows turning on the power when the flats are run and turning it off when done.

I can now run automatic flats anytime, except for Luminous which require me to manually adjust the panel intensity down.