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Drift alignment with PHD and your laser pointer

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#1 michael hester

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Posted 15 September 2012 - 07:10 PM

Recently I started practicing drift alignment for my imaging setup and I've gotten fairly decent results. The biggest issue I had though was adjusting the azimuth on the mount. You have to hit a spot when adjusting that that is literally sub millimeter in order to get a good 5 minute graph. I found a way to help myself get that subtle movement for alignment and i'll share it below.

PHD guiding
A 1 ft bungee cord
A small spring-loaded clamp to hold your laser pointer's button (home depot for $2)
your green laser pointer

1. Do a rough polar alignment with All star polar alignment, your polar scope, or whatever you used to use.
2. Use the bungee cord to tightly strap your green laser pointer to your mount. You may have to wrap the bungee more than once. The laser should point at the ground behind your telescope. (to the south)
3. Set your mount to aim at the meridian at about the level of the equator. The mount's counterweight should be on the east side of the mount. Turn on your mount's tracking.
4. Calibrate PHD. You don't need to align your mount's computer yet, just calibrate phd. Turn on the graph.
5. Under the brain menu, once calibration is complete, stop phd's image loop, open the brain menu and check "disable guide output".
6. Start guiding on a star again and turn on the graph.
7. (Azimuth correction) Watch the red line on the graph. It will drift up or down. If it drifts down the laser dot needs to move east, if it drifts up, the laser dot needs to move west.
8. Make the appropriate correction. Move the dot no more than 3 inches on the ground and do another test. Your calibration is good if you can get a drift less than 1 pixel over a 250 tick graph. Go back to step 7 until you get a flat dec line. Once you're satisfied, lock your azimuth and go on to step 9.
9. Now point the scope toward the east horizon. Start guiding again. So long as you don't change declination much the PHD calibration will hold. Now if the graph drifts up, the laser dot needs to be farther away from the telescope. If it drifts down, the laser dot needs to be closer to the telescope.
10. Make the appropriate correction. Once done go back to step 9 until you have a flat dec graph.
11. Alignment should be complete. Align your goto computer if you have one and happy shooting!

PS: if you have a west sky instead of an east sky, then you need to flip the corrections from step 9. That is, if the line drifts up the dot needs to be closer to your telescope, if it drifts down, the dot needs to be farther away from your scope.

PPS. This procedure can be done at twilight before you align you scope's computer, allowing you to put those twilight hours to better use. Turn off your laser when you're done alignment.

ppps: this procedure only works in the northern horizon Well the technique should work in the south but i don't know how the dot directions will work.

edit 3: laser directions are now in cardinal than left/right

#2 Raginar



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Posted 16 September 2012 - 02:45 AM

That's awesome Hestor :). Good way to visualize what's going on!

#3 michael hester

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Posted 25 March 2013 - 10:43 AM

I actually need to make an addendum to this that I hadn't found back in September...

It seems that Disable Guide Output may not always work (i think the guys that do PHD are aware of it) so an alternative is to set the dec guide pulse length to 1 (1 thousandth of a second). This will effectively disable the guide output so you can work using the guide.

#4 Gray


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Posted 26 March 2013 - 10:26 PM

The way I've always done it was after I calibrated, I turned the Auto to North, then switched RA/DC to DX/DY and try and flatten DY with AZ screws. Same thing in the west, but with the Alt screws. This looks like both of us doing the same thing a different way. Either way, the laser pointer idea is a good one :) I like your apartment obs box too. Got me thinking about something I might try out. Good ideas from you in summery.

#5 michael hester

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Posted 27 March 2013 - 04:08 PM

For the obs here's what I did:

Estimated total cost: $250
This cost is for the shed and pier only. What you put in it i can't tally up.

The box:

The lock: 1x master lock combo lock.
The pier: Find the slotted steel angles in lowes then pick up the following:
13x 1ft slotted steel angle
3x 2ft slotted steel angle

36 3/8 inch x 1/2 inch galvanized steel bolts
6x 3/8 inch x 1 inch steel bolts
36 1/2 inch nuts (to go on the bolts)

4x 2 inch furniture plates (with holes in em)
1x 12 inch square treated plywood section. I recommend a minimum of 3/4 inch thickness. Lowes will cut it to size for you.

For CGEM DX: 3x 1 1/2 inch corner brackets that can fit 1/4 inch bolts in the holes, 3x 1/4x20 screws, 3x washers 3x nuts to fit on the 1/4x20 screws.

1x 10mm x 3 inch galvanized steel bolt with washer (this is the center post from the CGEM dx. It helps to take one of the retaining knobs from the base in to lowes so you pick the right one). It should be at least 3 inches long so to reach through the plywood base into the CGEM DX base plate. You'll be reusing that for this pier.

1 box of 1/8th inchx 3/4 inch wood screws.
1 2 inch corner bracket

How to build it:
Assemble the suncast box according to the included instructions. When you get to the part where you install that brass rod, don't install it. It keeps the box from opening all the way, and we want it opened all the way. Place the box on a deck with the top door facing north. It doesn't have to be exact, just within 20 degrees, esp for a CGEM DX. If it's too far off of north the mount will hit the inside of the box as it slews.

Build the pier: Requires 2 socket wrenches.
Each of the 1 Ft steel angles is an edge to a 1ft cube with the 3 2 ft sections serving as the bottom stabilizers. There will be 4 sections of 1ft steel angles left over, these are stabilizers for the pier.

Build the 1ft square cube. Start with two of the 2ft sections. Attach one 1ft section between them at half way. Use the other 2ft section to complete the base. DO not tighten any bolts until the cube is finished.

Use 4 more sections of 1ft angles as the vertical edges of the cube. Finally use 4 more sections of 1ft angles as the top. Each corner of the cube should have 3 bolts in it. use the remaining 4 pieces of 1ft section as stabilizers. They'll fit at an angle and should fasten to the vertical bars in the Pier.

After all angles are in place torque all of the bolts until they're tight.

The plywood base: Requires 1/2 inch drill bit, marker
Remove bolts that face up from the top of the pier cube and place the plywood base on top of the pier. Mark 6 holes for where the base will be bolted to the pier. Drill these holes, then attach the base to the pier with the 1 inch bolts. Use washers to prevent splitting the wood.

Attaching the CGEM baseplate.
The CGEM DX needs 52 inches horizontal area for the mount to move freely without hitting anything and approximately 18 inches in front of the GEM Head. Place the pier in the suncast box. The 2 ft steel sections of the pier will form a T shape. The top of that T should be at the front of the pier over the suncast logo on the deck piece. Use extra bolts to prevent the back from wobbling then use the furniture plates and wood screws to bolt the pier to the deck of the suncast shed. The pier should be as centered as possible.

This works out such that the CGEM baseplate (what you attached to the big tripod it came with) will be right at the back edge of the plywood. Get a rough location of that and ensure that the plate is 26 inches from the side walls.

Mark where the center hole is plumb to the plywood, then drill this hole.

Using the small corners and the 1/4-20 bolts: Attach these corners to the cgem baseplate. They don't have to be too tight but they should be fairly tight. There will be play in the CGEM plate's holes. This is Okay. we need the corners to be flush to the bottom of the plate. They're not load bearing at all they just keep the plate from slipping when we install the CGEM DX head.

Align the baseplate assembly such that the north post on it points roughly north, then use the wood screws to bolt it to the plywood base. The pier is completed.

At this point install the CGEM DX head and attach your scope. Note that at higher latitudes the observatory box may not work as latitude increases the forward distance the CGEM needs to move freely. The box observatory will fit up to an 8 inch SCT. It will not fit a very long scope. It definitely didn't fit my C11. You may use longer scopes if you don't mind taking the scope off the mount every night.

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