dgpilot
super member
Reged: 09/25/09
Posts: 105
Loc: San Ramon, California
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Hey guys, before imaging M27 a couple weeks ago i decided to take a defocused shot to check for accurate collimation later. After dumping my money into all this equipment there is no room in my budget for a laser collimator right now.
After looking at this image, does it seem like the scope is in good collimation? You can see the obvious coma in the corners. This was taken with a 50D prime focus.
Defocused M27 image
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LXD75 8" SN f/4
Canon 50D, 350D (unmodded)
Orion ST80 Guidescope
Meade DSI II Pro
San Ramon, California 
My Astro Gallery
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hudson_yak
Pooh-Bah
Reged: 11/15/07
Posts: 1106
Loc: Hyde Park, NY, USA
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Neat picture.
Coma away from center is expected. Though I'm not sure if that's what we are seeing here, perhaps it's just the secondary obstruction shadow and the entrance aperture eclipsing the primary off axis. Depends on the defocus amount.
What you want is the donuts with the hole centered to be in the center of the field. It looks like currently the most-centered donuts are just above M27 so yes, collimation could be improved.
Mike
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Meade 8" LX10
Orion ED100 + Losmandy GM-8 + Berlebach UNI 24
Meade 12" LightBridge
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panhard
Mongo
Reged: 01/20/08
Posts: 5226
Loc: Markham Ontario Canada
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You are very close to being dead on. It would only take very minor adjustments from now on.
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gtown1
member
Reged: 11/07/08
Posts: 69
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When you can afford it, get a good collimator. Having dead on collimation, not just close collimation, will give you that much better of an image. Often times we have to take the hobby of astronomy one step at a time. Most of us cannot afford to buy everything we would like when we would like. Over the years, little by little, you will grow your add-ons such as as collimators, good eyepieces, and so on.
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Happy owner of an Obsession, 18 inch Ultra Compact and a Mag 1, 12.5 inch Porta Ball.
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dgpilot
super member
Reged: 09/25/09
Posts: 105
Loc: San Ramon, California
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Thanks.. I appreciate the tips  I knew it wasn't spot on perfect, but just wanted to check how bad it was for the time being without a collimator.
gtown1.. you're absolutely right, I already have a list of things i still need to get provided that 1--no budget disasters happen and 2--fiancee doesnt find out I'm buying them 
It's almost a good thing sometimes, that your significant other isn't familiar with this hobby. Last month I added an Orion ST80 guidescope on the OT and she didn't even notice the addition 
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LXD75 8" SN f/4
Canon 50D, 350D (unmodded)
Orion ST80 Guidescope
Meade DSI II Pro
San Ramon, California
My Astro Gallery
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Starman1
Vendor - Scope City
Reged: 06/24/03
Posts: 12230
Loc: Los Angeles
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The edge of field images display astigmatism on the right and some coma plus astigmatism on the left.
I would guess the collimation wasn't dead on.
If you can't afford a laser, don't worry. Get a combination sight tube + cheshire from Orion, Antares, Celestron, or others, and collimate easily. Should cost around $40+/-.
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Don Pensack
12.5" Truss Dob, 5" Maksutov, Fujinon Binos
Sustaining Lifetime IDA member
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sixela
Postmaster
Reged: 12/23/04
Posts: 10875
Loc: Boechout, Belgium
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Quote:
The edge of field images display astigmatism on the right
Actually, I think that's just clipping by the secondary. It looks like it's a really big sensor...
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400mm f/4.46 self made Dobsonian on Tom Osypowski equatorial platform
Orion Starblast (114mm f/4 reflector, Alt/Az)
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davidpitre
Carpal Tunnel
Reged: 05/10/05
Posts: 1826
Loc: Central Texas
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Quote:
The edge of field images display astigmatism on the right and some coma plus astigmatism on the left.
I would guess the collimation wasn't dead on.
If you can't afford a laser, don't worry. Get a combination sight tube + cheshire from Orion, Antares, Celestron, or others, and collimate easily. Should cost around $40+/-.
Agreed. You should be able to get excellent collimation without a laser.
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David
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deSitter
Carpal Tunnel
Reged: 12/09/04
Posts: 2928
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Quote:
Hey guys, before imaging M27 a couple weeks ago i decided to take a defocused shot to check for accurate collimation later. After dumping my money into all this equipment there is no room in my budget for a laser collimator right now.
After looking at this image, does it seem like the scope is in good collimation? You can see the obvious coma in the corners. This was taken with a 50D prime focus.
Defocused M27 image
Nice - by comparing the comatic edges you can see exactly where the sweet spot is (a little to the lower right of the nebula). Your collimation looks good at least on a cursory inspection.
The out of focus stars show the characteristic crescent shape on the inside edge. As you focus, that will go over into a bright 1st diffraction ring flared away from the center. By studying these patterns carefully you will be able to determine exactly where your sweet spot is. Bring it to the center with the primary adjusters. This works even if your secondary is not precisely collimated. You only potentially lose even field illumination at low powers.
-drl
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dgpilot
super member
Reged: 09/25/09
Posts: 105
Loc: San Ramon, California
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Technically then.. I can just use my 50D with a similar star field, defocus a bit more and zoom in with live view to the dead center and collimate until the stars have the secondary obstruction centered perfectly.
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LXD75 8" SN f/4
Canon 50D, 350D (unmodded)
Orion ST80 Guidescope
Meade DSI II Pro
San Ramon, California
My Astro Gallery
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Jason D
Post Laureate
Reged: 10/21/06
Posts: 3311
Loc: California
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Quote:
Technically then.. I can just use my 50D with a similar star field, defocus a bit more and zoom in with live view to the dead center and collimate until the stars have the secondary obstruction centered perfectly.
Just do not defocus too much. The defocus amount in your photo is good.
However, you are only bringing the primary focal point to the center of your view. This method will not flag or correct the tilt of the primary mirror focal plane with respect to your CCD plane. If it is too bad, then you can't bring the whole view to focus precisely.
Jason
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XT10 classic with premium optics
Tri-knob CR2 with compression rings
Round Table Platform
4.5" StarBlast
6" StarBlast6
TV EPs
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MitchAlsup
super member
Reged: 08/31/09
Posts: 186
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Quote:
After looking at this image, does it seem like the scope is in good collimation? You can see the obvious coma in the corners. This was taken with a 50D prime focus.
If this was taken at prime focus, then why are the stars 6 sided and not round?
The center of the optical axis near the top right corner of M27. I put a yellow cross on it, but the image looks like it did't come out so good.
Edited by MitchAlsup (11/03/09 03:45 PM)
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Jason D
Post Laureate
Reged: 10/21/06
Posts: 3311
Loc: California
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Stars wobble depending on seeing conditions. And if the stars happen to wobble along 3 difference axes, the final superimposing defocused images might resemble a hexagram.
The above is just an educated guess.
Jason
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XT10 classic with premium optics
Tri-knob CR2 with compression rings
Round Table Platform
4.5" StarBlast
6" StarBlast6
TV EPs
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dgpilot
super member
Reged: 09/25/09
Posts: 105
Loc: San Ramon, California
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Quote:
Quote:
After looking at this image, does it seem like the scope is in good collimation? You can see the obvious coma in the corners. This was taken with a 50D prime focus.
If this was taken at prime focus, then why are the stars 6 sided and not round?
The center of the optical axis near the top right corner of M27. I put a yellow cross on it, but the image looks like it did't come out so good.
Like Jason mentioned.. the seeing was about 2/5, so the stars were wobbly. I also believe at that time the scope wasn't cooled off yet
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LXD75 8" SN f/4
Canon 50D, 350D (unmodded)
Orion ST80 Guidescope
Meade DSI II Pro
San Ramon, California
My Astro Gallery
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Brian Schmidt
sage
Reged: 01/25/09
Posts: 204
Loc: Roswell, GA
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I was just looking at my 8 f/6 and had similar views. Mine also showed a little flat spot. Is that Coma or a misalignment? I guess a good way to check would be to throw in the paracorr that I use on my LB 12 and retry...
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Brian
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deSitter
Carpal Tunnel
Reged: 12/09/04
Posts: 2928
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Could be the drawtube impinging on the light path.
-drl
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Brian Schmidt
sage
Reged: 01/25/09
Posts: 204
Loc: Roswell, GA
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o yeah, never thought of that. I did receintly put on a 2 inch focuser. I'm going to check that when I get home. Thanks!
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Brian
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MitchAlsup
super member
Reged: 08/31/09
Posts: 186
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Having looked at the image several more times, I think the 8 sided aspect is simply the pixelization and that there is no obvious vignetting of the aperture.
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sixela
Postmaster
Reged: 12/23/04
Posts: 10875
Loc: Boechout, Belgium
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At least not on-axis.
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400mm f/4.46 self made Dobsonian on Tom Osypowski equatorial platform
Orion Starblast (114mm f/4 reflector, Alt/Az)
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