Hi.

clop the astrophotography cretin here.

I have my Canon 300D at the prime focus of my Celestron 8" SCT. I have done my drift alignment carefully. I have nailed the focus (I think) with DSLR Focus. Then why do all my (unclipped) star images look like this?

(magnified x5)



With a white middle bit, a blue bit on one side, and a red bit on the other. Why aren't my stars round and white?

Please help.

clop

From what I know of 'scopes, and from what I've seen of my own, it could be collimation problem: that is, how the primary and secondary mirrors align.

Others may know better.

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This looks like diffraction from the atmosphere. What was the altitude of the star in the image? The closer to the horizon you are, the more diffraction you would get in the image.

Since you are taking an RGB image in one shot rather than separate R / G / B images, it is not alignment of those sub images.

How are you guiding and how long are your exposures? What kind of a mount do you have? I ask from curiousity -- I don't think it is a guiding problem because there is a color gradient in the star. Bad guiding would just make fat and/or oblong stars of the same color. Although a small guiding problem might increase the effect of atmospheric diffraction as the star gets spread out over more pixels.

HTH,

--Andy
----------
http://www.obsballona.org/

Are the stars white points towards the center of the field? Or is the aberration consistent throughout?

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"Why do something now when you can put it off 'til later?" -- me :)

It's consistent throughout. I think Andy might be right - the star was only 10 degrees above my horizon. I'm using the Jewel Box cluster in Crux as a learning project, varying things to see what improves the image and what makes it worse. The trouble is, when you're starting out, you don't know whether you're doing well or not. It's frustrating not to know the limitations of the equipment in advance. If someone could show you the best possible image you could ever achieve with your equipment you would have something to work towards. Here's last night's effort (near full moon). It's better than previous attempts - I think mainly because the focus is better.



Canon 300D at prime focus of 8" SCT, 19x4"@ISO800, IRIS

How can I make it better?

I will try imaging something higher in the sky and see if the star images look better.

Thank you for your help.

clop

Clop,

I'd recommend shooting something as close to the zenith as possible. A good target might be the Ring Nebula (M57) in Lyra. It is relatively easy to find and a pretty target too. I don't think it is too low at sunset yet.

HTH,

--Andy

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Hello Clop

Go for something much higher, at that low altitude you are very likely experiencing atmospheric refraction - I get this when shooting planets low down.. Try something in the Scorpion, at least if you get the same results low & high, it rules out one problem. Your high focal ratio will also make it more obvious.

Shevill Tasmania 42 South

Clop, quite frankly I think you are doing well. Major limitations are atmospheric conditions especially low on the horizon. My telescope gives a trilobar appearance to the stars I image because of the difraction effect of the support of the secondary mirror ( so-called difraction spikes). Also check the alignment of your mirrors.
Here ar my best two afocal images of Antares and Alpha Centauri. Certainly not perfect but the best I can achieve with my equipment.

Attached Images

	Antares.jpg (10.0 KB, 32 views)
	Alpha Centauri.jpg (10.3 KB, 25 views)

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Awww thanks Bokmajierie! Your Alpha Centauri is good - you can definitely see that there are two stars there. I'm going to take the best possible shot I can of Alpha Centauri tonight. Should be a good exercise in focussing. Is it possible to capture Proxima using a modest ground-based telescope? Or is it too dim or something.

clop

Clop, I think it is too dim for our size telescopes.
I rather like your Jewel Box cluster. Problem is that its a bit low on the horizon at the moment. But take a few more shots.

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Clop. if you do not mind getting up at 4 am, the Pleiades makes an excellent target to try out your focussing on. If you can see the nebulosities around some of the stars, you know that you have arrived. I have not tried my afocal technique on it yet as my scope is to bulky to cart around the garden at 4 am and our weather has been too unsettled to leave it out all night. BUT I am waiting! I am waiting!
Regards

Philip ( Bokmakierie)

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Well gosh that was much harder than I was expecting.

Here's alpha Centauri, 8" SCT Canon 300D at 9.7mm eyepiece projection, 15 images stacked in low altitude, poor seeing and terrible wind. That's probably why they look like two planets instead of two stars




Edit: actually, having just seen tvdavis's M42 photograph I feel utterly unworthy to ever post anything here ever again. I've never felt so low.

Well here's my sorrowful attempt at 47 Tucanae tonight. I shudder to imagine what tvdavis would think of this.



Canon 300D piggybacked. 300mm, 12 x 20 seconds at ISO800, f/8. IRIS blah blah.

clop

Hi Clop,

Its your focus on the 47 Tucanea image......its not perfectly focused.

In fact, if you do manage to get perfectly focused....and the problem is still there....a large part of this is is probably field curvature of your optics, the stars near the edges will be shifted more than the ones in the center.

Concerning that closeup star you posted........the one with
the 3 colors slightly offset in your DSLR ...this might be able to be fixed...bring your image into Adobe Photoshop.....show the seperate rgb images by checking the channels option under the window menu.

Now click on blue channel and use your move cursor to slightly shift it to line up with the other colors or channels.

Do the same for the red channel.....this should help you align all your channels. and get rid of your color channel misregistration.

Did I read this correctly.....you are shooting through an eyepiece?

Eyepiece projection method....if so atmospheric turbulance is a real concern using eyepiece projection method.

This could be the problem as well...............

Try primefocus --- no eyepiece........just camera body through the scope.

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John Chumack
The Chumack Observatories
MPC 838 Dayton Research Station
MPC H66 Yellow Springs Research Station
www.galacticimages.com

Clop,

I think you have done very well!! Do not give up and do not stop posting your pics here. O like both your images!!

Phil

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Thanks for the advice Galactic2000.

The focusing is a real pain with my Canon 300mm telephoto lens. It's so loose and on full zoom it seems like you only have to turn the focusing ring a nano-micron and it causes a big difference. And the lens slops in the barrel too. I was worried about poor lens design causing the colour problems so I stopped it down from f/5.6 to f/8 to baffle it a bit and increase the depth of field. Well it didn't work did it.

I can't use prime focus for 47 Tucanae because my camera on the tele-extender bumps into the back of the mount anywhere close to celestial south.

Prime focus on alpha Centauri wouldn't split them. The eyepiece projection was the only way to see two stars.

Thank you again.

clop

Hi Clop,

I have a very basic digital camera (Fujifilm Finepix2600zoom with a 3x optical zoom.) I have by accident discovered that when I shoot through the eyepiece I set my camera on automatic ( auto flash and auto-exposure). When I click halfway the camera focuses on the image in the window and then, when I press the button the full way, the flash always goes off but has no influence on the image. More often than not my images are well focused especially of the moon and of Jupiter. Even my alpha Centauri image was well focused.
I do not know the Canon but maybe you could try this as well.
Phil

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Clop,

Here is an example of what my Jupiter images look like. I shot a whole series through my eyepiece and then selected the best and stacked them with Image Stacker ( 9 images) and the result is quite acceptable to me. All images were in focus and I stacked them to get rid of the noise. You must remember that I do not have a motor drive and just shoot at random through the eyepiece. The purists would certainly like to see more detail on the surface but then you need to take longer exposures which I cannot do.

Attached Images

Cutout 10a_Cutout 6a.jpg (12.4 KB, 25 views)

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This is really powerfully inspiring stuff.

Ta.

clop

Your picture is quite nice.

You can get outstanding planetary images without expensive equipment or long exposures. The secret is the web camera -- you take a short movie of the planet and user freeware (Registax) to sort and stack the images. I have attached a shot of Saturn I took with an 11" SCT and a Phillips web camera.

Ice in Space has a great tutorial on what you need to do.

Always more to explore!

--Andy

Attached Images

Saturn-f20-060313-04-st248.jpg (50.6 KB, 16 views)

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WOW!!!!! Double WOW!!!!!
That is fantastic!!
Thank you for sharing and the advice.

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If at first you fail, you're running average

andyschlei that is just amazing!! You certainly have to eat humble pie to post in this forum (me, not you).

Galactic2000 I went back and nailed the focus and did 47 Tucanae again. I'm think I'm really pleased with it this time.



30 images taken at 300mm, f/8, 30 seconds each at ISO1600, stacked in IRIS.

Thanks all for the kind words.

Clop -- looking better all the time!

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