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General Astronomy >> Beginners Forum

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osohumano
newbie


Reged: 09/30/12

Loc: Mesa, az
Beginning Astrophotography. new
      #5452532 - 10/02/12 07:36 PM Attachment (42 downloads)

hello all. I'm new here and i just wanted to get this question out. I have a Celestron 4se, a t-adapter from celestron, and a canon 300d. i hooked them together and came out with a beautiful photo of a doughnut of light. I'm thinking i need a focal reducer, but i don't know for sure. has anyone had this problem? if so what do you suggest. (a 4se is 4" diameter with a 1325mm focal length, f/13.

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Stargazer2012
super member


Reged: 09/01/12

Loc: Maine USA
Re: Beginning Astrophotography. new [Re: osohumano]
      #5452555 - 10/02/12 07:53 PM

Looks like lens flare to me. Were you shooting the full moon perchance?

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Midnight Dan
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Reged: 01/23/08

Loc: Hilton, NY, Yellow Zone (Bortl...
Re: Beginning Astrophotography. new [Re: Stargazer2012]
      #5452580 - 10/02/12 08:15 PM

That's what it looks like when things are out of focus. How did you focus? If you focused with an eyepiece and then just swapped the camera in, it will not be in focus.

-Dan


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drewp
professor emeritus
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Reged: 05/19/07

Loc: 41.5 deg north moline ill
Re: Beginning Astrophotography. new [Re: Midnight Dan]
      #5452591 - 10/02/12 08:22 PM

you definatly need to focus the scope for the camera, with a dslr goto a brighter object in the sky and focus looking threw the cameras finder. take a photo, if you can zoom it on playback and its in good focus you can then go to a dimmer object and it should remain in the focus it was in. obviously previewing focus and making adjustments on a brighter target first is easier than trying to do that on a dim object that probubly cant be seen well threw the cameras finder.

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osohumano
newbie


Reged: 09/30/12

Loc: Mesa, az
Re: Beginning Astrophotography. new [Re: drewp]
      #5452615 - 10/02/12 08:38 PM

it's prime focus through the back of the scope. no lens.

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davebuechler
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Reged: 08/21/11

Loc: Red River Gorge Kentucky
Re: Beginning Astrophotography. new [Re: osohumano]
      #5452644 - 10/02/12 09:02 PM

You haven't reached focus. If your focuser is fully racked out you need an extension. It looks like you are very close though.

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Raginar
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Reged: 10/19/10

Loc: Rapid CIty, SD
Re: Beginning Astrophotography. new [Re: davebuechler]
      #5452854 - 10/02/12 11:13 PM

Yup, camera isn't in focus.. if it stopped, you'll need some type of extension. Expect some pretty serious vignetting due to the CCD size of your DSLR. But, you can crop and still get some good images .

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WaterMasterAdministrator
Moat Keeper
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Reged: 02/17/10

Loc: Southeast Idaho, USA
Re: Beginning Astrophotography. new [Re: Raginar]
      #5452864 - 10/02/12 11:19 PM

Welcome to Cloudy Nights, you've come to the right place for answers.

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SteveNH
professor emeritus
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Reged: 07/10/11

Loc: Millbrae, CA
Re: Beginning Astrophotography. new [Re: osohumano]
      #5452904 - 10/02/12 11:50 PM

This is not lens flare - but as others have said, you are out of focus. If your sample picture is showing the full frame from your Canon camera, then according to my guestimates from this image, and measuring the size of your donuts, you are about 2.6 inches away from the focal plane.

If these are stars, then what you are seeing is the disk of your 4" mirror with a silhouette of your secondary inside it. (You can demonstrate this easily by putting your fingers in front and shooting another shot - you will see the shadow of your fingers!)

Your telescope focuses by moving its mirror back and forth, and so by turning your focusing knob, you can most likely move the focal plane a great enough distance so that it will match up with the sensor located in the back of your camera. You can check this easily, as mentioned above, by actually looking through the viewfinder of your 300D during the day, when an object like a distant telephone pole will make it easier to turn the focusing knob the right way and see that it is in focus. Then you can fine tune focus on the moon or stars.

I've gone through this many times with prime focus setups - it will be rewarding when you see the image finally pop into focus in the camera's viewfinder.


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Maverick199
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Reged: 02/27/11

Loc: India
Re: Beginning Astrophotography. new [Re: osohumano]
      #5453066 - 10/03/12 04:43 AM

That is indeed a good pic of a nice collimated scope. You just need to get the object into focus. Use your viewfinder to focus the object by turning the knobs. Use 'manual' mode at ISO 100 for the Moon with 1/80 shutter speed. For the stars, use ISO 800 at 10 secs to start with.

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Midnight Dan
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Reged: 01/23/08

Loc: Hilton, NY, Yellow Zone (Bortl...
Re: Beginning Astrophotography. new [Re: osohumano]
      #5453329 - 10/03/12 10:28 AM

Quote:

it's prime focus through the back of the scope. no lens.




Prime focus just means that you are using the scope's native focal length and not adding any additional optics like a barlow, focal reducer, or eyepiece. You still have to focus the system so that the image plane is at the sensor of the camera. Since the camera is at a difference distance than an eyepiece, you can't just pop the camera in and assume that it will be in focus.

If your camera has a live view mode, you should be able to focus on very bright targets using the LCD screen on the back to preview the image during focusing. Everything in the sky is at the same focal point so if you focus on, say the moon, and then move to your intended target, it will be in focus too.

If you don't have a live view mode, focusing is more difficult. You can use a software package like Backyard EOS or others that loop through repeated short exposures and then display them on a computer screen. You can then slowly change focus, wait for the computer screen update, and adjust accordingly. A Bahtinov mask helps with this task.

-Dan


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Phil Sherman
Pooh-Bah


Reged: 12/07/10

Loc: Cleveland, Ohio
Re: Beginning Astrophotography. new [Re: Midnight Dan]
      #5455004 - 10/04/12 12:21 PM

Try going out in the daytime and focusing on a distant target. It's a lot easier to do when the target isn't drifting through the imaging field. Make a record of how many turns of the focusing knob you needed from one end of the focusing range and you'll be set for night time use. You'll need a bit of touch-up on your focusing but you should be in the ballpark.

Focusing on stars is almost impossible to do. You should make your own Bahtinov mask using the generator at:
http://astrojargon.net/MaskGen.aspx
Print it, cut it out, tape it over the front of your scope, and you'll be able to get excellent focus using a bright star. You may even be able to focusing using the camera's viewscreen but it usually is easier to do when using a computer. You can use Canon's computer control for your camera or try a copy of Backyard EOS (BEOS), an inexpensive camera control program. You may be limited to 30sec exposures because your camera will require a separate shutter control cable for longer (bulb) exposures. Newer Canon cameras, with Digic III or later processors in them have a "live view" preview mode that helps focusing and can take long bulb exposures using the single USB control cable.

Take your astro images using the camera's RAW mode. In-camera JPEG processing will normally throw away at least 50% of your image data - a normal consequence of JPEG compression. Set the white balance to daylight and avoid AWB like the plague. The imaging chip in your camera is large enough that you'll see some vignetting in the image. You can learn how to do flats (and flat darks) to remove this and the effects of dust on the imaging sensor. You should also be taking darks, exposures of the same length (and temperature) as your lights (what you're imaging), whenever you're not imaging the moon or planets. Camera thermal noise will be removed by a good stack of darks.

Learn about calibration, stacking, and image enhancement. You can get excellent downloadable (free) programs to do calibration and stacking but may want to purchase software for image enhancement. Many astrophotographers use Photoshop but I've stayed away from it because it's expensive and usually requires additional purchased plug-ins to work well with astro images. My program of choice for image processing (and camera control) is Mike Unsold's ImagesPlus. It'll do just about anything you need for astro image processing, has good options for a novice using a DSLR camera (Canon or Nikon) and works great with any imaging camera that produces FITS images. Best of all, it's a lot less expensive than Photoshop.

Phil


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lamplight
Carpal Tunnel
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Reged: 09/18/12

Loc: western MA, U.S.
Re: Beginning Astrophotography. new [Re: Phil Sherman]
      #5455151 - 10/04/12 01:58 PM

that looks exactly like the view I had .. what you would see with no eyepiece.. try with the right angle removed? with the scopes visual back on (in my case) that did it for me. both with and without focal reducer. and focusing as said

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michael hester
professor emeritus


Reged: 11/28/08

Re: Beginning Astrophotography. [Re: Phil Sherman]
      #5455155 - 10/04/12 02:02 PM

At least your collimation is excellent

You're definitely out of focus there. Try to focus it. If you get all the way to one end and can't get a focus you'll have to modify your imaging chain. I would suspect you put the camera into a diagonal after you aligned something with your eyepiece. If you cant get it to focus then you need to attach the camera directly to the back of the tube without the diagonal. On the 4SE thats going to be a challenge since the back is so small. You'll slip the camera nosepiece in where the diagonal goes into the visual back. Thankfully you can still view through the scope with your DSLR and get a rough focus.


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