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the ultra low end of video astronomy

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#26 mclewis1

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Posted 19 March 2013 - 09:40 AM

Many folks will adjust an image slightly to make it more representative of what is seen on a high quality monitor (it's usually better than what shows up on a single image capture on a PC).

Viewing the video stream often does look better to me too but many video hosting sites seem to impart additional compression which doesn't help the images.

It's very clear that any more than slight tweaks can really make a substantial improvement in an image and this would now be something that isn't representative of the "live" image thus the comments about not posting images that have been extensively processed. ATL's great images show this progression very clearly as do many of Nytcam's images.

I think the best course of action is to post as many details about an image as possible. That way folks can better understand, interpret, and compare the images.

#27 Atl

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Posted 19 March 2013 - 09:46 AM

The processed M13 is "slightly" more detailed. It only has about a billion more stars visible...lol. The other images are completely representative of what can be seen on the screen with a camera and a c90. It would be pretty awesome to get my 12.5" dob up on a driven mount...oh well...

#28 Pharquart

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Posted 24 March 2013 - 04:25 PM

Atl,

Are you creating these images using prime focus, or are you using an afocal setup?

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#29 Atl

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Posted 24 March 2013 - 04:54 PM

I used a focal reducer for the nebula and prime focus for the cluster. Roughly 45 and 90x respectively.

#30 Pharquart

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Posted 24 March 2013 - 05:47 PM

How are you calculating the magnification? What is the equivalent focal length of the camera?

#31 Atl

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Posted 24 March 2013 - 08:33 PM

The ccd is .5" and the FL is 1200mm. So actually it is 96x and 43x. I have been wanting to add some more images but the moon has been early and bright lately.

#32 GlennLeDrew

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Posted 24 March 2013 - 08:47 PM

A magnification value ascribed to an image is arbitrary, for the result depends also on the angular size of the image on the viewer's retina. If you stick your face near your monitor, an image has a higher 'magnification' than when viewed from across the room.

To first order, one can say a detector is equivalent to an eyepiece whose focal length equals the detector's width. And so a 6mm wide CCD is about equal to a 6mm eyepiece. But this is open to debate, for one can choose a longer f.l. eyepiece having a small apparent field or one of a shorter f.l. but larger apparent field and have the same field of view.

Far better to simply state the field width, in degrees or arcminutes. This is unambiguous.

#33 Atl

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Posted 24 March 2013 - 09:13 PM

That is definitely precise. Now for my newbie question: How does one determine and state the field width given the information I have provided?

#34 GlennLeDrew

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Posted 24 March 2013 - 10:21 PM

Find the CCD's actual imaging area's dimensions, then divide this by the scope's focal length and take the inverse tangent.

Example, a 6mm wide chip with a 1,200mm focal length has a field width of:

ARCTAN ( 6 / 1200 ) = 0.286 degrees = 17.19 arcminutes.

Note that for such small angles it's valid to simplify like this. For larger angles, exceeding approximately 10 degrees, the correct formula is:

(ARCTAN ((CCD width / 2) / focal length)) * 2

In other words, you first calculate the semi-angle due to the tangent function, the multiply this by 2 to get the full angle.

#35 ccs_hello

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Posted 24 March 2013 - 11:31 PM

Also can try CCD Calculator:
http://www.newastro..../camera_app.php

Clear Skies!

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#36 Atl

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Posted 25 March 2013 - 04:18 AM

Nice! Thanks for the info!

#37 Atl

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Posted 31 March 2013 - 01:34 AM

I got the C90 back in action. I am focal reduced to about F6. I thought I might try a dimmer target so here is m51 at 128x sens up.

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#38 mclewis1

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Posted 31 March 2013 - 11:23 AM

That's an absolutely great shot for 2.1s with a 90mm scope. That EXview HAD sensor is really impressive.

#39 Atl

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Posted 31 March 2013 - 11:49 AM

I'm dumbfounded myself. When you look through a C90 the views are very dim. The camera is doing most of the work. Even in my big dob I just see a big blob and a little blob. The whole point of my thread is that this tech is accessible at nearly any price point...and it gives me a reason to play with my little telescope.

#40 Atl

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Posted 31 March 2013 - 05:55 PM

I implemented an upgrade. I ditched the C90. It seems to have a lot of internal reflections from bright stars. I replaced it with the 114mm Orion Imaging Newtonian today. It is F3.9...so I will probably be maligned for not using an f12.5 ...lol. It did cost me much less than the C90 so economically speaking I am even lower end than before. I am still using an eq1 with a 25$ drive.

#41 GlennLeDrew

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Posted 31 March 2013 - 07:58 PM

The faster Newt will be so much better for DSOs. And what a mere couple of seconds reveals through that 90mm Mak shows so clearly how utterly poor is the resolving power of our eyes at these low brightness levels. This is why even the low end of imaging gear can be superior for revealing subtler details which require much larger apertures for visual detection.

#42 Atl

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Posted 31 March 2013 - 09:11 PM

I agree. So now I am wondering what is the draw of the huge scopes and vast collections of eyepieces? This extremely modest gear is putting my 12.5" dob to shame. In any case the dob sold last night and an 8" on a cg5 goto just shipped...that will change my game just bit. In any case I will take some videos with the 114mm tonight since it is so clear tonight on the Mexican border.

#43 Atl

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Posted 01 April 2013 - 12:57 AM

Video of m1 tonight. 114mm at f3.9. I am not overly impressed, but here it is.

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#44 Atl

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Posted 01 April 2013 - 12:59 AM

...and Flame nebula...

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#45 mclewis1

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Posted 01 April 2013 - 09:58 AM

I'm pretty impressed ... I have to keep reminding myself that you are working with only 128x/2.1s integration times.

Since you obviously have good dark skies you might try pushing the f ratio down a bit again. On big objects like the flame you could try your .5x focal reducer, perhaps with some relaxed spacing (little closer to the sensor if possible). That way you won't need as much in focus travel (which will likely be the biggest obstacle to using the focal reducer with your new scope). Yeah there will be more background noise but a little tweaking of brightness and contrast on the display side will likely help.

The horizontal banding seen is likely electrical noise. What are you using for a power source for your Matrix video camera? What are you using to display/capture the images?

#46 Atl

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Posted 01 April 2013 - 04:32 PM

The power source is a 12V 1.5amp AC Adapter plugged to an outside outlet. The images are displayed onto a netbook running on battery. Any suggestions for reducing the electrical noise?

I do have dark skies for the most part. The city of Agua Prieta Mexico is about 6 miles away and it makes some light on the southern sky...but not too bad. I think I would be considered to be in a blue zone not far from green according to LP maps.

#47 mclewis1

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Posted 01 April 2013 - 05:31 PM

Electrical noise can be tough to troubleshoot. Grounding problems are the usual causes but so can be a failure of filtering capacitors in a power supply, another possible cause is bad or worn video cables (shielding failures). You have to work methodically, keep notes, and change one thing at a time.

I try and swap stuff wherever practical to see if that changes anything. Sometimes just re routing the power and video cables (keeping them more separate) can help.

Do you have a 12v battery or another AC 12v power supply that you could try with the camera?
A different video cable?
Do you have a cheap video monitor to try in place of the netbook?
Maybe a different USB frame grabber?

You can sometimes try flipping a two prong plug in an AC outlet around (if it doesn't have a wide and narrow prong which "keys" the plug to the socket) to change the grounding.

While quite rare a tougher cause to figure out is when there is something else running in your house (or your neighbour's) that is causing the electrical noise. Things like old fans and air conditioners (and compressors in fridges and freezers) are notoriously electrically noisy.

#48 Atl

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Posted 01 April 2013 - 06:34 PM

I've got extra video cables...actually the one I am using are pretty old looking but I have new ones to try. I do have a 12v 1amp power supply that I could go with. the 12v 1.5amp it came with has a slightly frayed cable (wire exposed) that I taped. Thanks for the suggestions!






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