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Analysis Paralysis

Big images of DSO

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Hi, I am trying to learn about AP. I understand that image size is dependent on pixel size on CCD and inversely proportional to focal length. Hope that's correct. I have. D7000 with 4.9 micron pixel size. Assuming I am limited to this CCD option, it seems to me that in order to increase image size I need longer focal length in order to reduce the arc seconds per pixel? So, let's use M51 as example. Am I right in thinking that with a 80mm ed with 0.87 FR, giving 522 mm FL, this would cover approx 10% of photo, but with say a 150 mm F12 mak, would cover about 35%? Assuming tracking was sufficient (heq5 with Rowan belt), would I get an good sharp image covering 1/3 of frame?

I use mak as example, because I'd like to use it for planets but have a option for bigger DSO image size.

I'm a newbie, so be gentle!!

Thanks

Mark

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An f12 mak would not be suitable for DSO AP. It's theoretically possible, and I have seen results, but I assume it would be very difficult as the high focal ratio would require very accurate tracking. Something like the ed80 would be much more forgiving.

I assume for an intermediate you could use a SCT for use on both planets and DSO's, using a focal reducer for DSO's. But I have heard about problems with that, dunno anything about it though.

HTH

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11 minutes ago, Galen Gilmore said:

An f12 mak would not be suitable for DSO AP. It's theoretically possible, and I have seen results, but I assume it would be very difficult as the high focal ratio would require very accurate tracking. Something like the ed80 would be much more forgiving.

I assume for an intermediate you could use a SCT for use on both planets and DSO's, using a focal reducer for DSO's. But I have heard about problems with that, dunno anything about it though.

HTH

Thank you. How would I obtain such very accurate tracking using the heq5 with belt mod? Can you think of an alternative way to get the equivalent larger image size without increasing focal ratio? I suppose I could zoom the image in Photoshop, but I suspect the detail would not be present to same degree.

Thanks

Mark

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I think there are two aspects thT are becoming confused.

Image scale i.e. how much detail you can theoretically see in the images is given by the formula : image scale(arcsec) = pixel size * 206.325/focal length

Field of view i.e. how much sky you can see in your image is given by the formula: FoV(degrees) = image scale * number of pixels/3600

To take your examples

Focal length = 650*0.87=567 mm,Pixel Size = 4.9 micron, Sensor = 4928 x 3264 pixels

Image Scale = 4.9*206.325/567 = 1.78 arcsec per pixel

FoV (horizontal) = 1.78 x 4928 = 2.4 degrees

FoV (vertical) = 1.78 x 3264 = 1.6 degrees

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11 minutes ago, catburglar said:

I think there are two aspects thT are becoming confused.

Image scale i.e. how much detail you can theoretically see in the images is given by the formula : image scale(arcsec) = pixel size * 206.325/focal length

Field of view i.e. how much sky you can see in your image is given by the formula: FoV(degrees) = image scale * number of pixels/3600

To take your examples

Focal length = 650*0.87=567 mm,Pixel Size = 4.9 micron, Sensor = 4928 x 3264 pixels

Image Scale = 4.9*206.325/567 = 1.78 arcsec per pixel

FoV (horizontal) = 1.78 x 4928 = 2.4 degrees

FoV (vertical) = 1.78 x 3264 = 1.6 degrees

Thank you. What I'm after is how much of my image would be covered by an object, let's take M51. Using 13' = 780" and 1.8 "/pixel, this would be 780/1.8 = approx 400 pixels. Assuming horizontal, this would be 400/4928*100% = approx 8% of horizontal image. So, on a 7.5 inch print it would be about 0.7 inch wide? 

Is this thinking correct?

Mark

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14 minutes ago, Analysis Paralysis said:

How would I obtain such very accurate tracking using the heq5

Hi. You'd need to guide the mount using an off axis guider and a very steady sky i.e. no twinkling stars... But definitely worth a try.

Maybe join a club so you can borrow the stuff you need and get help setting it up? 

HTH and good luck.

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48 minutes ago, Analysis Paralysis said:

Thank you. What I'm after is how much of my image would be covered by an object, let's take M51. Using 13' = 780" and 1.8 "/pixel, this would be 780/1.8 = approx 400 pixels. Assuming horizontal, this would be 400/4928*100% = approx 8% of horizontal image. So, on a 7.5 inch print it would be about 0.7 inch wide? 

Is this thinking correct?

Mark

Mark - that's correct.

Just noticed my answer was a bit wrong- because I used the wrong focal length for the ED80, but the rest of the cancellation was correct. 

I like to use https://dso-browser.com/ - it allows you to search for an object, set up your focal length and camera parameters and then gives a clear indication of how big something will look in your final image. It's a bit easier than in my view than the FoV Calculator @ 12DString

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4 hours ago, carastro said:

You might find this website useful:

http://www.12dstring.me.uk/fovcalc.php

Thank you. This may be what I'm after, but do you know how to see how much of the total image area is occupied by the image. I tried M51 but I can't tell if the whole black rectangle would be the image area or something else. Do you know?

Thank you

Mark

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11 hours ago, Analysis Paralysis said:

So, on a 7.5 inch print it would be about 0.7 inch wide? 

Here it is in 'real life'. So yeah. It looks about the right scale; a small blob.

80.JPG.3e011cd5054da6458974f87f11620b9f.JPG

And here it is with your Maksutov:

52.JPG.4bec6b7f8da3bc9cfa8e87da16289c4b.JPG

HTH.

Edited by alacant

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Quote

do you know how to see how much of the total image area is occupied by the image. I tried M51 but I can't tell if the whole black rectangle would be the image area or something else. Do you know?

I had the same question myself, but I think the whole black square is what your camera/scope combi will acquire but they don't always have the exact image to fit into the square, but you can at least see how big the main feature of the target will be.  Did you notice you can also add focal reducers and Barlows to the equation.

I use it all the time for planning imaging sessions so I know which camera and scope to use for each target.  

Carole 

Edited by carastro

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Essentially, you're looking to improve your resolution. As you correctly identified in your original post, what's important is arcseconds / pixel. That's determined by pixel size, focal length and the seeing conditions. Aperture can also limit your resolution - for example, the ED80 has a max resolution of ~1.43 arcseconds.

The long focal length of the F12 Mak you mention gives a resolution of 0.56"/pixel with your camera. I don't know where you're based, but that's a higher resolution than the average seeing conditions in the UK, so is not useful. Also, F/12 is very "slow", meaning you'll need very long exposures for DSOs like nebulae.

A 150mm F/5 Newtonian would be a good upgrade, giving you both faster optics and also 1.35"/pixel instead of your current 1.94.

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26 minutes ago, Shibby said:

Also, F/12 is very "slow", meaning you'll need very long exposures for DSOs like nebulae.

Hi. IIRC, on m51, the OP's f12 telescope will need less exposure time than his smaller telescope.

26 minutes ago, Shibby said:

higher resolution than the average seeing conditions in the UK

+1. That IMHO is the biggest factor. You may wait all year to get a night good enough.

Edited by alacant

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41 minutes ago, alacant said:

Hi. IIRC, on m51, the OP's f12 telescope will need less exposure time than his smaller telescope.

Ah yea, you're quite right actually; forget that! Scaled down 50%, a target with this would look the same as an f/6 with the same aperture, right? The FOV would be reduced, though.

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1 hour ago, Shibby said:

a target with this would look the same as an f/6 with the same aperture, right?

Hi. Not sure! I know that the bigger the aperture, the less time you need to image. It doesn't matter the f ratio. 

Just to clarify; I think the OP wants a larger m51 than his 80mm f6 will give. His f12 will give that if the sky is steady enough and the mount is in good condition.

HTH and clear skies.

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9 hours ago, Shibby said:

Essentially, you're looking to improve your resolution. As you correctly identified in your original post, what's important is arcseconds / pixel. That's determined by pixel size, focal length and the seeing conditions. Aperture can also limit your resolution - for example, the ED80 has a max resolution of ~1.43 arcseconds.

The long focal length of the F12 Mak you mention gives a resolution of 0.56"/pixel with your camera. I don't know where you're based, but that's a higher resolution than the average seeing conditions in the UK, so is not useful. Also, F/12 is very "slow", meaning you'll need very long exposures for DSOs like nebulae.

A 150mm F/5 Newtonian would be a good upgrade, giving you both faster optics and also 1.35"/pixel instead of your current 1.94.

Thank you. One thing I did notice when I was playing around with the camera options in the website another poster provided, was when I selected the atik 420 with 8 micron pixel size, the image size of M51 looked much bigger. Would you be able to advise on other camera options? Thanks for the 150 mm f5 suggestion. I was kind of hoping I could combine visual superiority of mak for planetary with bigger images of DSO. I have seen some very good images of M51 taken with 150mm Maks. I am based in UK. With a heq5 using Rowan belt mod and good seeing, I wonder if - with the minimum sub time- it would be possible to achieve similar results with imaging, whilst being able to use the Mak for visual- i.e. having my cake and eating it!

Thank you

Mark

Edited by Analysis Paralysis

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9 hours ago, alacant said:

Hi. IIRC, on m51, the OP's f12 telescope will need less exposure time than his smaller telescope.

+1. That IMHO is the biggest factor. You may wait all year to get a night good enough.

Hi. What would you say was a typical resolution limit for photography(arc seconds per pixel) in Midlands in UK? This would be useful in decision making. 

Thank you

 

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12 hours ago, alacant said:

Here it is in 'real life'. So yeah. It looks about the right scale; a small blob.

80.JPG.3e011cd5054da6458974f87f11620b9f.JPG

And here it is with your Maksutov:

52.JPG.4bec6b7f8da3bc9cfa8e87da16289c4b.JPG

HTH.

Hi,

Hope you don't mind my asking but how are you getting the FOV in Cart du Ciel, I've looked under all the settings and can't see it?

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12 minutes ago, Jkulin said:

getting the FOV in Cart du Ciel

Hi. Try:

Setup > Display > Finder rectangle > Compute

HTH

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3 minutes ago, alacant said:

Hi. Try:

Setup > Display > Finder rectangle > Compute

HTH

Thanks yep found it, so where are you getting the settings from, as I thought you might be inputting your scopes FOV? (Or maybe I am being thick)

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28 minutes ago, Jkulin said:

Hi,

Hope you don't mind my asking but how are you getting the FOV in Cart du Ciel, I've looked under all the settings and can't see it?

I will have to download this. It looks useful!

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56 minutes ago, Analysis Paralysis said:

resolution limit for photography(arc seconds per pixel) in Midlands

Phew, that's a tough one. Is there one I wonder? Probably best to go out and try it. FWIW, I quite often find the seeing gets better as the night progresses. Start with your refractor and if you see the guiding getting good enough, switch to your Mak. I often do that when trying to decide -striving similarly to yourself in getting a galaxy to look like a galaxy- whether it's worth going to the trouble of swapping to my 150 f8 @ 0.75spp. It gets to worth-a-try status when guiding RMS betters unity. HTH.

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If you examine (galaxy) images on astrobin, you'll notice that the better images often have a very high resolution (arcsecs/pixels < 1). It is generally better to oversample than to undersample if you want to go for high sharpness. I would recommend using a pixel scale that is better than your good seeing conditions. Remember that seeing, guiding and pixelscale all deteriorate the "ideal" image (whatever that is). If you go for high detail, you wouldn't want pixel scale to be the limiting factor. With moderation of course. There's equally no need to grossly oversample.

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