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Old 22-12-2014, 09:11 AM
LewisM
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Galaxy imaging

Is f/7.5 (106mm objective) going to be sufficient focal length for adequate galaxy imaging? OR should I bump it to f/8 with a 1.6x extender (vs the current ED 1.5x extender)
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Old 22-12-2014, 09:38 AM
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Either way that's pretty short FL so you'd want a small pixel camera. With a ICX-694 sensor the image scale would be 1.15 arcsec/pixel at f/7.5 and 1.08 arcsec/pixel at f/8. That would be adequate for larger galaxies.

Cheers,
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Old 22-12-2014, 09:45 AM
LewisM
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Using an FLI KAF 8300 chipped cam, so 5.4 Ám pixels. Adequate?
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Old 22-12-2014, 09:49 AM
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Quote:
Originally Posted by LewisM View Post
Using an FLI KAF 8300 chipped cam, so 5.4 Ám pixels. Adequate?
1.31 arcsec/pixel at f/7.5 and 1.4 arcsec/pixel at f/8. Would be a decent FOV for showing galaxies in context but not in close up detail. The KAF-8300 is a bit noisy so you'd need to go for longer exposures assuming your skies are dark enough to support that.
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Old 22-12-2014, 09:56 AM
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You'll need the Tak 1.6X Q extender or a barlow to capture most galaxies at that focal length.

Most galaxies usually require about 1200mm focal length with an 8300 chip. There are a few that are large like NGC253 that don't require as much.

Greg.
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Old 22-12-2014, 10:17 AM
LewisM
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I have the ED 1.5 extender, might spring a Xmas present for myself with the 1.6... or a TV or AP 2x Barlow...
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Old 22-12-2014, 11:06 AM
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Originally Posted by LewisM View Post
I have the ED 1.5 extender, might spring a Xmas present for myself with the 1.6... or a TV or AP 2x Barlow...
Just remember that you're extending the required imaging time as well as the focal length. To get high res views of dim galaxies there's no substitute for aperture.
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Old 22-12-2014, 07:48 PM
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I took this M104 with a TEC180 (1260mm focal length) and a 1.6X Q extender and a Microline 8300 camera:

http://www.pbase.com/gregbradley/image/123997685

Greg.
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Old 22-12-2014, 10:26 PM
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Lewis,

Where do you intend to be imaging from ?
. . . what is the light pollution like, what is the seeing like ?

The questions really are - what image scale is going to suit the galaxies you are interested in ? and therefore what are the seeing conditions you expect to be able to image in and
what sort of maximum exposure time is your light pollution going to allow you to capture with your given camera (well depth and noise) and will that be sufficient to capture the galaxies that you are interested in

Obviously imaging from within a light polluted capital city with the low well depth (and moderately noisy) chip like the KAF8300 in seeing that is rarely better than 1.7 arc secs is going to be limiting for getting any good detail in faint distant galaxies.

An ideal galaxy imaging setup is likely to have an image scale around 0.5 arc secs per pixel, probably have a CCD well depth of 50,000 to 150,000 e and low noise in a dark sky which supports seeing conditions that roughly matches your image scale - or an awful lot of images and lots of deconvolution !

Try using CCD calc as an intial test to see how the frame captures the galaxies you are interested in.

My guess is that the large galaxies will be OK with your rig but the small ones will be mostly unobtainable at any sort of quality.
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Old 23-12-2014, 08:54 PM
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Lewis,
Ask Peter Ward or Mike Sidonio why they both own premium quality refractors but prefer to use something else for imaging galaxies.
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Old 23-12-2014, 08:56 PM
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I know Clive, just hoping it can be done with reasonable results with a refractor of intermediate focal length. I am guessing mediocre is the word.

No matter, nebulae will do.
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Old 24-12-2014, 06:31 AM
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Herein lies the problem of using just one scope. No scope is ideal for all uses it seems. Some are more flexible than others. They tend to sit in either wide field, medium field or narrow field.

Wide field tend to be your 4 inch type refractors, medium field large refractors, Newts and narrow field are your RCs Dall Kirkhams.

Some APOs have multiple accessories to allow a bit of each world and so can Newts. RCs though tend to only be long focal length.

The Ceravolo 12 inch astrograph is an attempt to be both wide field and narrow field. AP Honders probably is another one.

Its better to have larger aperture and a faster F ratio like F5 or better and use size of sensors to gain the desired image scale. For example 12 inch F3.8 should be a fairly wide field scope and Mike uses both 16803 camera (gives a wide view) and a 694 chipped camera (gives a galaxy type field of view). That way you don't even need barlows etc and retain the F3.8.

So 900 to 1260mm focal length and 2 sized sensors is quite a flexible approach for fairly wide (not FSQ106 wide though) and for galaxies.

In other words, concentrate on aperture and F ratio at the highest level of optics you can afford.

Greg.
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Old 24-12-2014, 08:51 AM
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Quote:
Originally Posted by gregbradley View Post
Herein lies the problem of using just one scope. No scope is ideal for all uses it seems. Some are more flexible than others. They tend to sit in either wide field, medium field or narrow field.

Wide field tend to be your 4 inch type refractors, medium field large refractors, Newts and narrow field are your RCs Dall Kirkhams.

Some APOs have multiple accessories to allow a bit of each world and so can Newts. RCs though tend to only be long focal length.

The Ceravolo 12 inch astrograph is an attempt to be both wide field and narrow field. AP Honders probably is another one.

Its better to have larger aperture and a faster F ratio like F5 or better and use size of sensors to gain the desired image scale. For example 12 inch F3.8 should be a fairly wide field scope and Mike uses both 16803 camera (gives a wide view) and a 694 chipped camera (gives a galaxy type field of view). That way you don't even need barlows etc and retain the F3.8.

So 900 to 1260mm focal length and 2 sized sensors is quite a flexible approach for fairly wide (not FSQ106 wide though) and for galaxies.

In other words, concentrate on aperture and F ratio at the highest level of optics you can afford.

Greg.

Hi Greg,
Surely if Mike used the ASA 1.8 x barlow here:

http://www.teleskop-express.de/shop/...low--1-8x.html

He'd get more detail from small galaxys?

cheers
Allan
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Old 24-12-2014, 09:58 AM
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As kind of noted above, if you are pushing to 1 arcsec per pixel or lower, which is required for detail on smaller galaxies, then you typically need to 'beat the seeing'. A useful way of doing this is deconvolution, but for this to be most effective, it needs really low-noise data (so lots of subs and/or long ones - ideally subs that get you well above the noise floor, but not too long so as to introduce tracking problems etc.). There is no bootstrap here!

Drizzling can also help, especially if you have a larger image scale and yet your target object is relatively small. You can do this by cropping the frame down to focus just on your target, and then drizzling by 2-3 times. This requires lots of subframes (16+, more the better) and dithering. But it can be effective.

This is an example of NGC253 that I imaged at an ultra-short 384mm FL, using a CCD with 8.3 micron pixels (so the image scale was >4 arcsec/pix). Yet when 3x drizzled, it looked pretty decent!
http://www.astrobin.com/118924/
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Click for full-size image (NGC253_384mmFL.jpg)
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Old 24-12-2014, 10:01 AM
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Quote:
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I know Clive, just hoping it can be done with reasonable results with a refractor of intermediate focal length. I am guessing mediocre is the word.

No matter, nebulae will do.
even with only 100mm aperture, the resolution will mostly be determined by the atmosphere and will typically be about 2 arc seconds. To sample this properly, you need slightly smaller than 1 arcsec pixels, so you need to choose the focal length to do this (or use workarounds to get to the same sampling). If you do it right, the resolution will be almost as good as any other system.

However, the big disadvantage of a small aperture (as already pointed out) is that you have reduced sensitivity. For example, if you run your 100mm system at 1.2m fl, you will need to spend 9x as long getting data as you would if you had a 300mm aperture at 1.2m fl. Since galaxies are fairly dim, expect to spend maybe 100 hours to get the sorts of SNRs that Mike gets in 10 hours. In the end, it all comes down to how many photons get into the hole up front and there is no substitute for aperture.
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Old 24-12-2014, 10:19 AM
LewisM
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Been thinking of going with another VC200L again, just for galaxies.

Thanks all for the shake up - I thought it wouldbe pushing the friendship a touch to expect good detail from a 106mm refractor at f/7.5
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