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Old 22-04-2021, 08:43 AM
Emuhead
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Detail vs Long Focal Length

Hi all,

I'm interested in going deeper into space. I have been imaging at 800mm f/4 with my newt and very happy with the results so far, but long story short I'm looking to take the next step closer and try for something in the 1400 - 2400mm focal length range. Have an EQ6R Pro mount, 20kg payload and like to keep it under 13kg. Also whilst I'd love a CDK, or Dream Astrograph (or even an EQ8R mount with a 1200mm f4 newt) I'm not quite ready for that financial leap. So have been looking at GSO RC8 & Edge HD 8 & 9.25.

What's imaging at f7, f8, f10 like in practical terms. I know that f7 is twice as fast as f10 for example but how does that translate into the subs. Does an f7 sub appear twice as bright as an f10 sub?

My other question is which scope of those, and please suggest anything else I might have missed, would give the best image quality? I have been spending some time on Astrobin and looking at a selection of images from these scopes and they somehow seem less detailed & more blurry than id be happy with. To clarify it's almost like using digital zoom on a photo rather than optical zoom, it's like the target has increased in size but not increased in the same relative amount of detail as you might expect.

Is detail at that aperture and weight possible at long focal lengths or am I looking for something that is beyond what is technically achievable? As an amateur astrophotographer without limitless funds (*should have bought that Bitcoin), should I be realistic and just be content with detail at 1200mm or less?

Would appreciate people's thoughts on this.

Last edited by Emuhead; 22-04-2021 at 08:53 AM.
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Old 22-04-2021, 08:51 AM
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multiweb (Marc)
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Aperture will give you more light gathering and resolution. Focal length only gives you magnification.
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Old 22-04-2021, 09:59 AM
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Quote:
Originally Posted by Emuhead View Post
should I be realistic and just be content with detail at 1200mm or less?

Yes you should.
The detail is decided by the aperture but
is in practical terms controlled by "the seeing".
Your 200mm aperture is giving you a Rayleigh Limit of .69 arc seconds.
https://astronomy.tools/calculators/...e_capabilities
I know that in Melbourne the best seeing I ever measured -
by using the FWHM of stars was 2.8 arc seconds.
That is well short of the 0.69 arc seconds capability of your scope.
So - we are limited by seeing not by focal length.


cheers
Allan
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Old 22-04-2021, 10:43 AM
Emuhead
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Appreciate the responses.

The penny dropped finally on the seeing limit, that FWHM example really helped. So in Melbourne, even if i could get down to 2 arc seconds i would only need a fairly small aperture (70mm) to max out the detail level. Then it's down to f-ratio to determine the magnification & brightness of that image (given same sub exposure time).

So anything greater than about 70mm of aperture isn't providing more detail, it's simply allowing for a brighter & more magnified image potentially.

Or have i missed a few things? Just trying to find a scope that will give me highly detailed up-close images that'll suit my mount.

Last edited by Emuhead; 22-04-2021 at 11:13 AM.
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Old 22-04-2021, 11:43 AM
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For what my 2c is worth, I had an RC8 (FL 1500mm) and it was challenging to collimate, and fairly slow at f8. Paul (Discoduck) on the other hand gets consistently spectacular images from dark sites with his finely tuned RC8.

My former f4 10" newt on the other hand was a delightful light bucket that really sucked in those photons! With the paracoor the FL was 1148mm.
Contributors here on IIS like Lee Borsboom have done spectacular galaxy images with a beast such as this, but under darker skies.

Kevin (Kosborn) uses a 10" f5 Newt very well on an EQ6 too, his FL is a tad longer at 1270mm.

So yes, seeing is a big factor, and a road trip to LMDSS can help with that!
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Old 22-04-2021, 11:51 AM
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Quote:
Originally Posted by Emuhead View Post
Appreciate the responses.

The penny dropped finally on the seeing limit, that FWHM example really helped. So in Melbourne, even if i could get down to 2 arc seconds i would only need a fairly small aperture (70mm) to max out the detail level. Then it's down to f-ratio to determine the magnification & brightness of that image (given same sub exposure time).

So anything greater than about 70mm of aperture isn't providing more detail, it's simply allowing for a brighter & more magnified image potentially.

Or have i missed a few things? Just trying to find a scope that will give me highly detailed up-close images that'll suit my mount.



There is also the quality of the telescope to consider.
A Newt. with a small diagonal on its camera sensor
will work well but for a large frame camera - the small
spot size of say a Ritchey–Chrétien
will give smaller star sizes at the edges of the frame.
Refractors have a better contrast ratio but ones that have a high Strehl ratio
are very expensive and they don't have the light collecting power of a large Newt.


You also need to consider the application of Drizzle in stacking
and that will depend on your arc second per pixel count.
http://www.iceinspace.com.au/forum/s...d.php?t=158389


and here:
http://www.wilmslowastro.com/software/formulae.htm




cheers
Allan
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Old 22-04-2021, 07:30 PM
Startrek (Martin)
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I use an 8” f5 newt with focal length of 1000mm
With the right camera , good conditions , and some creative processing you can image fairly “deep” without going to the next step with long focal lengths , expensive mounts , expensive scopes etc... plus a requirement to guide a lot tighter than you would normal be comfortable with
Here’s some “Zoomed in images” of the outer arm of Carina nebula , Rosette nebula , Centaurus A Galaxy and Sombrero Galaxy showing a fair bit of detail and resolution at 1000mm focal length ( images were cropped a bit to “zoom in” close )
My imaging camera ZWOASI2600MC
Subs no longer than 3 minute
Nebula imaged with L Extreme filter under full moon
Galaxies imaged with no filter during New moon ( dark )
Hope this demonstrates what is achievable at 1000mm FL
Cheers
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Old 22-04-2021, 09:06 PM
Emuhead
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Thanks all for the helpful & informative responses with calculators & examples too. Much appreciated. I've only now got more questions though.

Andy I checked out both of the astrophotographers noted and their work is very impressive indeed. I don't think I'll have the patience that Discoduck has though (50hrs), and I'm not game like Kosborne to go a 10" mirror on an eq6r pro, at least not yet while im still playing nice with the mount.

Where does that leave me in terms of going deeper and getting the most detail possible.. 8" f5, or maybe even the elongated 8" f6, which is where the next question comes in..

3 scopes:
8" f4, 8" f5, 8" f6.
Given aperture determines resolution.. do they all provide the same level of detail ultimately? That is assuming the sensors pixel scale stays between 1 & 2 arc sec/pixel via binning & drizzling where needed, and just imaging longer on the slow scope vs the fast scope to ensure the overall SNR remains consistent across all 3. If I did all that, once in post if i cropped all 3 images to the same FOV would they all basically look identical or would there be more fine details on the f6 vs f4 image?

(Last question: Or based on actual experience with scope types, do refractors arguably provide the sharpest detail available and should I head in that direction..?)

Last edited by Emuhead; 22-04-2021 at 09:34 PM.
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Old 22-04-2021, 09:18 PM
Startrek (Martin)
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Some more information for your perusal

My example images were 3 minute subs , maximum 2 to 2.5 hours of integration on each object
No darks needed
Never take flats or bias
No camera binning, only software binned in Startools at 50%
My mount is an EQ6-R pro
PHD2 Guiding around 0.70 to 0.80 total rms ( Dec 0.45 to 0.50 and Ra 0.60 to 0.65 )
My image scale is around 0.80 arc sec per pixel
Cheers
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Old 22-04-2021, 10:11 PM
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Quote:
Originally Posted by Emuhead View Post
Thanks all for the helpful & informative responses with calculators & examples too. Much appreciated. I've only now got more questions though.

Andy I checked out both of the astrophotographers noted and their work is very impressive indeed. I don't think I'll have the patience that Discoduck has though (50hrs), and I'm not game like Kosborne to go a 10" mirror on an eq6r pro, at least not yet while im still playing nice with the mount.

Where does that leave me in terms of going deeper and getting the most detail possible.. 8" f5, or maybe even the elongated 8" f6, which is where the next question comes in..

3 scopes:
8" f4, 8" f5, 8" f6.
Given aperture determines resolution.. do they all provide the same level of detail ultimately? That is assuming the sensors pixel scale stays between 1 & 2 arc sec/pixel via binning & drizzling where needed, and just imaging longer on the slow scope vs the fast scope to ensure the overall SNR remains consistent across all 3. If I did all that, once in post if i cropped all 3 images to the same FOV would they all basically look identical or would there be more fine details on the f6 vs f4 image?

(Last question: Or based on actual experience with scope types, do refractors arguably provide the sharpest detail available and should I head in that direction..?)



I think you need to read it again:

I am using a KAF8300 sensor.
It has 5.4 um pixels.

My arc-sec/pixel = 1.11
http://www.wilmslowastro.com/softwar...m#ARCSEC_PIXEL


Let's round that off to 1 arc second per pixel.
If my best seeing is 3 arc seconds per pixel then

that 1/3rd of the best seeing is about right in terms of Nyquist sampling.
see here again:
http://www.iceinspace.com.au/forum/s...=158389&page=2


Nyquist says minimum sampling is a factor 2 ( but 3 is better. )
https://astronomy.tools/calculators/ccd_suitability

OK so - 10" f4 or f5 or f6 won't make any real difference to the detail
give that the pixel size is matched correctly as per the above.
A 10" f4 will work better for imaging as it's a faster scope.
Speed is important otherwise dim areas will be very noisy
unless you have large numbers of stacked frames.
Stacking with 3 x drizzle will improve everything but
not many people bother doing it.
It takes longer and is harder to do.



cheers
Allan
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Old 22-04-2021, 10:53 PM
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Atmos (Colin)
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This really is a bit of a minefield. One thing worth mentioning is that there can be some incredible seeing in Melbourne, I'm talking top shelf stuff. Take this image of 47 Tuc.
Full Resolution

It has been taken with a Mewlon 250 at F/10 with an ASI094 which results in 0.4"/pixel. This is about 40 minutes of 120s exposures, the final stack has a FWHM of ~1.4" with the best single exposure being closer to 1.1" seeing. That's about 3-4 pixels at 0.4"/pixel. Now 0.4"/pixel is a bit over the top but the best single sub I've managed from Melbourne as been 0.96" so that's a fraction under 1" seeing from my suburban backyard.

Don't get me wrong, I've had terrible nights of 4" seeing but my average is around 1.5-2".
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Old 23-04-2021, 12:08 AM
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Quote:
Originally Posted by Atmos View Post
This really is a bit of a minefield. One thing worth mentioning is that there can be some incredible seeing in Melbourne, I'm talking top shelf stuff. Take this image of 47 Tuc.
Full Resolution

It has been taken with a Mewlon 250 at F/10 with an ASI094 which results in 0.4"/pixel. This is about 40 minutes of 120s exposures, the final stack has a FWHM of ~1.4" with the best single exposure being closer to 1.1" seeing. That's about 3-4 pixels at 0.4"/pixel. Now 0.4"/pixel is a bit over the top but the best single sub I've managed from Melbourne as been 0.96" so that's a fraction under 1" seeing from my suburban backyard.

Don't get me wrong, I've had terrible nights of 4" seeing but my average is around 1.5-2".



Hi Colin,
I've never heard of seeing that good in Melbourne.
I think the best I ever measured from any single frame
was 2.8 arc seconds FWHM in Ha.

I have never measured a final stack - only individual frames.
Are you sure you had the correct information loaded
into Maxim DL or whatever you were using to make the measurement?
On some nights the seeing was as bad as 6 arc seconds FWHM.

The best seeing I've ever heard of in Melbourne
before tonight was at Mt Macedon -
close to Melbourne -
FWHM of 2.2 arc seconds.


cheers
Allan
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Old 23-04-2021, 07:54 AM
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Quote:
Originally Posted by Andy01 View Post
For what my 2c is worth, I had an RC8 (FL 1500mm) and it was challenging to collimate, and fairly slow at f8. Paul (Discoduck) on the other hand gets consistently spectacular images from dark sites with his finely tuned RC8.
Very kind words, Andy. Just a note that I use a focal reducer on my RC8, so run it at 1200mm (f6). At that focal length, my camera gives me 0.65" per pixel. I'd suggest you want to ideally get at least 2 * sqrt(2) pixels across your seeing distance. So 2" of seeing would need 0.7".

The RC8 was a serious challenge to collimate. I used a combination of a few techniques, but none of them I think guaranteed to converge! So some luck involved still!

Quote:
Originally Posted by Emuhead View Post
Andy I checked out both of the astrophotographers noted and their work is very impressive indeed. I don't think I'll have the patience that Discoduck has though (50hrs), and I'm not game like Kosborne to go a 10" mirror on an eq6r pro, at least not yet while im still playing nice with the mount.
Re the patience, LOL yeah that is needed for that. But that is for imaging from the suburbs - for which I need at least 5x more subs in my light polluted skies. From dark sky sites, 10-15 hours or so on an image seems to provide a good set of data - depending on the object of course.
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Old 23-04-2021, 11:35 AM
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Labouring this point.. so an 8" mirror can only ever give the same resolution, regardless of whether it's an f4 newt, or an f8 RC, or an f10 SCT, or any of the above with reducers or paracorrs?

The magnified result (determined by the f ratio) will only just increase the size (much like an eyepiece does) without providing any more or any less detail, so it wouldn't benefit me to look at any other 8" scope & expect more detail (unless I've been incorrectly sampling but with 3.8 micron pixels (around 1 arcsec/pixel) i think there's not much to be gained by going with smaller pixels) so i could consider the case closed.

Thanks all

Last edited by Emuhead; 23-04-2021 at 12:08 PM.
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Old 23-04-2021, 01:02 PM
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Quote:
Originally Posted by Emuhead View Post
Labouring this point.. so an 8" mirror can only ever give the same resolution, regardless of whether it's an f4 newt, or an f8 RC, or an f10 SCT, or any of the above with reducers or paracorrs?

The magnified result (determined by the f ratio) will only just increase the size (much like an eyepiece does) without providing any more or any less detail, so it wouldn't benefit me to look at any other 8" scope & expect more detail (unless I've been incorrectly sampling but with 3.8 micron pixels (around 1 arcsec/pixel) i think there's not much to be gained by going with smaller pixels) so i could consider the case closed.

Thanks all



3.8 um pixels will be slightly oversampling but that's OK.
It all boils down to what kind of seeing you'll get in Melbourne -
it's normally 3 to 6 arc seconds -

or consider some other place if you try imaging there.
Given that Mount Macedon can sometimes give
2.2 arc seconds of seeing then the longer focal length scopes would
give you some more detail or resolution.


Using a 10" f4 Newt. -

some of my examples from Melbourne are here:
https://www.flickr.com/photos/247194...in/photostream


and a crop using 3 x drizzle
https://www.flickr.com/photos/247194...in/photostream




cheers
Allan
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Old 01-05-2021, 10:31 AM
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Quote:
Originally Posted by alpal View Post
Hi Colin,
I've never heard of seeing that good in Melbourne.
I think the best I ever measured from any single frame
was 2.8 arc seconds FWHM in Ha.

I have never measured a final stack - only individual frames.
Are you sure you had the correct information loaded
into Maxim DL or whatever you were using to make the measurement?
On some nights the seeing was as bad as 6 arc seconds FWHM.

The best seeing I've ever heard of in Melbourne
before tonight was at Mt Macedon -
close to Melbourne -
FWHM of 2.2 arc seconds.


cheers
Allan
I make my decision as to whether I continue imaging early on in the night depending on how the auto focus is coming along but also how MaximDL deems the FWHM. When I do my proper measurements though it's all done in PI and done manually where I select at least a dozen stars. I know my pixel scale (0.4") and if a bunch of stars measure 3 pixels then I know I have 1.2" seeing.

Here are three crops. NGC 288 measures at 1.48" first off. Next is NGC 6752, a forgotten globular near NGC 253, it measures down to 1.42". Lastly is a crop of Eta Carina, actually cannot remember exactly what it was but it wasn't as low as those globulars; might have been around 1.8-1.9".
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Old 01-05-2021, 03:11 PM
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Originally Posted by Atmos View Post
I make my decision as to whether I continue imaging early on in the night depending on how the auto focus is coming along but also how MaximDL deems the FWHM. When I do my proper measurements though it's all done in PI and done manually where I select at least a dozen stars. I know my pixel scale (0.4") and if a bunch of stars measure 3 pixels then I know I have 1.2" seeing.

Here are three crops. NGC 288 measures at 1.48" first off. Next is NGC 6752, a forgotten globular near NGC 253, it measures down to 1.42". Lastly is a crop of Eta Carina, actually cannot remember exactly what it was but it wasn't as low as those globulars; might have been around 1.8-1.9".



Fair enough Colin.
As a standard candle.

Your Finger of God in Eta Carinae is amazing.
That certainly looks better than 2.8 arc seconds
which is all I ever got on that.


Your make my pics look blurry now - LOL


cheers
Allan
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Old 02-05-2021, 06:12 PM
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Quote:
Originally Posted by alpal View Post
Hi Colin,
I've never heard of seeing that good in Melbourne.
I think the best I ever measured from any single frame
was 2.8 arc seconds FWHM in Ha.

I have never measured a final stack - only individual frames.
Are you sure you had the correct information loaded
into Maxim DL or whatever you were using to make the measurement?
On some nights the seeing was as bad as 6 arc seconds FWHM.

The best seeing I've ever heard of in Melbourne
before tonight was at Mt Macedon -
close to Melbourne -
FWHM of 2.2 arc seconds.


cheers
Allan
I’m just over the next range (Cobaw Range) from Mt Macedon (approx 15km) and can confirm we get some cracking seeing here. 2.2 wouldn’t be deemed the best setting we get, definitely gets better than that..... although as others have said, some nights it’s best to stay inside.
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Old 03-05-2021, 06:54 AM
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Quote:
I’m just over the next range (Cobaw Range) from Mt Macedon (approx 15km) and can confirm we get some cracking seeing here. 2.2 wouldn’t be deemed the best setting we get, definitely gets better than that..... although as others have said, some nights it’s best to stay inside.



Wow - that's all news to me.
Looking at planets I've seen very clear images
from Melbourne but only fleeting for 1 second at a time.


cheers
Allan
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Old 03-05-2021, 10:53 AM
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Quote:
Originally Posted by Emuhead View Post

3 scopes:
8" f4, 8" f5, 8" f6.
Given aperture determines resolution.. do they all provide the same level of detail ultimately? That is assuming the sensors pixel scale stays between 1 & 2 arc sec/pixel via binning & drizzling where needed, and just imaging longer on the slow scope vs the fast scope to ensure the overall SNR remains consistent across all 3. If I did all that, once in post if i cropped all 3 images to the same FOV would they all basically look identical or would there be more fine details on the f6 vs f4 image?
A bit late to the discussion. Everything said so far matches either my understanding or experience.

However, I think there could be some difference in data quality between 8" f/4 and say 8" f/6. I'm not talking about quite obvious differences in SNR, but spot size. This is to do with the size of the secondary and also possibly some difference in field curvatures and the degree of coma across the frame (coma corrector reduces coma but I do not think it does eliminate it entirely, particularly further away from the optical axis). Yet these potential optical advantages could be offset by a more mechanically demanding accurate guiding of a longer tube, so in the real world it could be actually detrimental to data quality to use a slower Newtonian.

I feel that it is common to chase higher SNR with ever faster f-ratios, but having settled on the aperture size, perhaps we could be sacrificing a bit of detail with the faster optics. Or perhaps there is a potential for greater gains in resolution in images from a more precise guiding, be it a better mount or adaptive optics
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