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Focal length vs aperture

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#1 Paradoxdb3

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Posted 27 February 2020 - 09:36 AM

In astrophotography, which will result in more resolution? I know for visual, larger apertures have the ability to resolve more than a smaller aperture at the same focal length. Is this also true for photography?

I am using my Tamron 150-600mm lens and Sony a6500 on things like globular star clusters, and the core of the cluster usually ends up looking like a bit of a blob of stars, rather than thousands of tiny stars jammed together. If I want to make the core more defined, would I use my Celestron C5 at 1250mm, for example, to get closer in and see more stars, or would it be better to use an Orion 6" astrograph at 610mm? Thanks!

#2 ManuelJ

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Posted 27 February 2020 - 09:54 AM

Resolution is a factor of:

 

1) Aperture: depending on your seeing conditions, aperture reveals finer details. In regular 2" FWHM skies (better than most of us), a 6" is seeing limited.

2) Atmosphere: FWHM limits your resolution

3) Optical correction: aberrations limit your resolution

4) Image scale: Ideally, for the highest resolution, you need to image at about FWHM/2 - FWHM/3

 

So, for a 2.5" FWHM skies, imaging at 1" with 6" good corrected optics is as best as you can get if we are not talking about speed, only resolution. To gain speed, keep the image scale fixed and increase either in aperture, focal ratio or bigger pixels.


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#3 Garyth64

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Posted 27 February 2020 - 09:58 AM

Let's say 6" scope can resolve a certain double star, eg,  Going to a smaller aperture with a longer f.l. doesn't mean it will resolve that double star.  Increasing the f.l., or getting "closer" doesn't increase the resolving power.



#4 Paradoxdb3

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Posted 27 February 2020 - 10:00 AM

I'll have to take some moments (and research) to understand this. I appreciate your response, thank you!

#5 ngc7319_20

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Posted 27 February 2020 - 10:02 AM

It will depend on the pixel size of the sensor, and the focal length of the scope, if you want to fully resolve the cluster in say 1 arcsecond seeing.  So you have about 4 micron pixels in that camera.  To get Nyquist sampling you will want 1 arcsecond to map into 8 microns at the sensor.  So that implies a focal length of (8 microns)*57*60*60/1000/(1 arcsec)=1600mm.  So I would go with the C5.

 

You will also want to get short exposures, and do a so-called "high dynamic range" or HDR combination of the different exposure time images.


Edited by ngc7319_20, 27 February 2020 - 10:06 AM.


#6 Paradoxdb3

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Posted 27 February 2020 - 10:03 AM

Let's say 6" scope can resolve a certain double star, eg, Going to a smaller aperture with a longer f.l. doesn't mean it will resolve that double star. Increasing the f.l., or getting "closer" doesn't increase the resolving power.


Thank you for the response. I was kind of wondering about this.

#7 Paradoxdb3

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Posted 27 February 2020 - 10:05 AM

It will depend on the pixel size of the sensor, and the focal length of the scope, if you want to fully resolve the cluster in say 1 arcsecond seeing. So you have about 4 micron pixels. To get Nyquist sampling you will want 1 arcsecond to map into 8 microns at the sensor. So that implies a focal length of 1600mm. So I would go with the C5.

You will also want to get short exposures, and do a so-called "high dynamic range" or HDR combination of the different exposure time images.


I have a C5 and a 6" Mak. I have had good success with the Mak on targets like M109, so I may actually try it on a star cluster. Thank you.

#8 bobzeq25

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Posted 27 February 2020 - 11:09 AM

It's really complicated, many things are important in getting sharp images.  More focal length can give more resolution, but for many beginners, that just doesn't work.  Key issues are how good your mount is, and how good your autoguiding is.  Longer focal lengths magnify tracking errors, you'll get little benefit from long focal lengths if you don't address tracking first.  Also, slower scopes require longer subexposures, also making tracking more important.

 

A factor some beginners underestimate is the importance of good signal to noise ratio in getting good images.  In general shorter focal lengths give better snr, although that too is complicated.

 

Most beginners underestimate the importance of the mount.  It's more important than the scope.  This is _not_ like visual.

 

Suggestions.  Start short, and get that working well.  600mm before 1250.  Get better snr with more total imaging time.  Work on autoguiding well.

 

Get this book.  There's far more to learn than you'll get from short posts here.

 

https://www.amazon.c...d/dp/0999470906


Edited by bobzeq25, 27 February 2020 - 11:15 AM.


#9 Paradoxdb3

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Posted 27 February 2020 - 11:18 AM

It's really complicated, many things are important in getting sharp images. More focal length can give more resolution, but for many beginners, that doesn't work. Key issues are how good your mount is, and how good your autoguiding is.

A factor some beginners underestimate is the importance of good signal to noise ratio in getting good images. In general shorter focal lengths give better snr, although that too is complicated.

Suggestions. Start short, and get that working well. Get better snr with more total imaging time. Work on autoguiding well.

Get this book. There's far more to learn than you'll get from short posts here.

https://www.amazon.c...d/dp/0999470906


My mount is sturdy enough, but my issue probably comes down to autoguiding, as I do not have an autoguider. I rely on shorter exposures (30s) to keep stars round. With my Tamron lens at 300mm, and in calm nights, I should be able to get up to 2 minute exposures. But I want to try my 6" Mak on my new mount (EQ6-R). I'm actually hoping for 60s subs on a very calm night, but I don't know for sure. 30s for sure.

#10 Paradoxdb3

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Posted 27 February 2020 - 11:20 AM

Also, is that the Flaming Star Nebula on the cover of that book? Funny thing, I just tried imaging it, and it turned out HORRIBLE! lol

#11 bobzeq25

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Posted 27 February 2020 - 11:37 AM

My mount is sturdy enough, but my issue probably comes down to autoguiding, as I do not have an autoguider. I rely on shorter exposures (30s) to keep stars round. With my Tamron lens at 300mm, and in calm nights, I should be able to get up to 2 minute exposures. But I want to try my 6" Mak on my new mount (EQ6-R). I'm actually hoping for 60s subs on a very calm night, but I don't know for sure. 30s for sure.

You will need to autoguide at that focal length.  Otherwise, little benefit from the big scope.  Tracking errors will just blur the image, you'll get little or no increase in resolution.  This is a (complicated) system.

 

Doing a lot of short subs might work, but then you'll get increased read noise.  Complicated.

 

Also, is that the Flaming Star Nebula on the cover of that book? Funny thing, I just tried imaging it, and it turned out HORRIBLE! lol

Orion, through a camera lens.

 

Bottom line.  Start slow, do the right things (are you doing bias, flats, darks?), get better images.  Then is the time to try a big scope.  Otherwise you'll get the horrible images, and won't know what the problem is.  Makes fixing it tough.  <smile>

 

There are excellent reasons why the overwhelming majority of imagers in your situation autoguide.
 


Edited by bobzeq25, 27 February 2020 - 11:46 AM.


#12 TOMDEY

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Posted 27 February 2020 - 11:44 AM

In summary, larger aperture is inherently capable of better resolution, in exact proportion to diameter. But, in order to actualize that, the other properties of your rig must also be favorable: stability, tracking, array finesse, aberrations, wavefront, seeing, etc. That is to say, on a decent night, a 6-inch scope can comfortably resolve an arc-sec; a 60mm absolutely cannot, ever.    Tom



#13 Paradoxdb3

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Posted 27 February 2020 - 12:39 PM

This was with my 6" Orion on my SkyView Pro. Of course, it may not be the same with star clusters.

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#14 bobzeq25

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Posted 27 February 2020 - 03:37 PM

This was with my 6" Orion on my SkyView Pro. Of course, it may not be the same with star clusters.

Nice start. 

 

But note the fairly low resolution and signal to noise ratio on the galaxy.  It's exactly what I'd expect at this stage in your career.

 

Here are the key lessons.

 

This is a complicated and difficult business.  It takes time, study, and attention to detail, to get good at it.  In five years, I've dragged myself up to a bit above average.  <smile>

 

This is a system.  One thing rarely drives the train.  If it does, it's far more likely to drive things down, rather than raise them up.

 

It's not all about equipment.  Rory McIlroy's clubs will not put you on the PGA tour.  <smile>

 

The following image is not intended to diminish your fine effort.  It's intended to illustrate the key points.

 

Check out this M106 with a 3.2 inch, 510mm scope.  And, no doubt, a _very_ experienced and skillful imager.  32 darks.  10 flats. 20 bias.  Autoguided.  8 hours total imaging time.

 

https://www.astrobin.com/346542/

 

Keep at it.  The suggestions in #11 remain.  Start out short focal length.  Do longer total imaging times.  Do bias, flats, darks.  Autoguide.

 

This is a lot of fun.  It's not easy, part of the reason why it's fun.  <smile>


Edited by bobzeq25, 27 February 2020 - 04:16 PM.

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#15 Madratter

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Posted 27 February 2020 - 04:11 PM

In astrophotography, which will result in more resolution? I know for visual, larger apertures have the ability to resolve more than a smaller aperture at the same focal length. Is this also true for photography?

I am using my Tamron 150-600mm lens and Sony a6500 on things like globular star clusters, and the core of the cluster usually ends up looking like a bit of a blob of stars, rather than thousands of tiny stars jammed together. If I want to make the core more defined, would I use my Celestron C5 at 1250mm, for example, to get closer in and see more stars, or would it be better to use an Orion 6" astrograph at 610mm? Thanks!

I would use the telescope with the higher image quality, whether it  was the 5" or the 6". Either is capable of resolving the stars BUT you will need to do everything right. Globular clusters are an excellent canary, in the sense they will show problems with your system. You need good seeing, good optics, good focus, good guiding, and and appropriate image scale to really do them justice. It doesn't take a lot of slop in any of those things to end up with mush.


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#16 Paradoxdb3

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Posted 27 February 2020 - 04:30 PM

I would use the telescope with the higher image quality, whether it was the 5" or the 6". Either is capable of resolving the stars BUT you will need to do everything right. Globular clusters are an excellent canary, in the sense they will show problems with your system. You need good seeing, good optics, good focus, good guiding, and and appropriate image scale to really do them justice. It doesn't take a lot of slop in any of those things to end up with mush.


In my limited experience, the 150 Mak does a better job. It holds collimation really well, and makes stars look sharp. I'm going to play around with it tonight, actually.

#17 Paradoxdb3

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Posted 27 February 2020 - 04:37 PM

Nice start.

But note the fairly low resolution and signal to noise ratio on the galaxy. It's exactly what I'd expect at this stage in your career.

Here are the key lessons.

This is a complicated and difficult business. It takes time, study, and attention to detail, to get good at it. In five years, I've dragged myself up to a bit above average. <smile>

This is a system. One thing rarely drives the train. If it does, it's far more likely to drive things down, rather than raise them up.

It's not all about equipment. Rory McIlroy's clubs will not put you on the PGA tour. <smile>

The following image is not intended to diminish your fine effort. It's intended to illustrate the key points.

Check out this M106 with a 3.2 inch, 510mm scope. And, no doubt, a _very_ experienced and skillful imager. 32 darks. 10 flats. 20 bias. Autoguided. 8 hours total imaging time.

https://www.astrobin.com/346542/

Keep at it. The suggestions in #11 remain. Start out short focal length. Do longer total imaging times. Do bias, flats, darks. Autoguide.

This is a lot of fun. It's not easy, part of the reason why it's fun. <smile>


That is too funny! I was planning on shooting M106 tonight! I'm still going to shoot it, weather permitting. My M109 image above was three hours of integration time at 30s exposures, ISO 10000, I believe. The Orion scope has a focal ratio of f/12. I never go by my histogram, since I'm under Bortle 2 or 3 skies, so I expose just until I can faintly see the core of the galaxy in one of the subs. I'll probably drop the ISO down to 6400 for this one, and try to shoot for four hours. Clouds are forecast later in the night. I'm thinking the EQ6-R will handle the Mak better than my SkyView Pro did. Looking forward to trying this Mak on a star cluster.

#18 RJF-Astro

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Posted 27 February 2020 - 05:16 PM

That Mak is really good for solar system imaging and star targets, like globulars. It is not that good at dimmer DSO's, because it is rather slow at f/12. Compared to an f/6 scope, you will need to expose 4 times as long for the same result. If 1 hour wil get you a decent image on the f/6-scope, the f/12-scope will need 4 hours for the same result.

 

Your bortle 2-3 helps a lot, but I would seriously speed up if you are going to invest a lot of time in astrophotography. An f4 newton is pretty fast, but collimation is harder. A refractor will be easier to handle, but either more widefield or more expensive. Not an easy choice, but most go for the refractor first because there are enough difficulties to overcome anyway.

 

By the way, that Mak is a nice scope to do a bit of everything. WIth a planetary camera like the ASI290MM you can do great things with the moon. I have read that you need to insulate a 6" Mak though, because of thermal issues. Otherwise the air inside will not get stable enough. This happens faster compared to an SCT because of the thick corrector plate.


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#19 17.5Dob

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Posted 27 February 2020 - 07:37 PM

The best thing would to be to dump the "Tamzooka" and use anything else besides a "super zoom" camera lens...


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#20 BQ Octantis

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Posted 28 February 2020 - 01:46 AM

If I want to make the core more defined, would I use my Celestron C5 at 1250mm, for example, to get closer in and see more stars, or would it be better to use an Orion 6" astrograph at 610mm?

The astrograph is at f/4, whereas the C5 is at f/6.3 with a focal reducer/coma corrector; both are about the same magnification (at ~610mm vs. 630mm). The astrograph collects light 2.5× faster than the SCT and has 20% more resolution; the tradeoff is the diffraction spikes off the secondary mirror of the astrograph.

 

If you want globulars instead of blobulars, reduce your exposures; I can get decent results with 15 sec subs at f/6.

 

BQ


Edited by BQ Octantis, 28 February 2020 - 01:49 AM.

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#21 Paradoxdb3

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Posted 28 February 2020 - 02:16 AM

That Mak is really good for solar system imaging and star targets, like globulars. It is not that good at dimmer DSO's, because it is rather slow at f/12. Compared to an f/6 scope, you will need to expose 4 times as long for the same result. If 1 hour wil get you a decent image on the f/6-scope, the f/12-scope will need 4 hours for the same result.

Your bortle 2-3 helps a lot, but I would seriously speed up if you are going to invest a lot of time in astrophotography. An f4 newton is pretty fast, but collimation is harder. A refractor will be easier to handle, but either more widefield or more expensive. Not an easy choice, but most go for the refractor first because there are enough difficulties to overcome anyway.

By the way, that Mak is a nice scope to do a bit of everything. WIth a planetary camera like the ASI290MM you can do great things with the moon. I have read that you need to insulate a 6" Mak though, because of thermal issues. Otherwise the air inside will not get stable enough. This happens faster compared to an SCT because of the thick corrector plate.


I HAD the Mak insulated once, but I just didn't find it helped. I live in a cold climate, sometimes reaching as low as -45°C (Although I haven't been out past -35). Insulation just made things less efficient, I found.

By the way, I shot Venus this evening. 3 minutes at 30fps, and then I accidentally deleted the video file after Venus was too low. ARRRG! lol

#22 Paradoxdb3

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Posted 28 February 2020 - 02:18 AM

The astrograph is at f/4, whereas the C5 is at f/6.3 with a focal reducer/coma corrector; both are about the same magnification (at ~610mm vs. 630mm). The astrograph collects light 2.5× faster than the SCT and has 20% more resolution; the tradeoff is the diffraction spikes off the secondary mirror of the astrograph.

If you want globulars instead of blobulars, reduce your exposures; I can get decent results with 15 sec subs at f/6.

BQ


I'll try the shorter subs, thanks for the suggestion. I love those diffraction spikes. Considering the Orion 6" astrograph because of the f/4 and spikes. Lol

#23 bobzeq25

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Posted 28 February 2020 - 12:55 PM

I'll try the shorter subs, thanks for the suggestion. I love those diffraction spikes. Considering the Orion 6" astrograph because of the f/4 and spikes. Lol

When you're starting out faster optics are better.  F12 is awfully slow.


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#24 Paradoxdb3

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Posted 28 February 2020 - 01:40 PM

You're definitely not wrong, but I'm up for a challenge! Last night, I used the Orion 150mm Mak on M106. No guiding, just really good Polar Alignment and short subs. Nearly four hours on this target. I'm happy enough. Not pristine, but for an amateur like myself, I was thrilled to see this in the final image!

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#25 RJF-Astro

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Posted 28 February 2020 - 03:45 PM

Nice! Imagine what you could do with a faster scope ;). No, as long as the results keep you going, why look for something else?


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