Imaging small galaxies with focal length under 800mm

[TheDadure]

Hi everyone,

I was just looking up some images of galaxies taken by amateurs,and they all seem to be taken with large telesopes.I mean,i know why that is.Except for andromeda and triangulum,galaxies are really small,so we are better off taking frames of close ups with large focal lengths.But since i don't have equipment like that,and i am getting my old SA tracker back in the middle of the galaxy season,i would like to see some of your results and opinions about this.Till this day i only captured pinwheel and andromeda galaxy,but i also had one failed attempt to capture whirlpool galaxy,but because of bad wheather,pollution and poor tracking it was very disappointing.

That's it from me,i would like to hear anything you think about this,any sort of advice,picture,thoughts... is welcome

Especially for imaging m51 with such a small rig,since it is my favourite dso and it always bugs me i haven't capture it yet

-Clear skies,David

 

[tooth_dr]

4 minutes ago, TheDadure said:

i would like to hear anything you think about this,any sort of advice,picture,thoughts... is welcome

 

Hi David

I just wanted to drop in some images I've taken at 500ish mm FL, with either a ED80 or 180ED, its very possible.   But a lot of them are pretty small, maybe just a case of choosing your targets sensibly.

 

 

All the best

Adam.

 

 

M51:

 

NGC3521:

 

M63:

 

Leo Triplets:

 

M81 M82 NGC3077

 

Widefield around NGC3718:

 

M106:

[TheDadure]

Thanks Adam,this is exactly what i was looking for,great shots,i don't really mind galaxies being smaller in wide shots,i was just wondering if you can pull out the detail from them,since that limits the number of pixels you are working with

-David

[vlaiv]

Nothing wrong with imaging smaller galaxies with focal length under 800mm.

It is not focal length alone that determines scale of object in the image.

There are two different metrics that are important - one is FOV and other is pixel scale.

Field of view is determined by focal length and camera sensor size. You can have small FOV on short focal length scope if you use very small sensor. You can have largish FOV on a long focal length scope if you use large sensor.

In any case, field of view, or proportion of field of view to object imaged gives how large object will appear when image is viewed on screen size. Screen size is size that we see here on forum or when you view your image with some image viewing application and you "fit image to screen". This is in fact often "scaled down" version of the image as image can have a few thousand pixels in height and width and still it is displayed on screen that has less pixels than that (like 1920x1080 or similar).

Pixel scale or sometimes referred to as sampling resolution is number that represents mapping of angular sizes in the sky to pixel. It depends on focal length and actual physical pixel size on sensor. This metric is important when you view image on 1:1 zoom - or often referred as 100% zoom setting. In this case one pixel of image is mapped to one pixel on screen - you can usually pan around images that are larger than screen in pixel size.

Back to the imaging. It is important to understand that in most cases either sky or mount is the limit in how high sampling rate you can get. With modern pixel sizes and 800mm focal length - you will probably be at sky limit. This means that you need mount good enough to support this.

Aim at about 1.2"-1.4" per pixel to be safe (or just use camera that you have with scope that you have), but make sure your mount can guide at half that in RMS.

[tooth_dr]

7 minutes ago, TheDadure said:

Thanks Adam,this is exactly what i was looking for,great shots,i don't really mind galaxies being smaller in wide shots,i was just wondering if you can pull out the detail from them,since that limits the number of pixels you are working with

-David

Someone with better processing skills will be able to do that

As Vlaiv points out it's imaging scale thats important.  These were taken with a cameras of 5.4 and 5.6um pixels, so giving a resolution of 2.2"/px.   I can guide comfortably at 1.1" RMS so that was a pretty good match

Edited April 23, 2020 by tooth_dr

[JamesF]

I have more data for this and need to brighten the colours up a bit as it looks quite dull compared with my laptop, but this is my M51 with an ED80:

And whilst I haven't processed the data yet, this is a crop from the same rig:

Tonight I am working on NGC2403, which also occupies the frame quite nicely.

James

[vlaiv]

Here are a few with very short focal length - 384mm - but also small sensor and small pixels:

[TheDadure]

4 minutes ago, vlaiv said:

Nothing wrong with imaging smaller galaxies with focal length under 800mm.

It is not focal length alone that determines scale of object in the image.

There are two different metrics that are important - one is FOV and other is pixel scale.

Field of view is determined by focal length and camera sensor size. You can have small FOV on short focal length scope if you use very small sensor. You can have largish FOV on a long focal length scope if you use large sensor.

In any case, field of view, or proportion of field of view to object imaged gives how large object will appear when image is viewed on screen size. Screen size is size that we see here on forum or when you view your image with some image viewing application and you "fit image to screen". This is in fact often "scaled down" version of the image as image can have a few thousand pixels in height and width and still it is displayed on screen that has less pixels than that (like 1920x1080 or similar).

Pixel scale or sometimes referred to as sampling resolution is number that represents mapping of angular sizes in the sky to pixel. It depends on focal length and actual physical pixel size on sensor. This metric is important when you view image on 1:1 zoom - or often referred as 100% zoom setting. In this case one pixel of image is mapped to one pixel on screen - you can usually pan around images that are larger than screen in pixel size.

Back to the imaging. It is important to understand that in most cases either sky or mount is the limit in how high sampling rate you can get. With modern pixel sizes and 800mm focal length - you will probably be at sky limit. This means that you need mount good enough to support this.

Aim at about 1.2"-1.4" per pixel to be safe (or just use camera that you have with scope that you have), but make sure your mount can guide at half that in RMS.

Hvala na odgovoru,kamera je canon 1300d,krop faktor na senzoru je 1.4,to objašnjava što je na prijateljevoj kameri field of view znatno veći

-David

[TheDadure]

Thanks for the response everyone,this really gives me hope

[vlaiv]

4 minutes ago, TheDadure said:

Hvala na odgovoru,kamera je canon 1300d,krop faktor na senzoru je 1.4,to objašnjava što je na prijateljevoj kameri field of view znatno veći

-David

Don't worry much about crop factor, you can check field of view for your particular camera and scope here:

https://astronomy.tools/calculators/field_of_view/

You can always crop further your image - it will make it smaller but will also change FOV and therefore object of interest can appear larger (but not more detailed in 1:1 view) on screen size.

[TheDadure]

In case you are wondering how some of my pictures look like here is one taken with 300mm f4 lens.I don't really remember how many light and calibration frames i took.

ps. this was actually a failed attempt,since clouds covered the sky,just after 15 minutes.But it gives you a clue.

Edited April 23, 2020 by TheDadure

[ollypenrice]

I'm perfectly happy shooting galaxies with a focal length of a metre. I don't think I would lose much if I came down to 800mm provided I used slightly smaller pixels. You can crop images so that they instantly appear at, or close to, full size when opened on a forum like this. These are at about a metre.

 

Olly

[TheDadure]

4 minutes ago, ollypenrice said:

I'm perfectly happy shooting galaxies with a focal length of a metre. I don't think I would lose much if I came down to 800mm provided I used slightly smaller pixels. You can crop images so that they instantly appear at, or close to, full size when opened on a forum like this. These are at about a metre.

 

Olly

WOW olly,those are trully some extraordinary shots.

[ollypenrice]

9 hours ago, vlaiv said:

 

There are two different metrics that are important - one is FOV and other is pixel scale.

 

We should be careful here because beginners often feel that a smaller chip, framing an object more tightly, means that they have the object more 'zoomed in.' I realize that you don't think this but it's a mistake we see very regularly in discussions. Like most other imagers I'll crop an image of a galaxy so that, without clicking to enlarge, it will appear at an appropriate size on screen when opened.

So as far as capture is concerned I don't think the FOV metric is important. It only becomes important at the presentation stage when a crop is all you need.

Olly

[michael.h.f.wilkinson]

I have captured M51 with my 80mm F/6 scope and ASI178MM camera, and got some pretty decent results at 480mm focal length

This was just an hour or so of L data. The small pixels of the ASI178MM do help in this case. I do prefer the result I am getting on galaxies at the longer focal lenght of the Meade 6" F/5 Schmidt-Newton at 762mm focal length.

 

[vlaiv]

2 hours ago, ollypenrice said:

We should be careful here because beginners often feel that a smaller chip, framing an object more tightly, means that they have the object more 'zoomed in.' I realize that you don't think this but it's a mistake we see very regularly in discussions. Like most other imagers I'll crop an image of a galaxy so that, without clicking to enlarge, it will appear at an appropriate size on screen when opened.

So as far as capture is concerned I don't think the FOV metric is important. It only becomes important at the presentation stage when a crop is all you need.

Olly

I this particular case, the concern that I was addressing was that of shorter focal length coupled with DSLR type sensor.  In this case FOV might look like too much - but like you mentioned, it is just the way image is presented and you can have different FOV by just cropping.

What I really wanted to point out as important bit in small vs large scope, but somehow run out of steam and was distracted by other things is that people need to realize that speed depends on scope size in a certain way and this is why most images of small galaxies are taken with larger scope.

If we take small scope and small sensor or large sensor and crop to target and compare it with large scope and large sensor (here small sensor won't do) - you'll get roughly the same FOV, but aperture will be different and large scope will win in light collecting ability and hence speed of the system will be greater.

This is why smaller galaxies are more often imaged with larger scopes - not because small scopes can't do it.

[ollypenrice]

1 minute ago, vlaiv said:

 

This is why smaller galaxies are more often imaged with larger scopes - not because small scopes can't do it.

I agree with your main points but there is an increasing movement towards galaxy imaging with 5 to 6 inch refractors. Quite a few imagers here and eleswhere have chosen this path. Being plug and play they save time while reflectors are arguing with their owners! 🤣

Olly

[michael.h.f.wilkinson]

2 minutes ago, ollypenrice said:

I agree with your main points but there is an increasing movement towards galaxy imaging with 5 to 6 inch refractors. Quite a few imagers here and eleswhere have chosen this path. Being plug and play they save time while reflectors are arguing with their owners! 🤣

Olly

I am bickering much more with the mount than with the Schmidt Newton's collimation, I find

I really need to get a permanent set-up

[Skipper Billy]

You could always look at groups of Galaxies such as the Leo Triplet or Markarian's Chain ???

These were taken with a 530mm FL scope and a large sensor (Atik 16200)

[gorann]

I like to test out different approaches, and for galaxy hunting this spring I have been using two big SCTs (Meade 14" ACF and EdgeHD 11") but a year ago I dedicated my galaxy season to imaging with my Esprit 150 and ASI071 OSC. OK, not really <800 mm, but not that far off. The FL of the Esprit 150 is 1050mm. What I like with imaging galaxies with shorter FL scopes is that you can fit in more of them. Here is 7 hours on M100 and a few others that would not have ended up together with my SCTs. But on nights with very good seeing the SCTs will pick up a bit more detail than the smaller refractor (at least in the same amount of time).

[ollypenrice]

4 hours ago, gorann said:

I like to test out different approaches, and for galaxy hunting this spring I have been using two big SCTs (Meade 14" ACF and EdgeHD 11") but a year ago I dedicated my galaxy season to imaging with my Esprit 150 and ASI071 OSC. OK, not really <800 mm, but not that far off. The FL of the Esprit 150 is 1050mm. What I like with imaging galaxies with shorter FL scopes is that you can fit in more of them. Here is 7 hours on M100 and a few others that would not have ended up together with my SCTs. But on nights with very good seeing the SCTs will pick up a bit more detail than the smaller refractor (at least in the same amount of time).

Yup! Very good.

😁lly

[michael.h.f.wilkinson]

4 hours ago, gorann said:

I like to test out different approaches, and for galaxy hunting this spring I have been using two big SCTs (Meade 14" ACF and EdgeHD 11") but a year ago I dedicated my galaxy season to imaging with my Esprit 150 and ASI071 OSC. OK, not really <800 mm, but not that far off. The FL of the Esprit 150 is 1050mm. What I like with imaging galaxies with shorter FL scopes is that you can fit in more of them. Here is 7 hours on M100 and a few others that would not have ended up together with my SCTs. But on nights with very good seeing the SCTs will pick up a bit more detail than the smaller refractor (at least in the same amount of time).

Very nice indeed. I am thinking of going for this region with my 6" F/5 one of these days

[Ships and Stars]

23 hours ago, ollypenrice said:

I'm perfectly happy shooting galaxies with a focal length of a metre. I don't think I would lose much if I came down to 800mm provided I used slightly smaller pixels. You can crop images so that they instantly appear at, or close to, full size when opened on a forum like this. These are at about a metre.

 

Olly those photos are spectacular! May I ask what galaxy this is above? I'm fascinated by the interacting galaxies in the lower left. My novice brain is drawing a blank. 

[vlaiv]

6 minutes ago, Ships and Stars said:

Olly those photos are spectacular! May I ask what galaxy this is above? I'm fascinated by the interacting galaxies in the lower left. My novice brain is drawing a blank. 

Top right - NGC7331 (main) - Deer Lick group (with smaller surrounding galaxies)

Bottom left - Stephan's Quintet

[Ships and Stars]

1 minute ago, vlaiv said:

Top right - NGC7331 (main) - Deer Lick group (with smaller surrounding galaxies)

Bottom left - Stephan's Quintet

Thanks Vlaiv, Stephan's Quintet is incredible...👍