‘Scope for smaller galaxies.

[Grierson]

My SW ED80/reducer is limited for imaging smaller galaxies. I wonder if anyone has any suggestions for a suitable reasonably priced alternative which can be carried by an HEQ5. Thanks - John

[Skipper Billy]

1370mm focal length

£375

8kgs

20" long

https://www.firstlightoptics.com/ioptron-telescopes/ioptron-photron-6-ritchey-chretien-telescope.html

[vlaiv]

I second above recommendation, but do have question for you:

When you say that your scope is limited for imaging smaller galaxies - what exactly do you mean?

If you are referring to resolution - ASI1600 (from your sig) coupled with ED80 at native focal length will provide you with 1.3"/px. With Heq5 it is very unlikely that you will want to go any lower than that regardless of the scope that sits on it.

For example, if you choose to go with above scope (and I think that you should) - you will be working on native 0.57"/px with ASI1600 - that is too much, and you will want to bin in software by factor of at least x2.

That will give you resolution of 1.14"/px - which is still too much I think.

Now if you don't have enough light grasp to go for fainter things, they yes - going with 6" scope will give you more light gathering capability. Just don't forget to adjust your sampling rate to something reasonable (1.2"-1.5" for high power work with 6" scope and heq5).

[Grierson]

3 hours ago, vlaiv said:

I second above recommendation, but do have question for you:

When you say that your scope is limited for imaging smaller galaxies - what exactly do you mean ..............

Thank you gentlemen. My simple issue is that when imaging many galaxies I find that the actual galaxy is very small in the frame. (The ‘scope is fitted with a reducer and equates to a focal length of 510mm) Cropping and enlarging the image is clearly not the best approach! 

I will have a look at the iOptron spec. John

[dannybgoode]

On 22/01/2020 at 18:54, Grierson said:

Thank you gentlemen. My simple issue is that when imaging many galaxies I find that the actual galaxy is very small in the frame. (The ‘scope is fitted with a reducer and equates to a focal length of 510mm) Cropping and enlarging the image is clearly not the best approach! 

I will have a look at the iOptron spec. John

Actually cropping and enlarging is probably as good a route as any. It’s hard to stop thinking in the same manner as normal photography but if your resolution is correct for the camera / scope combo then that is what matters. 
 

I think I’m right in saying (and I’ll happily be corrected on this) but if you have two camera / scope combos, both with a resolution of 1” per pixel then the target will be the same size for both. 
 

If one of your scopes is of a longer focal length and it reduces the field of view accordingly then all that is doing is ‘cropping’ the image with the scope rather than in software. 

[vlaiv]

4 hours ago, dannybgoode said:

Actually cropping and enlarging is probably as good a route as any. It’s hard to stop thinking in the same manner as normal photography but if your resolution is correct for the camera / scope combo then that is what matters. 
 

I think I’m right in saying (and I’ll happily be corrected on this) but if you have two camera / scope combos, both with a resolution of 1” per pixel then the target will be the same size for both. 
 

If one of your scopes is of a longer focal length and it reduces the field of view accordingly then all that is doing is ‘cropping’ the image with the scope rather than in software. 

You are certainly correct, but issue is that there are people that don't quite understand what is going on because the way images are displayed by devices.

If you view your image at 1:1 (1 screen pixel for one image pixel, or 100% zoom level), then you are absolutely right - object will have dimensions that correspond to resolution "/px times size of object in arc seconds.

Problem comes when images are displayed how they are usually displayed - to fit screen of display device (be that computer screen or smart phone or tablet or whatever) - then size of object is determined by FOV. This is where cropping starts to change things - it will not change resolution or original object size in the image - it will change FOV (decrease it) and that in turn will make relative size of object larger when viewed on "fit to screen" zoom setting.

[dannybgoode]

2 hours ago, vlaiv said:

You are certainly correct, but issue is that there are people that don't quite understand what is going on because the way images are displayed by devices.

If you view your image at 1:1 (1 screen pixel for one image pixel, or 100% zoom level), then you are absolutely right - object will have dimensions that correspond to resolution "/px times size of object in arc seconds.

Problem comes when images are displayed how they are usually displayed - to fit screen of display device (be that computer screen or smart phone or tablet or whatever) - then size of object is determined by FOV. This is where cropping starts to change things - it will not change resolution or original object size in the image - it will change FOV (decrease it) and that in turn will make relative size of object larger when viewed on "fit to screen" zoom setting.

Thanks Vlaiv. It’s something I’ve been slowly trying to get my head around and what tot say makes perfect sense. It’s very hard when I’ve spent so long doing terrestrial photography to think differently but it’s an enjoyable journey learning :)

[Whirlwind]

The other benefit of longer telescopes is that you are usually getting a larger aperture.  Therefore if you are binning to the same resolution as a smaller 80mm refractor then you get the benefit of the extra aperture overall (barring some noise differences from binning).  Your field of view will be overall smaller though.  For example a C8 edge with reducer or 200mm F4 reflector would give you a similar 1.1" resolution.  Ideally you'd want the largest telescope with a short focal length but they are usually quite expensive... 

So it depends on how much you want to spend!

[ollypenrice]

If you decide, first, on the resolution in arcseconds per pixel which your sky's seeing and your mount's accuracy under guiding will support then that's a solid start. So... if you use PHD and key in your guider's pixel size and focal length it will, in operation, give you a guide RMS in arcseconds per pixel. For a rule of thumb, multiply this by two to get your maximum useful resolution in arcseconds per pixel. An RMS of 0.6 arcseconds will support an imaging resolution of half that, so 1.2"P/P. These are just numbers but they are not entirely meaningless. They will help you approximate reality.

You can get to 1.2"P/P in any number of ways by varying focal length and pixel size (or by binning to change effective pixel size.) Just don't expect a free lunch. If you go for larger aperture you must remember that optical production costs are proportional to that aperture and are also proportional to focal ratio, faster being more expensive for quality 'x.' For what it's worth I've personally called a halt to my search for resolution by using 5.5 inch refractors  and small pixel cameras to give me 0.9"P/P but I have exceptionally good mounts. I could probably squeeze a bit more resolution out of something larger but at the cost of a lot of fiddling. In your shoes a scope I would consider is the Skywatcher ED120 Pro. https://www.firstlightoptics.com/pro-series/skywatcher-evostar-120ed-ds-pro-outfit.html  I think the 6 inch RC would beat it if you managed to get it just right - but would you? Those who succeed in collimating RCs say it's easy. Those who don't say it's difficult!

Olly

Edited January 25, 2020 by ollypenrice

[Grierson]

Again gentlemen tank you for taking the trouble to provide some further food for thought. Although I have been imaging for some time I am still climbing the learning curve and at my extended age it is getting more difficult rather than easier.

My first thought Olly, was the ED120, but as the responses have rightly implied I come from a photography hobby and I was sure that it was perhaps not as simple as just investing in a longer 'scope. - John

Edited January 26, 2020 by Grierson