Why is 80mm APOs most common for AP?

[Luke]

The ED80 can cope with a fair bit of wind. I used my ED80 more than my 120 and C11 partly because of that. Also, I liked targets that suited the short focal length, and it was less hassle to set up from scratch than the C11 due to my lack of an obsy.

The C11 I wanted to do galaxies with so tended not to use on moonlit nights, doing narrowband instead on nebulae with the shorter fracs.

[alienf0rm]

Thank you for the replies. 

I think I'm getting a hang of it. F ratio is more important compared to aperture when shooting DSOs. 80mm tubes generally have shorter focal length than their 100mm counterparts. This means wider swaths of the sky and shorter exposure times. 

However since we are trying to capture point or multiple point sources of light, won't that extra bit of aperture help resolve those really faint sources? 

[ollypenrice]

Thank you for the replies. 

I think I'm getting a hang of it. F ratio is more important compared to aperture when shooting DSOs. 80mm tubes generally have shorter focal length than their 100mm counterparts. This means wider swaths of the sky and shorter exposure times. 

However since we are trying to capture point or multiple point sources of light, won't that extra bit of aperture help resolve those really faint sources? 

shorter exposure times.    Have a care here. The F ratio (not the focal length) does determine the exposure time but it is very misleading to compare two instruments of different F ratio unless they have the same focal length. This comes up in regard to focal reducers, which don't necessarily reduce exposure times. If the object will fit on the chip reduced and unreduced then exposure time on just the object itself will not be reduced. There are no new photons brought in by the reducer. The available photons are just put onto fewer pixels which fill faster, but the object is smaller on the chip.

The effect of aperture on imaging resolution is a little underwhelming. Comparing the Taks in my earlier post the 85 resolves to 1.5 arcsecs and the 106 to 1.09 arcsecs. How much difference does this really make? Not a lot. Few deep sky imagers are trying to resolve double stars, for instance. This is best done at the eyepiece. What really matters is the ability to control star bloat, especially in the blue channel, and to give clean stars corner to corner.  A scope which can do this, and do it at a fast F ratio, is a good imaging scope.

I like the Tak 85 very much.

It was such an easy scope to use. Focus was unambiguous, the field was flat, the F ratio fast, the stars clean. It held focus and was very rarely troubled by off-shot flares or reflections. These are the real-world issues in deep sky imaging.

Olly

[Scott]

This leads to the question of why do so many then go out and buy a 200P or a 9.25 and then ask about AP.

For me, it (200p) was my first scope. Ap was not on the agenda when I kicked off. very soon after granted but by then the scope was here. I've since changed scopes, mounts, cameras, scopes,mounts and again cameras but the one constant is my 200p. don't think I'll ever part with it

[barkis]

Yep, I have the ed80 and the zs 66 Apos, and fully intend to keep them.

My arsenal is supplemented with the MN190.

These are my Modern units, But as with Scott, my heart still  lies with my late 8.75" F6 Newt.

Sadly, I sold it  to a colleague at my local Astro. Society, and as far as I know, it tours around Various functions in Cumbria,

showing the nebulae, clusters, and moon and planets to eager young astronomers to be.

I dearly miss that telescope, many because I gave birth to it. I made it's mirror, and initial hardware and drive system  too.

It is in a Dobsonian configuration now, for ease of use, but still going strong.

Apologies to the OP for the little deviation here.

Ron.

[gorann]

A comment om mikeobrien's comparison pictures taken with the Skywatcher ED80 and the 300 pds. I think it is a bit unfair to the ED80 because that shot must be heavily cropped and then blown up since the focal lengths are so different between these two scopes (600 mm vs 1500 mm). Still, of course the 300 pds pic is really pretty!

Another comment to alien0form. I would chose an even faster 80 mm than the Skywatcher ED80 (f 7.5). There are several around. For example the Explore Scientific ED80 is f 6.0 (480 mm focal length) and it is a triplet apo (=probably better colour correction) and costs about the same as the Skywatcher.

[LukeSkywatcher]

Interesting read as i had always wondered why so many imagers use such scopes as the 80ED. I can only concur that small refracs are indeed compart,light,portable and robust (yeah yeah i know............i only have a 70mm travel scope), but its still a scope that i use and can agree with its specs being favourable for ease of use. 

[alienf0rm]

The effect of aperture on imaging resolution is a little underwhelming. 

Would it be wrong to say that aperture makes more of a difference for visual observation, where we are limited to what 'adjustments' our eye can make? 

What really matters is the ability to control star bloat, especially in the blue channel, and to give clean stars corner to corner.  A scope which can do this, and do it at a fast F ratio, is a good imaging scope.

How does one find a scope like this? Based on user reviews, sample pictures and vendor data? Or can you just take any scope which has good quality optics and a fast F ratio?

Another comment to alien0form. I would chose an even faster 80 mm than the Skywatcher ED80 (f 7.5). There are several around. For example the Explore Scientific ED80 is f 6.0 (480 mm focal length) and it is a triplet apo (=probably better colour correction) and costs about the same as the Skywatcher.

What I had in mind was a Espirit 80mm (400mm FL, f/5). Trying to read up more about this. I'm wondering if the 400mm is 'too wide' for galaxies, and make small objects very tiny in the final image.

[gorann]

The Esprit sound like a good choice. Some galaxies will be small but excellent for the Andromeda and big nebulas. Here is a pic I took of M81 & M82 with my Explore Scientific with a focal length of 480 mm, to give you an idea (there are some more pics in my album of various objects taken with the same scope). With 400 mm these galaxies would be 20 % smaller, but you can use 30% shorter exposure time with f5 compared to f6, so a little bit less risk for star trailing. Do not forget that for AP you also need to buy a field flattener for the scope.

Here is a useful site for calculating effects of focal ratios and focal lengths: http://www.astrosurf.com/luxorion/report-formulae.htm

[gorann]

Here is a calculator that shows your field of view for various objects, very cool:

http://www.skyatnightmagazine.com/field-view-calculator

[Starlight 1]

You don't need to cheat by looking for diffraction spikes!

 15 mins to 1 hour say it all.

[mikeobrien]

I take the point about the scopes f ratios being different but I like others have mentioned started off with a scope for deep sky observation with imaging being an option for later on, so aperture was important. The comment about image crop would be more valid if they were cropped the other way around but that is not the case and the image was mainly to illustrate noise levels resulting from light gathering capabilities. So to sum up will I continue to use the 12" for most of my imaging you bet!

[Paultesla]

I bought a Megrez 72 as a beginner as it mainly came down to cost and hopefully something to get into AP with.  To be honest it has been more than enough to handle and I'm glad I did not go any bigger.  I still have lots to learn and really enjoy using it.  A 33mm eyepiece on it when sweped thought the milky way is amazing, and still one of my favorite things to do with it!

Best wishes,

Paul

[ollypenrice]

Would it be wrong to say that aperture makes more of a difference for visual observation, where we are limited to what 'adjustments' our eye can make? 

Yes, aperture matters hugely in visual observing and far less in imaging because the camera collects data over time, unlike the eye.

How does one find a scope like this? Based on user reviews, sample pictures and vendor data? Or can you just take any scope which has good quality optics and a fast F ratio?

To avoid any possibility of disappointment buy a Takahashi FSQ85. At more reasonable prices you do, indeed, scour the net for reviews and, above all, images. When looking at images note that narrowband images don't test a refractor to any great extent. Almost any old rubbish can take a good narrowband image (I'll be in trouble for this but it's true!!!) but LRGB separates the sheep from the goats.

What I had in mind was a Espirit 80mm (400mm FL, f/5). Trying to read up more about this. I'm wondering if the 400mm is 'too wide' for galaxies, and make small objects very tiny in the final image.

Other than M31, M101 and M33 galaxies need a long focal length and, to be honest, longer than any any refractor any of us is likely to buy. Most galaxies strut their stuff at no less than 1.5 metres and 2.5 or more is best. But that means a serious mount to get the accuracy and payload. Note accuracy. That's the killer.

M31 at 530mm FL; http://ollypenrice.smugmug.com/Other/Best-of-Les-Granges/i-3D2Hw7s/0/L/M31%20Outer%20Halo-L.jpg

But for M51 2.5 metres is a better idea!  http://ollypenrice.smugmug.com/Other/Best-of-Les-Granges/i-Sc3kgzc/0/X3/M51%20DEC%20VERSION%20clip-X3.jpg

Olly

[jetstream]

Olly, what scope, camera etc did you use for that M51 image above? That is a superb image.

[alienf0rm]

The Esprit sound like a good choice. Some galaxies will be small but excellent for the Andromeda and big nebulas. Here is a pic I took of M81 & M82 with my Explore Scientific with a focal length of 480 mm, to give you an idea (there are some more pics in my album of various objects taken with the same scope). With 400 mm these galaxies would be 20 % smaller, but you can use 30% shorter exposure time with f5 compared to f6, so a little bit less risk for star trailing. Do not forget that for AP you also need to buy a field flattener for the scope.

Here is a useful site for calculating effects of focal ratios and focal lengths: http://www.astrosurf.com/luxorion/report-formulae.htm

Thanks for you input and those links. 

To avoid any possibility of disappointment buy a Takahashi FSQ85. At more reasonable prices you do, indeed, scour the net for reviews and, above all, images. When looking at images note that narrowband images don't test a refractor to any great extent. Almost any old rubbish can take a good narrowband image (I'll be in trouble for this but it's true!!!) but LRGB separates the sheep from the goats.

Thank you for this bit of information. I'm guessing that the technique used by each astronomer will produce varying results for the same scope.

In the M51 pic, the galactic centre  itself is sharp but the surrounding stars seem a bit out of focus. Was this shot this way?

[ollypenrice]

Olly, what scope, camera etc did you use for that M51 image above? That is a superb image.

This was with Yves Van den Broek's 14 inch Optimized Dall Kirkham, F6.8 and focal length just shy of 2.5 metres.

Great scope but you can also see why people often use 80mm refractors! (Portabilty and pocket depth do play a part...)

Olly

[ollypenrice]

Thanks for you input and those links. 

Thank you for this bit of information. I'm guessing that the technique used by each astronomer will produce varying results for the same scope.

In the M51 pic, the galactic centre  itself is sharp but the surrounding stars seem a bit out of focus. Was this shot this way?

Sure, image processing is a big factor in determining how an image comes out. Capture is, essentially, a simple, predictable and mechanical process. Frame, focus, guide and shoot a lot of data (then double it.) That's about it.

Processing is something which you go on learning forever and involves you in hundreds of decisions each of which will make your result your own.

The stars in the image are in focus but haven't been given any software sharpening. The core has been through unsharp masking and deconvolution. Like a lot of imagers I never sharpen stars. I think it makes them look un-natural and draws the eye away from the main target.

Olly

[barkis]

Is the jury still out on PI and or CS?. I am aware they are two different processing packages, and each have their champions.

So, the 64k Dollar question, if only one can be afforded, how would one jump?

PS was on the scene long before Pixinsight, but then the latter burst onto the scene with a fanfare, 

and established image processors lauding it's capability.

Do those opinions still hold good, or has the bubble burst, and has PI been relegated to the "It does an OK job"  Category?

Both these Software tools are expensive, and for some. it has to be one or the other because of the price tags.

So Gentlemen, and Ladies of course, which one would it be for you if those restrictions were in place for you?

 Ron.

[Earl]

Pi is very good, and is the better choice for a pure astro processing standpoint, if you are willing to learn to do thing the way the software does it.and there is a huge amount to learn its a new language to the usual PS stuff.

however it lacks the all round versatility of PS

If you can only afford one id say get a sub based CS at £8.99  a month if you want to take processing to the next level thee are very useful tools in PI that are worth it DBE for instance.

I do hoever feel that both are valid together as I want to do thing i dont know how to do in PI, it would be good if PI would take on board editable layers like PS to offer a greater level of making it pretty versatility.

[Anas Albounni]

shorter exposure times.    Have a care here. The F ratio (not the focal length) does determine the exposure time but it is very misleading to compare two instruments of different F ratio unless they have the same focal length. This comes up in regard to focal reducers, which don't necessarily reduce exposure times. If the object will fit on the chip reduced and unreduced then exposure time on just the object itself will not be reduced. There are no new photons brought in by the reducer. The available photons are just put onto fewer pixels which fill faster, but the object is smaller on the chip.

The effect of aperture on imaging resolution is a little underwhelming. Comparing the Taks in my earlier post the 85 resolves to 1.5 arcsecs and the 106 to 1.09 arcsecs. How much difference does this really make? Not a lot. Few deep sky imagers are trying to resolve double stars, for instance. This is best done at the eyepiece. What really matters is the ability to control star bloat, especially in the blue channel, and to give clean stars corner to corner.  A scope which can do this, and do it at a fast F ratio, is a good imaging scope.

I like the Tak 85 very much.

It was such an easy scope to use. Focus was unambiguous, the field was flat, the F ratio fast, the stars clean. It held focus and was very rarely troubled by off-shot flares or reflections. These are the real-world issues in deep sky imaging.

Olly

This photo is ABSOLUTELY stunning     

can you give us few specs please

i just ordered my Esprit 100mm  to mount it on my AZ-EQ5, but I'm still trying to find the best guider, bought an Equinox 66mm for it, any recommendations?

Many Thanks

[johnrt]

Is the jury still out on PI and or CS?. I am aware they are two different processing packages, and each have their champions.

So, the 64k Dollar question, if only one can be afforded, how would one jump?

PS was on the scene long before Pixinsight, but then the latter burst onto the scene with a fanfare, 

and established image processors lauding it's capability.

Do those opinions still hold good, or has the bubble burst, and has PI been relegated to the "It does an OK job"  Category?

Both these Software tools are expensive, and for some. it has to be one or the other because of the price tags.

So Gentlemen, and Ladies of course, which one would it be for you if those restrictions were in place for you?

 Ron.

For me it's both, I can't live without either. PI has some things which it excels at - DBE, colour calibration & LHE for example, but there are things where it cannot compete with PS, sharpening is a very good example and the ability to use layers and brushes. It's not so simple as one or the other!

[ollypenrice]

This photo is ABSOLUTELY stunning     

can you give us few specs please

i just ordered my Esprit 100mm  to mount it on my AZ-EQ5, but I'm still trying to find the best guider, bought an Equinox 66mm for it, any recommendations?

Many Thanks

Glad you like it. Thanks. It's an image which has grown over the years but it is Tak FSQ85ED/Atik4000 monochrome CCD in LRGB plus Ha blended with red. I combined an M42 shot at native focal length with a wider field shot through the 0.73 reducer. This enhanced the resolution in the core. Sub lengths were 11 seconds for the Trapezium, 50 seconds for the area around it and 10 minutes for the main field. The Ha would have been in 30 min subs from memory. The different sub lengths were combined using Layer Masking in Photoshop CS3. Total exposure time will be a lot, in the order of 15 hours or so.

Regarding Ps and PI, I agree 100% with John, above. I couldn't manage without both and use them very much as John says he does. I would find trying to process a whole image in PI to be like tying my shoe laces with chopsticks. I rely on layers in Ps and make heavy use of the eraser to select what part of each layer I want to keep. In PI you have to use masks to do equivalent work in particular zones and the masks never go where I want them to go! But for colour calibration and flattening and gradient removal and losing green noise PI is king.

Olly

[alan potts]

Olly,

A superb image in every way, you sure don't get an image like that after 10 minutes work. One of the best I have ever seen of M42 complex. You really are rather good .

alan

[ollypenrice]

Olly,

A superb image in every way, you sure don't get an image like that after 10 minutes work. One of the best I have ever seen of M42 complex. You really are rather good .

alan

Thanks Alan. The real point , though, is that you don't need a big scope for this kind of imaging. Here's the little fella who did the business.

(Don't guide with a solar scope. It doesn't work.      )

Olly