Why is 80mm APOs most common for AP?

[alienf0rm]

Hello everybody

I am reading up on AP and have noticed that a lot of the forum members choose 80mm EDs for their work. Why is 80mm so common? Why not something larger in aperture like 100 or 102mm? Is this a cost vs utility factor? Or is it because you can get pretty decent pictures with the 80mm? 

Davis

[Qualia]

I don't take images and know very little about AP, but I imagine an 80mm ED fills a good number of ticky boxes: relatively fast, CA is controlled, they're light and small so easier on the mount and for tracking, and have a nice field of view to capture many DSO targets     

[Ibbo!]

Fairly light - does not tax mounts too much

reasonable lenght

easy to handle

good price

f ratio not short so relatively distortion free, not too long to have guiding problems and a decent FOV

[ronin]

Throw in that aperture is not what gives a bright image, the f number does that. So a 102mm f/6 is dimmer then an 80mm f/5.5.

The focal length gives the image size, and "generally" that relates to the objective diameter.

A ccd camera is not big, unless you really want to spend money, so a smaller image (small focal length) can means less cost a the camera purchasing end.

So you do not need a bigger scope.

Also with bigger diameters come additional problems or potential problems, weight is one and aberrations caused by the wider (edges) lens.

In the end I suspect a bigger aperture (and so bigger scope) delivers very little of use in AP and many aspects of a larger aperture will be negative in regards AP. So the 80mm comes out about optimum when all aspects are considered.

Bit like binoculars - why do birdwatchers not use 12x50's ?

Enough light from the sun and the bird is rendered bigger/closer.

However 8x42's are by far the chosen and preferred instrument.

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.

[MikeD]

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.

Simple, Detail, the more aperture you have, the more resolution you have and for me it's all about the detail.

For  example, I have an 80ED and a 10" newt, I could pretty much fit M33 in the FOV of the 80ed but would rather take the time to shoot a mosaic at a higher resolution.

may take a few years to complete but for me it's worth it.

Mike.

[gorann]

I see it just like ordinary nature photography. If you want wide field (landscape) you put a wide angle lens on your DSLR, which in the AP world would correspond to a small 80 mm refractor (about 500 mm focal length and f6) and if you want a pic of a bird in a tree you use a telephoto lens. In AP terms that would correspond to something like a 200 - 300 mm Schmidt-Cassegarin (2-3 m focal length, f10) that you can use for taking pictures of small things (as seen from earth) like the majority of galaxes. So, even if I am new to astrophotography I soon realized that I needed at least two scopes to cover most of the deep sky objects. I ended up with an Explore Scientific ED80 (the cheapest 80 mm triplet apo with good reviews that I could find) and a Celestron Edge HD8. I could have bought a hyperstar adapter to my Celestron to get wide field but that thing costs more than many decent 80mm ED refractors (still I may very well end up buyng a hyperstar one day if budget allows...). A plus with the 80 mm refractor is of course its small size so you can have it on a small and less expensive mount.

[gorann]

Mike D of course has a pont about the aperture, but initially in astrophotography I think it may be important to be encouraged by getting good results with not too much efforts, and then progress to more advanced and time consuminng endevours later on. When I bought my NEQ6 mount I got a Skywatcher 250p as a package deal. I still have not come around to use it for AP since the 80 mm apo give such pleasing results quickly and I am still in the early stages of exploring good wide field targets to shoot (or was, my Swedish sky is too bright now for anything but the moon). However, in the autumn when it starts to darken again I aim to try AP with the big Newtonean - I am sure it will be exciting.

[MattJenko]

 I think it may be important to be encouraged by getting good results with not too much efforts,

I think this should not be underestimated. It is all well and good looking back with hindsight saying if it was to be done again then it would go a different way, as not everyone gets to the point where they look back and say those things. Many people might find the route taken with hindsight would mean not getting results up front, finding the curve too steep and/or just not enjoying it the way they actually did it. I know that is the case for me, in that if I required all the skills I have now to simply get anything, then I would not have gotten here in the first place.

[MikeD]

I fully understand that the 80mm is the most common scope used for AP because of all the reasons given so far to the OP (ease of use ect) I was just answering the question posed by ronin "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." 

Mike.

[Coastliner]

I think this should not be underestimated. It is all well and good looking back with hindsight saying if it was to be done again then it would go a different way, as not everyone gets to the point where they look back and say those things. Many people might find the route taken with hindsight would mean not getting results up front, finding the curve too steep and/or just not enjoying it the way they actually did it. I know that is the case for me, in that if I required all the skills I have now to simply get anything, then I would not have gotten here in the first place.

I agree with this, as a relative newbie I am just getting over the planet thing and am now going for DSOs.  I started with the Beehive a couple of nights ago, simply because someone pointed out that it was "just to the right of Jupiter" at the moment.  I located Jupiter in my 200p, scanned around a bit, and got fed up looking. It has just been pointed out to me that I won't see it in a 200p it's way too big, use 10x50 binoculars instead.  So, start simple and work up.

Neil.

[alienf0rm]

I guess what I'm asking is why not choose a slightly larger 100 or 102mm? Is it just the price and f stop?

[gorann]

100 mm costs more, weigh more and usually has a smaller field of view than 80 mm. The reolution of a 100 mm refractor is only about 25 % higher than an 80 mm so I expect that you need a really clear and steady sky to notice the difference, particularly on wide field shots.

[rowan46]

its a cost benefit thing fast  scopes are what you need for imaging  an 80mm is relatively cheap to get a reasonable focal ratio a 100mm costs an awful lot more. take the ed80 at 7.5 its a relatively slow scope add the focal reducer and you get 6.35 which makes it just within the realms of easily usable for imaging. If you take the 100 ed at f9 and add the reducer you get f7.65  which is still a slow scope for imaging this means you need  longer exposures maybe a better (more expensive mount) take a look at the esprit one of the cheaper good 100mm scopes optimised for imaging you are talking about  £1400 Nb people are recommended the ed 80 not because its the best but because its the easiest cheapest scope to  use. the usual question we get is "whats the cheapest easiest scope to start imaging with?"

the cheapest imaging scope is now the 130 pds although some would argue that the ed80 is easier

[dph1nm]

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.

Never forget that the larger the aperture the more photons you get from a particular object. If 15th mag galaxies are your thing, an 80mm is really not the optimum scope.

NigelM

[laser_jock99]

The 80mm ED is recommended as it is a good beginners scope. Astrophotography is already complicated enough so a straightforward scope like the ED80 is the ideal learning platform.

If you started off with something like a 10" F4 Newtonian the added grief might put many people off for good!

That said- a season or two with the humble ED80 might soon have you yearning for something bigger/faster........ 

[Earl]

Mostly due to the ease of use and not been very demanding on the mount this leads to the good price point the ED80 comes in at.

The generic ED80 that celestron and Skywatcher  / Orion do has its drawbacks , focuser and a tad slow, (these can be dealt with to some extent)  but at the price its does do the job well and your looking to spend at least twice as much for an improvement.

The ultimate 80 mm range scope is the FSQ85 with its fast performance and very well corrected colour the results are awesome... costs 7 times as much though.

I went for the 106 as one came up at the right price when i was looking to buy.

The starter package of a reduced ED80 on a HEQ5 with a DSLR and finder guider is not excessive in price and a good introduction to see if you want to take it further without digging deep.

[mikeobrien]

For me it boils down to this 1 hour from a dark site.

[wavydavy]

Id say its price as well as all the above, an 80mm is fairly reasonable price, compared to any size up.......

[mikeobrien]

15mins from a heavily light polluted site guess which is the ED80 and which is the 300pds!

[mikeobrien]

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

[RobertI]

It may be worth pointing out for the benefit of the OP that some very experienced imagers also use 'small' aperture high end imaging scopes (eg: the Tak FSQ85) for widefield imaging. Edit - some other posters got in before me so my comments are s a bit redundant now!

[gorann]

Interesting! I just started with AP this spring and bought an Explore Scientific 80 mm triplet apo (got good reviews and costs about the same as SWs doublet 80mm) and I have been very pleased. However, I got a SW 250 Newtonean as a packet deal with my NEQ6 mount (a SW 250 costs even less than the 80 mm triplet). The Newtonean is so bulky so I have so far not got around to use it for AP, but your pics and that by Red Dwarf further up in this string convince me that it would be worth while and I will surely try this autumn (when the sky gets dark again up here in Sweden). Still, for a beginner I think the small apo makes life easier.

[ollypenrice]

Why 80 rather than 100mm is a good question and this may be the answer; maintaining refractor colour correction at a given F ratio gets harder as the aperture increases. As we all know, 'harder' is optics-speak for more expensive!!!

At the budget end we have the Skywatcher ED80 and ED100, both excellent for the price. The 100 is F9 but the 80 is F7.5 which means that the smaller one is a hefty 1.44 times faster. No imager is going to ignore this fact and the increase in resolution is, quite honestly, trivial at this scale. (When we take a huge leap to 10 inches, as Mike does, then the increase is not trivial and Mike is also imaging at a longer focal length.)

At the 'money is no object' end of the scale we turn, as usual, to Takahashi and compare the FSQ85 with the FSQ106. I have a lot of experience with both. Tak have kept the F ratio almost the same, the 85 being F5.3 and the 106 a tad faster at F5. But look at the price; £2,280 for the 85 versus £3498 for the bigger one. A difference of  £1218. Say it quickly! 

I only changed from the 85 to the 106 because I started using a full frame chip which the 85 did not quite have the image circle to cover. The pixels of the new camera are also large and are better suited to the longer focal length of the 106.

Beginners using DSLRs have small pixels which are too small for long focal lengths and cannot be binned up 2X2 because they are one shot colour. DSLRs are, in my view, a bad choice for long focal lengths and many beginners do make the mistake Ronin implies of going for long FL SCTs before doing all the homework.

Newts do make good imaging scopes but are bigger, needing better mounts, and they do need more tuning. The also give diffraction spikes which, on widefield images, I can't claim to like. Some do though.

Olly

http://ollypenrice.smugmug.com/Other/Best-of-Les-Granges/22435624_WLMPTM#!i=2266922474&k=Sc3kgzc

[Traveller]

Focal length is another factor, or rather, the forgiveness of an ED80 with FL of 600mm compared with 900mm with an 100 f9. Shorty FL will allow a certain degree of tracking error etc, which is more pronounced at longer FLs. I can easily get away with a 50mm mini guidescope with my ED80 but on longer FL, I will need a longer FL guidescope or an OAG.

Bo

[orley]

If bought new (and quite often second hand) the ED100 comes with a reducer which takes it down to F7.65, same as the ED80. That said, you can add a reducer to the 80 as well, but it is an extra purchase. 7.65 is regarded by many as slow, but still quite practical. I used to use a 150mm F5 newt for imaging but TBH I get better results now. There is the added benefit of the extra aperture compared with the 80 for visual use.

In terms of tracking I've had no issues with my reduced ED100 and using a 9x50 finderscope for guiding. Lovely round stars.