ED80, 81 Or 100?

[Jon Sales]

I like to do a lot of reading up and research before making purchases and was wondering what the difference would be between an 80 or 81 ED Refractor and a 100ED?

I will be using this with a Nikon D5100 DSLR or a Canon EOS1100D.

I won't be using it with a CCD camra for the foreseeable future.

Which would be better for DSO AP?

Or should I get something smaller?

Cheers.

[nmoushon]

There really wont be much difference between the 80/81 ED. The only difference that will be noticeable is the focal length depending on the manufacturer and there for the focal ratio. The 100ED will have a bit longer focal length probably by around 200-300mm, again depending on who is the manufacturer. Do you have any links to the exact scope you are looking at? 

The bid difference between the 80/81ED and the 100ED for DSO AP will be the focal length and thus the focal ratio which is the more important thing for AP. The 100EDs tend to have a higher f/ratio than the 80EDs. You will want as small of a f/ratio as is reasonable for the telescope quality. For doublets that will be about F/5 ish. The native f/ratios, which will be around F/6.5 or so, will be a bit high but you WILL NEED a focal reducer/field flattener to give you the best results. At least a field flattener is nothing else. The 80EDs will tend to get you in this range very easily. The 100EDs have a tendency to have a higher native focal ratio, which will be around F/7 to F/9, and thus even with a reducer could be a bit high. 

But what even more important is the mount. There is nothing in your sig about what mount you have or dont have. Do you have an EQ mount already?

Also if you haven't already, buy the book Make Every Photon Count. It will explain everything you need to know about getting into AP. It really will make things more clear and explain what you need and dont need and why. 

[Jon Sales]

Thanks for the reply.

The mount I have chosen is the Celestron AVX mount.

I've been looking at the Skywatcher ED's and the William Optics ED's.

I also like the look of the William Optics 71 ED as it's quite a bit cheaper but don't know what difference it'll make to AP.

Both the WO's have quite fast f ratio's.

[ronin]

The only 81 I am aware of that is "common" is the WO as either GT or GTF and that is a triplet apo, the ED's are doublets.

So you could be comparing different items.

[Jon Sales]

Just looked on altair astro's website and they have some nice Starwave and Lightwave scopes on there.

This one looks good for the price.

http://www.altairastro.com/product.php?productid=16496&cat=0&page=1

or this

http://www.altairastro.com/product.php?productid=16707&cat=270&page=1

[Jon Sales]

These are the WO ED's I've been looking at

http://www.firstlightoptics.com/william-optics/william-optics-gt-81-triplet-refractor.html

http://www.firstlightoptics.com/william-optics/william-optics-zenithstar-zs71-ed-2013-reducer-bundle.html

[nmoushon]

Oh you are looking at a triplet? Then that has the advantage hands down over the doublets. If you have the budget for it then that's the one you would want. Remember you might want to budget for a reducer. You wont need a flattener as the triplets are already flat. So you could image with or without a reducer to change your focal length and thus your FoV to allow larger objects to fit in on your sensor. Do forget the T-adaptor to hook up the DSLR as well. 

I'm not sure about this particular scope but what is a common trend among most scopes is the focuser usually isn't up to par for AP. It tends to slip under the weight of the camera, especially when imaging high targets. Now since you are only using a DSLR you probably shouldn't have to worry but if you switch to CCD you might need to upgrade the focuser. 

Another thing you will want to budget for, eventually if not very soon, is guiding. Even at the short focal lengths you will be limited to about 2min subs without guiding and that will require spot on polar alignment and balance. I worked like this for a year and got some great shot so you can do a lot without guiding but you'll get to a point where you will want it. Especially if you ever want to image the faint nebula and the detail as that require longer subs. The cheapest is a modded 9x50 finder scope with a modded web cam attached to it or if you can push (or save up) the budget put a dedicated guide camera on the back. Thats what I did and got 15min subs with my kit in my sig.

[Jon Sales]

Thanks for the info. Very helpful

[ollypenrice]

Oh you are looking at a triplet? Then that has the advantage hands down over the doublets. If you have the budget for it then that's the one you would want. Remember you might want to budget for a reducer. You wont need a flattener as the triplets are already flat. So you could image with or without a reducer to change your focal length and thus your FoV to allow larger objects to fit in on your sensor. Do forget the T-adaptor to hook up the DSLR as well. 

I'm not sure about this particular scope but what is a common trend among most scopes is the focuser usually isn't up to par for AP. It tends to slip under the weight of the camera, especially when imaging high targets. Now since you are only using a DSLR you probably shouldn't have to worry but if you switch to CCD you might need to upgrade the focuser. 

Another thing you will want to budget for, eventually if not very soon, is guiding. Even at the short focal lengths you will be limited to about 2min subs without guiding and that will require spot on polar alignment and balance. I worked like this for a year and got some great shot so you can do a lot without guiding but you'll get to a point where you will want it. Especially if you ever want to image the faint nebula and the detail as that require longer subs. The cheapest is a modded 9x50 finder scope with a modded web cam attached to it or if you can push (or save up) the budget put a dedicated guide camera on the back. Thats what I did and got 15min subs with my kit in my sig.

Careful, triplets aren't flat. No refractor with a front-only objective can give a large flat field. A rear element is essential. The triplet advantage comes in colour correction. While a good triplet beats a good doublet a moderate triplet doesn't have to beat a good doublet. I think you have to look at scopes on an one by one basis. A TV85 doublet with flattener would, in my view, easily beat many cheaper Chinese triplets.

What about the TS/Altair etc 65 quad? This is now performing well, it seems, is good value and, being a Petzval, does away with the hassle of getting the chip distance just right for the reducer. You just pop the camera on the back and focus. Very relaxing! 

Olly

[nmoushon]

Careful, triplets aren't flat. No refractor with a front-only objective can give a large flat field. A rear element is essential. The triplet advantage comes in colour correction. While a good triplet beats a good doublet a moderate triplet doesn't have to beat a good doublet. I think you have to look at scopes on an one by one basis. A TV85 doublet with flattener would, in my view, easily beat many cheaper Chinese triplets.

What about the TS/Altair etc 65 quad? This is now performing well, it seems, is good value and, being a Petzval, does away with the hassle of getting the chip distance just right for the reducer. You just pop the camera on the back and focus. Very relaxing! 

Olly

Thanks for bring that up. I honestly thought the third element was a flattener. My mistake.

[Jon Sales]

Thanks for the advice Olly.

Want to photograph the main Messiers etc, not interested in the the fainter stuff at the moment.

Would anything less than an 80 have enough light grasp? Or is it about the f ratio?

I ask this because there is a 70 ED I've seen advertised as an AP package.

[nmoushon]

For AP aperture doesn't mean you will get more light faster to your camera sensor. For visual more aperture will get more light to your eye and really improve things. For AP f/ratio is what you are wanting. 

[ollypenrice]

You'll find that you can go nice and deep with small apertures and a fast F ratio on extended targets. Essentially it's small targets occupying less than the full chip which benefit from aperture.

However, from experience, when you start to go below about 80mm the star sizes tend to go up perceptibly. (They continue to go down the larger you go in aperture but on the popular targets it's a case of when it starts be become obtrusive.) Used in narrowband the small apertures (like my WOZS66) produce very tight stellar images. But in broadband, one shot colour etc, the stars are large and rob the image of sophistication. 

By far the best thing is to look at the posted results but beware of the effects of the processing, of course.

[martin_h]

I have both the ED80 and the WO ZS71, of the two the ED80 is the better corrected, the WO suffers from 'blue bloat' quite a bit, as I mainly narrowband image that doesnt bother me too much tho.

[Skyline]

If your on a budget, you cannot go far wrong with a 80ED Doublet, even at f6 or f6.37. 

Even though decent small aperture triplets are the holy grail. I would like to think a somewhat tried & tested 80ED doublet would be better then a poor/mediocre performing triplet.

Al.

[Alien 13]

The choice of scope has a lot to do with what you enjoy imaging the smaller fracs tend to have larger FOV much like a camera lens they do suffer more from blue bloat but i have never found it much of an issue and is no worse than some top end camera lenses.

The benifit for me is nice widefield shots which i love its easy on the mount and i can take it out birding etc.

Alan

[Jon Sales]

I'll have to do a search online and see if I can compare some images taken through them.

I know the 80ED is very popular for AP, just thought an extra lens would make things better all round.

Thanks for all the input everyone.

Might have a closer look at the Altair Astro ED's too.