Refractors - large diameter vs high quality glass

[hjw]

Hi,

I have started to read about telescopes and I am interested in imaging of DSOs. When it comes to refractors I noticed that you can get a 150mm achromatic refractor for about the same price as a 80mm doublet APO refractor (3.5 x the opening area!).

I do understand that the APO lenses reduce the chromatic aberration but on the other hand with DSO you want as much light as you can gather. I think I know the answer will be go for the better glass but I would like to know why. Is the CA really that bad that it will ruin the image. Since I can't compare the two personally it would be great if someone had images to compare the qualities.

Cheers

HJW

[Dave In Vermont]

Chromatic aberration - CA - from a refractor can be removed during the processing of the images. The amount of CA that a given refractor will be prone to introduce is dependent mostly on the F/ of the instrument. An F/5 will have more than an F/8, etc. If the amount of CA present while you use the scope for visual observing, there are specialized filters available to reduce this. People have their preferences. I personally use a Baader Fringe Killer. It works very nicely on my ST80 F/5 scope.

Hope this helps,

Dave

[Moonshane]

The problem with big scope is you need a huge mount to maintain stability for the long periods needed for imaging. The main things to get right with dso imaging are mount, speed of scope (faster is usually better) and the focal length - try to match image scale with chip size.

I am no imager but recommend you do more reading on why the vast majority of good imagers use small fast refractors

[D4N]

Larger apertures tend to have a longer focal length too, this make guiding and alignment more critical, short focal lengths are much more forgiving.

A 70mm F5 APO will gather light just as fast as a 150mm F5 achromatic frac, the 150mm will just give a smaller FOV. Basically the 70mm may have a much smaller aperture but it is gathering light from a larger area of sky.

Sent from my iPad using Tapatalk

[ronin]

For imaging another factor comes in and that is that an achro tends to have a longer depth of focus so there is no nice sharp focal plane at which to position the sensor. Useful to have a sharp image when imaging.

One other factor is that a doublet even if ED is not an apo.

An ED doublet will likely still show some CA especially if you have a nice bright star in the image.

The requirement for an apo is to control 3 wavelengths and being a doublet an ED doublet can only control 2 - one wavelength per bit of glass. I have the impression that saying apo allows them to add 30% to the cost of something that is not.

Also to make matters more amusing I am not aware of where ED glass starts and non-ED ends. It depends on the Abbe number but no-one has drawn a line saying above this they at  ED and below they are not. Generally people accept FPL-53, FPL-51, FCD1 and H-FK61 as ED glass, not sure what the Russian glass designations are but they make some nice optics.

Over time what we have as ED glass has been called LD (low dispersion) SLD (I guess Super) and FPL-53 is at times called Super ED, probably in an attempt to seperate itself from the common everyday ED glasses, and then add 30% to the cost as you now have a Super ED glass not an ED glass.

So make sure you know what you are getting and eventually get what you want.

[michael.h.f.wilkinson]

The key thing is focal ratio when imaging resolved (larger) DSOs. What counts is the amount of light per pixel and that is given by the focal ratio. An F/8 150mm achromat will be a lot worse than e.g. my 80mm F/6. The latter requires just over half the exposure time of the former. Add a focal reducer (0.8x) and my scope is F/4.8 making exposures shorter and guiding even easier. A 150mm F/5 like the ST150 is nice and fast of course, but will show horrible CA.

Chromatic aberration - CA - from a refractor can be removed during the processing of the images. The amount of CA that a given refractor will be prone to introduce is dependent mostly on the F/ of the instrument. An F/5 will have more than an F/8, etc. If the amount of CA present while you use the scope for visual observing, there are specialized filters available to reduce this. People have their preferences. I personally use a Baader Fringe Killer. It works very nicely on my ST80 F/5 scope.

Hope this helps,

Dave

I disagree with this. CA is not easily and certainly not totally removed during processing. You can mitigate the effects by only going for narrow-band imaging (H-alpha, O-III, S-II is a common combination). Each of the narrow bands will not show CA, but when taking the shots care must be taken to refocus when switching bands. When combining them into a colour image, you may need to scale the images with respect to each other, to counteract slight differences in focal length (and therefore magnification or image scale) at different waevlengths.

[Zakalwe]

Chromatic aberration - CA - from a refractor can be removed during the processing of the images.

Im very doubtful of this claim. especially if you are using a colour cam. An achromat cannot bring all colours into focus on the same plane, therefore one wavelength will be in focus when others aren't. Out of focus isn't something that can be fixed in post-processing.

If an achro could be used for imaging then why would anyone bother buying an apo??

[hjw]

Thanks for the replies,

when I was looking at telescopes I was tossing up between a SW ED80 APO with 600mm focal length (f/7.5) or the ED100 APO with 900mm focal length (f/9). With the latter I would probably need a focal reducer at times. Than I came across another Skywatcher scope with 150mm and 750mm focal length (f/5). The last one got me thinking... If can get the fastest scope for the lowest price, how bad will the added CA effect the shots. As a mount I have settles on the HEQ5 (I think ). The mount should be able to handle them all.

Cheers

HJW

[Christopher Davenport]

Hey I use the ed80, to good effect.

You will definitely also need a reducer flatterner with it. SW make a optically matched one with is pricey.

I use a generic one as it is cheaper and faster but I do need to crop my images and do lens correction in processing.

[michael.h.f.wilkinson]

The SW80ED is a great scope by all accounts. You can also put a focal reducer in that one to gain speed. The ST150 (150mm F=750) has the worst CA of the entire ST line (CA increases rapidly with aperture). Nice scope for visually sweeping DSOs at low magnification, but not good for imaging, or even planetary viewing. A 6" F/5 Newtonian with coma corrector would do better, but it would need collimation and does give diffraction spikes.

[DaveS]

Or you could look at the Equinox apo which is a bit faster. I'd steer clear of the 150s, especially for imaging, and even more so on a HEQ5. Yes it will handle them for visual but with that mass and moment arm, not a chance for imaging.

There's a reason why so many go for the 80-90mm ED on an HEQ5, it just works.

If you can push the boat out look for a 80mm triplet, something around f/6 perhaps.

[Tinker1947]

You could go for a Newt like the 150mm size it would need a Coma Corrector but there not to heavy, fast'ish F/length cheap and if you don't get the imagine bug easy to sell....plus a HEQ5 Mount.....

http://www.firstlightoptics.com/reflectors/skywatcher-explorer-150p-ds-ota.html

[hjw]

Thanks again,

it seems it is back to Plan A. If the 80mm is the ideal size for the mount I might have a look what else is there. The only problem is, that there is only a limited variety available in Australia. Thank god for the internet! I had an idea from the start that the better optics would win at the end, however, it was good and very informative to read your comments. And now, that we have against expectations a clear night in Melbourne, I shall try to take some more pictures.

Cheers

HJW

[ollypenrice]

I can't process out CA from images and I pretty much do it for a living these days. I can reduce it but the loss of resolution is still there across the board, not just in star haloes.

Michael's point that it's the light per pixel that counts is an elegant explanation and avoids the pitfalls of discussing the matter via F ratio, which is a minefield.

Olly

[Dave In Vermont]

I read an article that said that you can - but it didn't go into specifics. It was found in another forum. But I'll withdraw my former statement. Sorry!

But I'll try retracing my steps and will post same if I can find it again. I seem to recall it spoke of monochrome being used for the initial imaging, and then applying colour.

Clear Skies,

Dave (Finally a beautiful night coming up!)

[Ruud]

I often do a chromatic aberration correction in Photoshop for my camera's lenses. It makes for a better fit of the red, green and blue channels of the image. It can only fix lateral chromatic aberration, not longitudinal.

With lateral chromatic aberration, the red, green and blue images have a different image scale and/or different image distortion. With longitudinal chromatic aberration, the red, green and blue images are focussed at different focal lengths.

The other day, I was looking through this 80mm f/7 ED doublet and could not find a trace of CA with the 8mm eyepiece I was using. It was really much better than this beautifuuly built 71mm f/6 ED, which threw a rainbow at every contrasty edge.

I would like to have an ultra portable 70mm or so refractor, but have a hard time finding a good one.

[rwilkey]

I get absolutely no CA when using my ED100 doublet, but I do just visual, whether this would be different for photography I don't know.  I don't understand why some say there is CA in doublets, one supposes they are just guessing.

[ollypenrice]

FIrstly cameras are more sensitive down towards the UV than the eye, so a scope wich is visually apochromatic may be far from that photographically. My Mk 1 F5 Pearl River TV Genesis was a classic example, stunning visually but not photographcally corrected to the same degree. It was never designed for digital imaging.

The colour correction of doublets also breaks down in proportion to the speed, so the slow ED100 is particularly good visually but the speed is against it photographically. The faster the refractor the harder it is to correct, and indeed the bigger it is the harder it is for a given F ratio.

I think that when folks talk about processing the artefacts from badly corrected scopes they are talking about correcting the stars, in the main. But all the detail is out of focus in whatever channel we are talking about. (Usually blue.) Using a programme which resises and re-curves layers when it registers them is a help. Pixinsight and Registar come to mind.

Best to get a real apo though! It comes at a price but there really is only one top dog and it is way ahead of the rest in its aperture class. The Takahashi FSQ85ED. Very few scopes have no rivals but I know of none for this one. (Nor does any other come at such a price!)

Olly

[hjw]

Thanks everyone,

Just a word of caution, related to my personal set of circumstances. This will be my first "decent" set of gear. While I can appreciate the beauty of a real triplet APO, my wife can not and neither will my bank manager. I guess it is a bit like buying your first car - you go for something you can afford and any argument that a merc or a bmw is a better car does not make it a more suitable car for this moment. There has to be room for improvement .

[DaveS]

OK, so that we don't go recomending the Ferraris of the telescope world rather than the Fords, what sort of budget can you stretch to?

[DaveS]

One of these perhaps?

http://www.altairastro.com/product.php?productid=16493&cat=270&page=1

Plus a flattener perhaps.

http://www.altairastro.com/product.php?productid=16597

[Macavity]

I have started to read about telescopes and I am interested in imaging of DSOs. 

If you want to image DSO's in a conventional (progressive) way, there seems little escape

from convention! (Roughly) Get a big EQ mount and a small (rather expensive) APO? 

If you could be satisfied with lesser imaging achievements, but want to (seriously) dabble, 

you then have access to a wider *variety* of scopes that might be useful in other areas...

I started astronomy in a different way to many? I eschewed (heheh) a standard Dobsonian, 

and wanted to see e.g. what a fast Achromat (ST102) and a Makutov (MAK127) might do.

Via these, I "worked my up" to an "interesting" Fast Newt - As VIDEO imaging "light bucket".

Nothing is simple! But nothing is cast in stone? Some experiment might be inevitable...

As a general idea re. relative performance of achromatic refractors:

(False colour might not be the only problem with FAST achromats)

An F8 ST150 might be of similar performance to an F5 ST102!

[RikM]

Skywatcher ED80 plus flatner reducer is a real peach. You have to pay a lot of money to get better results. If you are using a DSLR rather than CCD, you might also consider a 130P or 150P Newtonian with a coma corrector. The faster focal ratio offers some benefits for the lower sensitivity cameras but they do require a bit of tinkering to get the best out of them.

[hjw]

Skywatcher ED80 plus flatner reducer is a real peach. You have to pay a lot of money to get better results. If you are using a DSLR rather than CCD, you might also consider a 130P or 150P Newtonian with a coma corrector. The faster focal ratio offers some benefits for the lower sensitivity cameras but they do require a bit of tinkering to get the best out of them.

I think, that will be my setup (SW ED80 + HEQ5mount). I would like to stay away from Newtonians because the way I understand it, they are either set up for imaging or for observation, but not for both. In addition collimating them frequently sounds daunting. I would then sink additional funds into guiding optics. By now I feel guilty that I caused so much fuss. Thanks to everyone who answered my query. You get all these puzzle pieces and eventually you see the picture  !

[RT65CB-SWL]

OK, so that we don't go recomending the Ferraris of the telescope world rather than the Fords, what sort of budget can you stretch to?

I think someone beat you to it...