If you don't see the merit in this then - that's okay. Just don't spoil it for me, 'kay?
I didn't try to spoil it for you..
I just tried to stress the fact that no instrument is ideal, but all of them are useful for this or that purpose.
You will have to decide for yourself what's good enough for you - so going for cheap(er) option is not necessarily bad step for the start.
The worst scenario is spending a lot of money and discovering that even APO has limitations.. because of expectations set too high.
I am still using my MTO1100A for nebulae and I am happy with results.
I can't post full frames here because of the 200kb limit. The full frame tiffs are about 100mb!
They are not single frames but the circumstances are similar. Same goes for post processing.
Since we are comparing apples to oranges anyway, the thing that matters most is to get focal length and f ratio as close as possible, which I have done, eg: the 71mm is 400mm f5.6 and the ED80 440mm f5.5. The 4 inch achro is 550mm f5.5 roughly. The 4 inch achro is a crop of another image to illustrate red halos.
There was no filter in the focal path of any of the scopes, all straight through.
Here's a full frame from the 4 inch achro. It's a Ricoh. It's also > 20 years old. But.... the 4 inch has a Lumicon Deep Sky filter in the focal path as I shoot mostly with filters.
Using the same photo, I've split the channels into red and green to explain more about focus and achromatic refractors.
You will see in the red channel the stars are all bloated and look like blobs. They are out of focus.
With the green channel the stars are all tiny and well focussed. Why? Well human eyes have peak sensitivity at green, so I focussed my camera for my eyes.
If I had somehow managed to focus the red channel properly, (which I could have by taking short exps to test) then the green channel would be out of focus.
Such is the life of a fast (F5) achromatic refractor. They improve as the F ratio rises, say to F15, or if special glass is used to make the objective. Hence why everyone loves APO's.
Edit, an achromatic refractor can still take good pics with all colours focussed, but you have to shoot each colour separately and combine them later in software. Or be happy with monochrome.
thanks a lot for the explanation and the uploads, too.
Your words together with your examples already help a lot.
while "20 years" is not "new-ish"
and the fact that these are stacked and post-processed images from long exposures do not meet my criteria,
(I would not be able to reproduce your skills this well so I can't tell whether or not I'd be happy with my result - hence the "single frame" etc. criteria)
I don't dislike the full frame image. Very useful for me. Thank you.
- on that note:
could you guys please help in the way I asked for help? thanks a million.
Now, I'm really - and even hopeful - looking forward to images from new-ish achro scopes in comparison to APO.
Mostly unedited,
single frames
of short-ish exposures
taken through new-ish achro
and apo refractors
of 80mm +
with little to nothing else in the imaging path. If anything, please specify.
Object: Anything goes.
And if you could list the model and age of your refractors, too? Awesome!
while "20 years" is not "new-ish"
and the fact that these are stacked and post-processed images from long exposures do not meet my criteria ..
You have to understand what simple processing (stacking) does to a picture:
1) it reduces the noise introduced by camera
2) it reduces the effects of the atmospheric turbulence
3) it does not affect much the local visibility (LP, transparency and so on)
4) it does not affect the optical operformance of the system
Also, you will find (in time) that 20 years or even 50 does not mean much when optical design is concerned - APO from 20 years ago is very similar if not the same performance-wise to APO design done with the help of all that high performance computer software from today - because it uses the same mathematical solutions, that were done 100 year ago, and it uses the same glass materials...
The difference from 20 years ago and today is, today the design of optical systems is faster (because it runs on computers.. 20-30 years ago it was done on a piece of paper with the help of slide rule and pencil and book with logarithmic tables) and manufacturing is today somewhat cheaper. But the assembling and testing is not - it is still done manually and individually for each product.
Also, some new exotic optical materials are available today that were not on the market 50-100 years ago, but optical elements made of those materials are still very expensive, and most likely not affordable by most (myself included).
So, reasonably processed images will show you what you want and need to know - is APO better than achromat and by how much.
Kevin's images (post #39 of this thread) are exactly what you needed - side by side picture of the same object (star-like) and there you can see chromatic aberration of the achromat at work.
What is the exact reason you want the comparison? Do you want to save money buying a new achromatic scope instead of an APO? Do you think a new achromat will be better than a cared for 20 year old achromat? Do you want to use it for visual, photographic or both?
For me, I know what I'm in for if I buy a new fast f ratio achromat. I reckon I could live with it for wide field visual use if combined with a fringe killer or semi-APO filter. But for me, for single shot colour photography they are not good enough. Your taste may be different. If you can live with one colour being constantly out of focus like in post #45 then go for it!
Just to clear things up, basically you believe a new achromat will be superior to an older model. You want to use it for both visual and photographic use. You will be shooting single shot colour. You want performance similar to an APO.
Just to clear things up, basically you believe a new achromat will be superior to an older model. You want to use it for both visual and photographic use. You will be shooting single shot colour. You want performance similar to an APO.
Correct?
I think Silv wants to know if the new-ish achromats can will give her the results (when compared to a "more expensive" APO of similar size/FL) that she can live with primarily for imaging as well as some visual work
I think her request is pretty good as it allows newbies to get a "feel" of how things work out in the real world without having to make the "mistake" by spending the money first to find out that an ahromat will not cut it based on one's own expectations - however there is nothing wrong at all if one is happy to accept that certain colors will always be out of focus and the $$ spent is within one's means!
That I understand, and is fair enough. My story is similar. Let me explain from my point of view.
Optical physics hasn't changed over the last 20 years. Present day achromatic doublet refractors bear the same disadvantages that they did 20 years ago. If this wasn't so we wouldn't bother buying APO scopes. If everyone hasn't done so already, google "refractor chromatic aberration".
In past years of "classic" refractors, we would never see fast F5 optics. They were usually F11 - F15. But now China has decided to flood the world with big lens diameter cheap F5 refractors. While they are awesome to look at, they are not so awesome for astronomy! I think they may be more directed at terrestrial use.
In the days of film we could get away with it. Film was slow, grainy and lacked the resolution of digital. The funky coloured bloats were absorbed in the process and never made it to print. Enter the digital world and everything has become more critical.
Basically that's my story. I used to shoot deep sky on ISO 800/1600 film with fast optics. The fringes never showed up on print. Then I went digital and the chromatic aberration of the achromats was there in all their glory!
Like Silv I wanted a cheap way out. Because I've been doing astrophotography for something like 30 years+, I didn't want to have to abandon my old achromats. They cost too much back then! I did some major research to find a loophole for single shot colour. What I found was this. There are compromises. One, forget fast F5 optics and go with something with a higher F/D ratio, the higher the better to reduce CA. Problem. F11 - F15 has reduced CA but is painfully slow for photographing deep sky. Great for visual use though. Some claim a good F15 achromat has similar CA to a F7.5 APO.
If you want a cheap achromatic scope for visual use only, get a long tube high F/D ratio type. They work better at higher power and have less CA.
For single shot colour photographic use for deep sky, there is no loop hole. I buckled and bought an ED80.
however, since I have read enough elsewhere about how bad achros are, oh, so bad!, but never seen images made with achros that are currently on the market, and can't compare those never seen images to never seen images taken with Apos,
I'd like to propose a change of rules in my thread:
attach an image to each of your posts.
an image that meets the help requirements:
Quote:
Mostly unedited,
single frames
an uncropped version, too,
of short-ish exposures
taken through new-ish achro
and apo refractors
of 80mm +
with little to nothing else in the imaging path. If anything, please specify.
Object: Anything goes.
And if you could list the model and age of your refractors, too? Awesome!
