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An APO triplet might be worse than an achro doublet for imaging?

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#1 AP-Photo

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Posted 23 February 2021 - 12:13 PM

Ok, controversial topic title aside, I was thinking...if someone is imaging EXCLUSIVELY in narrowband (mono sensor, narrowband filters), it seems to me that there is no advantage to an APO triplet, since focusing all wavelengths of light at the same point with an APO is a moot point when you only care about a specific wavelength for each sub exposure. Furthermore, assuming air-spaced doublet/triplet, the triplet would have more surfaces for reflection/transmission loss, and therefore might be at a disadvantage relative to a doublet of otherwise equal quality.

This argument obviously falls apart if you use the same scope for OSC/wide band images, and probably also for LRGB images with a mono camera (especially the luminance part), but if someone was shooting hubble palette only, for example, it would seem that they might be able to save some cash without compromising on image quality based on this logic. Am I crazy? I'm new at this, so I just might be, but for someone in a light polluted area shooting only narrowband this topic might be informative.

Thanks for any thoughts!
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#2 Robindonne

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Posted 23 February 2021 - 12:36 PM

im not sure if you end up with all "equal" images


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#3 sg6

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Posted 23 February 2021 - 12:37 PM

Trouble is why make a bad triplet, so bad that the usual doublet is worse.

Any idiot can make a bad triplet, just take the triplet you have an alter the element spacing say 0.5mm and you have a bad triplet.

 

The catch is many do use doublets, I know one that does and he understands and accepts the "negatives"  of it, but at some stage you have to halt it all and refocus. If say OIII and Ha are at the same focal plane then the mid of 550nm will be out of focus. So you have to redo the focus. It takes time and can lead to something going wrong - of course it NEVER does lol.gif  lol.gif  lol.gif Have you ever seen a Youtube video of anything going wrong?

 

Save money get a nice doublet and expect to in effect pay some other way - time being the common one. It is your time.

The old saying is "You don't get anything for nothing" is one of the few really applicable rules in astronomy and AP especially. People still try.


Edited by sg6, 23 February 2021 - 12:37 PM.

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#4 Tapio

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Posted 23 February 2021 - 12:39 PM

Not the first time this has been discussed.

Here's one of the threads from CN:
https://www.cloudyni...dard-refractor/

 

So yes it can be done up to a certain point but there might be a problem finding a flattener for it.


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#5 drd715

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Posted 23 February 2021 - 12:39 PM

Achros work well for narrow band imaging. It still comes down to quality of the lens correction and smoothness of optical figure. So a super cheap achro may not make the grade of optical perfection needed. Other considerations are the quality of the focuser and the draw tube size (larger for lower F numbers) needed for imaging. And considering the cost of narrow band filters plus accessories, the lower cost of a simple achro may not be a huge savings. But it should work well with a high quality achro with supior optics and better focuser. My choice would still be a doublet using ED glass and a higher standard of construction/figure. Reference the Stellarview 102D and 127D scopes coming out this year. Feather touch focusers and .99+ strel figure.

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#6 SeattleScott

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Posted 23 February 2021 - 12:56 PM

The problem is the high quality achros tend to be slow. Best example of a good achro for narrowband is probably a Japanese Vixen NA140. But then it isn’t all that cheap.

Scott
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#7 AP-Photo

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Posted 23 February 2021 - 01:47 PM

Thanks all for the replies! I actually had the Stellarvue doublets in mind when asking the question. (Actually I have an SVX102T-R, so I'm not really asking on my own behalf, but for a friend who lives in a Bortle 7, and is interested in Stellarvue.) With these scopes in particular, it seems like good optics and accessories are reasonable assumptions. I haven't looked at the speed/focal ratios of those doublets, though, and faster is obviously better for imaging.

Clear skies!
Adam
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#8 SeattleScott

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Posted 23 February 2021 - 01:57 PM

Thanks all for the replies! I actually had the Stellarvue doublets in mind when asking the question. (Actually I have an SVX102T-R, so I'm not really asking on my own behalf, but for a friend who lives in a Bortle 7, and is interested in Stellarvue.) With these scopes in particular, it seems like good optics and accessories are reasonable assumptions. I haven't looked at the speed/focal ratios of those doublets, though, and faster is obviously better for imaging.

Clear skies!
Adam

Yes that’s the catch, the doublets tend to run slower. A good solution is the Vixen doublets because they have an aggressive 0.67x reducer that gets down to around F5-5.5. So you get a doublet with an excellent figure and it is fast enough for imaging. The current SD models support full frame imaging.

Looks like the SV doublets are F7 versus Vixen at F7.7 so with reducer you end up in about the same place. The Vixens should have less CA in theory but doesn’t really matter for narrowband.

Scott

Edited by SeattleScott, 23 February 2021 - 02:02 PM.

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#9 GSBass

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Posted 23 February 2021 - 02:03 PM

Things seem to be changing rapidly with the quality that can be achieved with mass produced telescopes, we may not be many years away from all of them being near perfect..... as is I think the general assumption is the higher dollar the glass used, the better chance of the figuring will be done right.... and that probably applies more to high end doublets than triplets since many manufactures still make triplets with cheap glass, they probably can get away with more errors and just go for good enough


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#10 ngc7319_20

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Posted 23 February 2021 - 03:41 PM

Two more points to mull over:

 

Field-flattened -- whichever you choose, you will need a good flattener for it.  

 

Wavelength-dependent spherical aberration:  even if you can re-focus the achromat, you may still have spherical aberration to worry about.  Not saying it will be a problem, but it needs to be modeled and considered.


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#11 SeattleScott

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Posted 23 February 2021 - 06:06 PM

Things seem to be changing rapidly with the quality that can be achieved with mass produced telescopes, we may not be many years away from all of them being near perfect..... as is I think the general assumption is the higher dollar the glass used, the better chance of the figuring will be done right.... and that probably applies more to high end doublets than triplets since many manufactures still make triplets with cheap glass, they probably can get away with more errors and just go for good enough

Mass produced refractors are probably about as good as it will get, barring a future price reduction in FPL53 equivalents.

The process is already automated. The premium brands use a computer controlled grinding machine to polish lenses. So do Meade, Celestron and Skywatcher. Do you think a boutique refractor company can afford a better or smarter grinding machine than Skywatcher? Not likely. So why do expensive premium brands have better polish. Because they grind the lenses longer. Then they test them and touch up any weak spots until it meets a quality standard. Really probably the same thing Skywatcher does, just with a different quality standard. Then there is the tweaking of the spacing of the lenses with premium brands to get that last little bit of perfection. So with a premium brand, you are paying for potentially better components/build quality, more polishing time, and more manual intervention to evaluate and tweak the performance to a higher standard. None of this is stuff that can easily be automated. What can be automated has pretty much already been automated. It isn’t a matter of Skywatcher hacking TEC and stealing the secret formula for precision optics. Skywatcher and other mass production telescope makers have all the same tools at their disposal already. They can already make premium quality Apos that compete with premium brands. Problem is they end up costing about as much as the premium brands.

So there probably isn’t much point in waiting around for the mass produced scopes to catch up to the premium brands, but at more typical mass produced pricing. Maybe there will be some cheap glass developed that rivals Fluorite? I suppose anything is possible. But probably not in the next decade. Sort of like Gen III night vision. If the price was $500 rather than $3,000-4,000, I would be all over it. A decade ago it was $2,500-3,000. I thought that’s too expensive, maybe wait and see if the price drops. Nope, just more expensive now, and I missed out on a decade of using night vision. Not that I have it now, just using NV as an example. If you really want a premium Apo, your best bet is to just go ahead and buy one rather than waiting a decade to see if the Chinese will be able to make something just as good for a fraction of the cost. Because most likely, a decade later, the Chinese will be able to make something about as good. For about the same price as premium brands. Just like now.

Scott

#12 GSBass

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Posted 23 February 2021 - 06:45 PM

I was not suggesting anyone wait, just seems inevitable the cheap scopes will continue to improve, technology marches on

#13 SeattleScott

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Posted 24 February 2021 - 01:18 AM

It does, although generally slowly. Are mass produced Apos (or scopes in general) better today than in 2011? I don’t know that they are. A few new offerings. You see some FPL53/La doublets that didn’t seem to be around much before. But in terms of optical quality, most stuff seems about the same to me. Now compared to 2001, yeah, I would say things are noticeably better now. Compared to 1991, things are quite a bit better.

Scott

#14 Jon Isaacs

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Posted 24 February 2021 - 05:21 AM

Thanks all for the replies! I actually had the Stellarvue doublets in mind when asking the question. (Actually I have an SVX102T-R, so I'm not really asking on my own behalf, but for a friend who lives in a Bortle 7, and is interested in Stellarvue.) With these scopes in particular, it seems like good optics and accessories are reasonable assumptions. I haven't looked at the speed/focal ratios of those doublets, though, and faster is obviously better for imaging.

Clear skies!
Adam

 

A doublet is not necessarily an achromat.  In general usage, an achromat is a crown-flint doublet with a longitudinal color error of about 1 part in 2000.  Many of the replies to this thread seem to be discussing these standard achromats.

 

ED doublets can have longitudinal color errors from about 1 part in 6700 to maybe 1 part in 12000. These are very different scopes.

 

Early on, StellarVue sold standard achromats but that's been years.

 

Color correction overall depends on many factors including aperture and focal ratio, smaller and slower make for better color correction.

 

A small ED doublets have been used for many fine photographs.

 

Jon


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#15 GSBass

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Posted 24 February 2021 - 06:11 AM

If I had to guess I think the next breakthrough will be what you eluded to earlier, Inexpensive glass that shares or exceeds what we consider the best today, once that happens it may justify the the labor involved to improve the specs. It does come down to economics. As far as price goes though, we may be entering a period of inflation soon, the actual dollar amounts we consider good deals may go up substantially

It does, although generally slowly. Are mass produced Apos (or scopes in general) better today than in 2011? I don’t know that they are. A few new offerings. You see some FPL53/La doublets that didn’t seem to be around much before. But in terms of optical quality, most stuff seems about the same to me. Now compared to 2001, yeah, I would say things are noticeably better now. Compared to 1991, things are quite a bit better.

Scott



#16 GSBass

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Posted 24 February 2021 - 06:37 AM

A doublet is not necessarily an achromat.  In general usage, an achromat is a crown-flint doublet with a longitudinal color error of about 1 part in 2000.  Many of the replies to this thread seem to be discussing these standard achromats.

 

ED doublets can have longitudinal color errors from about 1 part in 6700 to maybe 1 part in 12000. These are very different scopes.

 

Early on, StellarVue sold standard achromats but that's been years.

 

Color correction overall depends on many factors including aperture and focal ratio, smaller and slower make for better color correction.

 

A small ED doublets have been used for many fine photographs.

 

Jon

it’s been many years since I researched refractors, if you had asked me a couple of months ago I would have thought all apo scopes were triplets, it’s pretty amazing to me that many doublets can perform so well. But what’s also clear is that a perfect scope is still unaffordable to most and the prices of the premium scopes are insane now, I’ve seen many I use to look at sell for over 1k more than they did not that long ago. New people getting in to the hobby probably don’t realize how lucky they are that the mass produced scopes have gotten so good. I still find it almost humorous that you can walk in to Walmart and by a tv for 300 dollars that outperforms in every metric what people paid 10,000 for 20 years ago


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#17 zirkel 2

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Posted 24 February 2021 - 08:41 AM

For my Achromat refractor i chose a model better finished in optics and mechanics, all with a controlled budget.

My choice : TS 102 f / 11.

Mechanically nothing wrong with a CNC crayford focuser and retractable dewcap (plastic not present anyway!).

Optically a doublet with Ohara glasses, probably S-BSL-7 Crown and S-SIM-2 Flint, optics in a compensated aluminum cell with lateral collimation possible (but not easy to access) .

I do not regret my choice, in terms of quality we are above the standard range for this type of instrument.

Juste visual use.


Edited by zirkel 2, 24 February 2021 - 08:44 AM.


#18 Alterf

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Posted 24 February 2021 - 05:55 PM

I used to do narrowband with an Orion 120 achromat f/5 reduced to f/4. Here are a few examples.

 

https://polarisb.blo...-12-1-2010.html

 

https://polarisb.blo...-12-1-2010.html

 

https://polarisb.blo...and-9-2010.html

 

One problem I ran into when switching colors: The focal length of deep red (Ha, SII) and blue-green (OIII) was different enough that stars had halos; the different colors were at a different scale enough to cause that.  I got so I could process it out, but it was a pain.  Monochrome imaging worked great, though.

 

Val


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