Triplet Lens -vs- Doublet Lens
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acf900gto@...
As we know, one of the benefits of a triplet lens is that it brings the three primary colors to a single focus while a doublet lens can only bring two of the primary colors to a common focus. The third forms a kind of blur circle around the other two. However, if I am doing narrow band imaging with something like a 5nm band pass Ha filter, I am only bringing a single "color" to a focus. So, it would seem that it would be possible to do quality narrow band imaging with a good quality ED doublet lens. A 6" ED doublet refractor is certainly a lot more affordable than a 6" ED triplet! Is there any reason to believe an Ha image from a doublet would be of lower quality than an Ha image taken with a triplet? Charlie |
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I would think if you plan to combine the NB data later into a false colour image you’ll have the same problem. If your goal is pure B&W then I’d guess you’d be fine. Stuart
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Stuart Heggie |
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George OBrien
I don’t mean to butt-in, but I think Charlie is on to something. Each NB image will have been focused independently. There would be no chromatic aberration because each set of images will be in focus, albeit on what would have been different focal planes. Registering and combining the images would seem to work fine.
I think this makes sense, coming from an old physicist. Clear skies, George From: Stuart Heggie stuart.j.heggie@... [ap-gto] Sent: Friday, August 17, 2018 2:08 PM To: ap-gto@... Subject: Re: [ap-gto] Triplet Lens -vs- Doublet Lens I would think if you plan to combine the NB data later into a false colour image you’ll have the same problem. If your goal is pure B&W then I’d guess you’d be fine. Stuart On Fri, Aug 17, 2018 at 12:43 PM acf900gto@... [ap-gto] <ap-gto@...> wrote: As we know, one of the benefits of a triplet lens is that it brings the three primary colors to a single focus while a doublet lens can only bring two of the primary colors to a common focus. The third forms a kind of blur circle around the other two. However, if I am doing narrow band imaging with something like a 5nm band pass Ha filter, I am only bringing a single "color" to a focus. So, it would seem that it would be possible to do quality narrow band imaging with a good quality ED doublet lens. A 6" ED doublet refractor is certainly a lot more affordable than a 6" ED triplet! Is there any reason to believe an Ha image from a doublet would be of lower quality than an Ha image taken with a triplet? Charlie -- Stuart Heggie http://www.stuartheggie.com/featured.html [Non-text portions of this message have been removed] |
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George
Remember…since you are refocusing for each image the image scale is changing.
Regards,
George
George Whitney Astro-Physics, Inc. Phone: 815-282-1513 Email: george@...
From: ap-gto@... [mailto:ap-gto@...]
Sent: Friday, August 17, 2018 1:47 PM To: ap-gto@... Subject: RE: [ap-gto] Triplet Lens -vs- Doublet Lens
I don’t mean to butt-in, but I think Charlie is on to something. Each NB image will have been focused independently. There would be no chromatic aberration because each set of images will be in focus, albeit on what would have
been different focal planes. Registering and combining the images would seem to work fine.
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George, I was thinking the same thing. The image scale changes very slightly. Stuart
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Stuart Heggie |
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Michael Fulbright <mike.fulbright@...>
Any decent software for image registration and stacking will rectify
any slight image scale difference.
toggle quoted message
Show quoted text
Michael Fulbright On 8/17/2018 3:11 PM, Stuart Heggie
stuart.j.heggie@... [ap-gto] wrote:
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My memory of using Maxim was that it only handled rotation and translation. Registar handles scaling. Not sure about PixInsight. Stuart
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Stuart Heggie |
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Antoine PAVLIN
PixInsight does rescale images.
Clear skies
Antoine
De : ap-gto@...
My memory of using Maxim was that it only handled rotation and translation.
Registar handles scaling. Not sure about PixInsight.
Stuart
On Fri, Aug 17, 2018 at 3:13 PM Michael Fulbright mike.fulbright@... [ap-gto] <ap-gto@...> wrote:
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George
…but does it rescale radially???
Regards,
George
George Whitney Astro-Physics, Inc. Phone: 815-282-1513 Email: george@...
From: ap-gto@... [mailto:ap-gto@...]
Sent: Friday, August 17, 2018 3:21 PM To: ap-gto@... Subject: RE: [ap-gto] Triplet Lens -vs- Doublet Lens
PixInsight does rescale images.
Clear skies
Antoine
De : ap-gto@... <ap-gto@...>
My memory of using Maxim was that it only handled rotation and translation.
Registar handles scaling. Not sure about PixInsight.
Stuart
On Fri, Aug 17, 2018 at 3:13 PM Michael Fulbright mike.fulbright@... [ap-gto] <ap-gto@...> wrote:
-- Stuart Heggie
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George OBrien
PixInsight has a function called DynamicAlignment for correction of image scale. I suspect the scale change due to slight differences in wavelength would be miniscule, however.
Also, to be clear the refocusing is only necessary when you change NB filters, not for each subframe. George From: Stuart Heggie stuart.j.heggie@... [ap-gto] Sent: Friday, August 17, 2018 3:11 PM To: ap-gto@... Subject: Re: [ap-gto] Triplet Lens -vs- Doublet Lens George, I was thinking the same thing. The image scale changes very slightly. Stuart On Fri, Aug 17, 2018 at 3:07 PM George george@... [ap-gto] <ap-gto@...> wrote: Remember…since you are refocusing for each image the image scale is changing. Regards, George George Whitney Astro-Physics, Inc. Phone: 815-282-1513 Email: george@... From: ap-gto@... [mailto:ap-gto@...] Sent: Friday, August 17, 2018 1:47 PM To: ap-gto@... Subject: RE: [ap-gto] Triplet Lens -vs- Doublet Lens I don’t mean to butt-in, but I think Charlie is on to something. Each NB image will have been focused independently. There would be no chromatic aberration because each set of images will be in focus, albeit on what would have been different focal planes. Registering and combining the images would seem to work fine. I think this makes sense, coming from an old physicist. Clear skies, George From: Stuart Heggie stuart.j.heggie@... [ap-gto] Sent: Friday, August 17, 2018 2:08 PM To: ap-gto@... Subject: Re: [ap-gto] Triplet Lens -vs- Doublet Lens I would think if you plan to combine the NB data later into a false colour image you’ll have the same problem. If your goal is pure B&W then I’d guess you’d be fine. Stuart On Fri, Aug 17, 2018 at 12:43 PM acf900gto@... [ap-gto] <ap-gto@...> wrote: As we know, one of the benefits of a triplet lens is that it brings the three primary colors to a single focus while a doublet lens can only bring two of the primary colors to a common focus. The third forms a kind of blur circle around the other two. However, if I am doing narrow band imaging with something like a 5nm band pass Ha filter, I am only bringing a single "color" to a focus. So, it would seem that it would be possible to do quality narrow band imaging with a good quality ED doublet lens. A 6" ED doublet refractor is certainly a lot more affordable than a 6" ED triplet! Is there any reason to believe an Ha image from a doublet would be of lower quality than an Ha image taken with a triplet? Charlie -- Stuart Heggie http://www.stuartheggie.com/featured.html [Non-text portions of this message have been removed] -- Stuart Heggie http://www.stuartheggie.com/featured.html |
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My comment was specific to combining multiple filter images for a false colour image. Not monochrome. Anyway as long as you use a registration package that deals with slight changes in image scale I’m guessing you’d still be okay. Stuart
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Stuart Heggie |
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acf900gto@...
The S2 and O3 filters would behave the same as the Ha. Each would focus at a slightly different point but that happens anyway. I believe the pixel scale would have an infinitesimal difference between the three masters.
I was specifically looking at a particular 152mm f/7.9 ED doublet as the basis for building a new scope. Based on some of the published ray trace diagrams for lenses at this size and f/ratio, the RGB focuses are separated by about 1 mm. This means you have a focal variation of 1mm in a 1200mm focal length. I see a lot more focus spread than that from temperature variations when imaging with my 130mm f/6 APO. Since a 152mm ED doublet lens is about $2,000 and a 152 ED triplet is about $6,000, it made me think about the real benefit of the triplet when considering NB imaging. It is likely a triplet does a better job of correcting field curvature and other off axis issues but enough to count it out? With RGB imaging, the filter band passes are way bigger. Probably enough to show problems but in NB, maybe not. I guess all this could be tried with a smaller ED doublet refractor but I know there are optical engineers out there and was hoping for some feed back on the idea. Or even info and pix from someone already doing this. Charlie |
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I'm guessing you're right about this. An extreme example of what you're describing would be shooting with camera lenses. Richard Crisp has shot a lot of NB images with camera lenses. They're far from APO typically yet the results in NB turn out great. Stuart
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Stuart Heggie |
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Antoine PAVLIN
PixInsight can align images made with different instruments at different scales. For example you can combine images at longer focal length of an object core with a wider field image to get fine details in the core together with the widefield view of the entire object. Doing soi t compensates for translation, rotation and scale. You can also check a box allowing distorsion correction like barrel, pincushion, differential curvature when doing mosaics. You can even define customized distorsion models and apply it to the images to be aligned. So yes I would say it will certainly properly align images taken with the same instrument at different focus point, making slight difference in F ratio.
Clears skies
Antoine
De : ap-gto@...
…but does it rescale radially???
From: ap-gto@... [mailto:ap-gto@...]
Sent: Friday, August 17, 2018 3:21 PM To: ap-gto@... Subject: RE: [ap-gto] Triplet Lens -vs- Doublet Lens
PixInsight does rescale images.
Clear skies
Antoine
De : ap-gto@... <ap-gto@...>
My memory of using Maxim was that it only handled rotation and translation.
Registar handles scaling. Not sure about PixInsight.
Stuart
On Fri, Aug 17, 2018 at 3:13 PM Michael Fulbright mike.fulbright@... [ap-gto] <ap-gto@...> wrote:
-- Stuart Heggie
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Roland Christen
As we know, one of the benefits of a triplet lens is that it brings the three primary colors to a single focus while a doublet lens can only bring two of the primary colors to a common focus. This is not strictly true. A doublet lens can be constructed that brings more than 3 colors to a common focus. It requires special glasses that are more expensive than common ED combinations, but can be done.
An inexpensive ED doublet can do narrowband imaging quite nicely and can be as sharp if made properly.
Rolando
-----Original Message-----
From: acf900gto@... [ap-gto] <ap-gto@...> To: ap-gto Sent: Fri, Aug 17, 2018 12:46 pm Subject: [ap-gto] Triplet Lens -vs- Doublet Lens As we know, one of the benefits of a triplet lens is that it brings the three primary colors to a single focus while a doublet lens can only bring two of the primary colors to a common focus. The third forms a kind of blur circle around the other two.
However, if I am doing narrow band imaging with something like a 5nm band pass Ha filter, I am only bringing a single "color" to a focus. So, it would seem that it would be possible to do quality narrow band imaging with a good quality ED doublet lens. A 6" ED doublet refractor is certainly a lot more affordable than a 6" ED triplet!
Is there any reason to believe an Ha image from a doublet would be of lower quality than an Ha image taken with a triplet?
Charlie
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Roland Christen
Filed curvature is a function of the focal length and has nothing to do with the number of elements if they are all in the same place up front. Typically the field curvature is around 1/3 of the focal length. There is also off-axis astigmatism which is similar in triplets and doublets. The only way around those two aberrations is to use a field flattener.
Rolando
-----Original Message-----
From: acf900gto@... [ap-gto] To: ap-gto Sent: Fri, Aug 17, 2018 5:56 pm Subject: Re: [ap-gto] Triplet Lens -vs- Doublet Lens The S2 and O3 filters would behave the same as the Ha. Each would focus at a slightly different point but that happens anyway. I believe the pixel scale would have an infinitesimal difference between the three masters. I was specifically looking at a particular 152mm f/7.9 ED doublet as the basis for building a new scope. Based on some of the published ray trace diagrams for lenses at this size and f/ratio, the RGB focuses are separated by about 1 mm. This means you have a focal variation of 1mm in a 1200mm focal length. I see a lot more focus spread than that from temperature variations when imaging with my 130mm f/6 APO.
Since a 152mm ED doublet lens is about $2,000 and a 152 ED triplet is about $6,000, it made me think about the real benefit of the triplet when considering NB imaging.
It is likely a triplet does
a better job of correcting field curvature and other off axis issues but enough to count it out?
With RGB imaging, the filter band passes are way bigger. Probably enough to show problems but in NB, maybe not. I guess all this could be tried with a smaller ED doublet refractor but I know there are optical engineers out there and was hoping for some feed back on the idea. Or even info and pix from someone already doing this.
Charlie
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acf900gto@...
Rolando:
Thanks for the clarification on the differences between a triplet and doublet refractor related to narrow band imaging. I have been able to find several sample images taken in NB using doublet refractors and some were very good. So, good to know that a well made doublet would work as a good NB imaging platform -- especially if used in conjunction with a reducer / flatener. Charlie ---In ap-gto@..., <chris1011@...> wrote :
Filed curvature is a function of the focal length and has nothing to do with the number of elements if they are all in the same place up front. Typically the field curvature is around 1/3 of the focal length. There is also off-axis astigmatism which is similar in triplets and doublets. The only way around those two aberrations is to use a field flattener. Rolando -----Original Message-----
From: acf900gto@... [ap-gto] <ap-gto@...> To: ap-gto <ap-gto@...> Sent: Fri, Aug 17, 2018 5:56 pm Subject: Re: [ap-gto] Triplet Lens -vs- Doublet Lens The S2 and O3 filters would behave the same as the Ha. Each would focus at a slightly different point but that happens anyway. I believe the pixel scale would have an infinitesimal difference between the three masters. I was specifically looking at a particular 152mm f/7.9 ED doublet as the basis for building a new scope. Based on some of the published ray trace diagrams for lenses at this size and f/ratio, the RGB focuses are separated by about 1 mm. This means you have a focal variation of 1mm in a 1200mm focal length. I see a lot more focus spread than that from temperature variations when imaging with my 130mm f/6 APO. Since a 152mm ED doublet lens is about $2,000 and a 152 ED triplet is about $6,000, it made me think about the real benefit of the triplet when considering NB imaging. It is likely a triplet does
a better job of correcting field curvature and other off axis issues but enough to count it out? With RGB imaging, the filter band passes are way bigger. Probably enough to show problems but in NB, maybe not. I guess all this could be tried with a smaller ED doublet refractor but I know there are optical engineers out there and was hoping for some feed back on the idea. Or even info and pix from someone already doing this. Charlie |
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