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DIMM seeing test, seeking prism source
A question came up in the STV group (SBIGSTV) at Yahoo regarding
the DIMM measurement. DIMM = Differential Image Motion Monitor, and it appears to be the present de facto method to evaluate sites for astronomical use as I've found in 100s of papers and journals over the past 5 days. The SBIG STV device has a DIMM operational mode, but it seems everyone who has attempted it has run into some difficulties, and it doesn't appear to be due to the algorithms but, instead, to the aperture mask and whether one should have a prism over one of the sub-apertures or not (with "not" being a slightly defocused image). In brief, the mask is a Hartmann mask and the goal is to have 2 images of the same star appear on the CCD chip after passing through the mask whose baseline is typically 6 to 10 inches depending on the scope used. After a lot of web surfing and document search, the consensus is that a prism "should" be used in one of the sub-apertures of the DIMM mask for optimal results. SBIG's docs make no mention of a prism being used or needed for this application. Everywhere I've read about the prism, a 30 arcsecond deviation is what's being used whether with the SBIG STV or a custom, home-brew imaging station at some of the world's major observatories. The prism must be custom made since I've found nothing like it in the Edmund Industrial Optics 2004 catalog 41B or elsewhere. Some have reported minor success with slight defocusing and with both sub-apertures "clear" (i.e., no prism or beam-splitter). I'm still experimenting with DIMM and would like to continue the tests this time using a prism. Does anyone have any idea where such a prism (roughly 2 to 3 inches square or circular) with a 30 arcsec "bending" can be found without paying an arm and a leg? For these purposes "an arm and a leg" can be defined to be no more than US$200. :-) If anyone's interested in further reading, some of the better references site testing and DIMM usage are in the following list; most of the scopes used are SCTs from 8" to 12". Prisms used for DIMM in mask's sub-apertu http://www.ctio.noao.edu/telescopes/dimm/dimm.html http://www.ing.iac.es/Astronomy/development/hap/dimm.html http://www.astro.virginia.edu/~cp8h/dimm/ http://www.astro.washington.edu/rest/dimm/dimmtext.htm http://www.astro.caltech.edu/~george/celt/schoeck.ps http://www-astro.unice.fr/Concordia/sitetesting.html http://www-astro.unice.fr/Concordia/Documents/dimm.pdf The above PDF document (finally!) describes the prism: 7cm x 7cm with 30 arcsec deviation angle on one of the sub-apertures. Note this is being done in Antarctica and also referenced in the paper below: http://sait.oat.ts.astro.it/MSAIS/2/PDF/146.pdf http://galaxy.ps.uci.edu/~celtsite/equipment.html Also at the above site ("uci.edu"), click on the link for "Sarazin & Roddier 1990" for the original definitive article about DIMM. Defocused DIMM: http://www.iac.es/proyect/sitesting/PDF/defocus.pdf Defocused and with prism: http://www.tug.tubitak.gov.tr/gozlemler/SDIMM/ Hartmann-DIMM (without prism): http://astro.uchicago.edu/cara/research/papers/bob/ |
#2
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DIMM seeing test, seeking prism source
A question came up in the STV group (SBIGSTV) at Yahoo regarding the DIMM measurement. DIMM = Differential Image Motion Monitor, and it appears to be the present de facto method to evaluate sites for astronomical use as I've found in 100s of papers and journals over the past 5 days. SNIP The prism must be custom made since I've found nothing like it in the Edmund Industrial Optics 2004 catalog 41B or elsewhere. Some have reported minor success with slight defocusing and with both sub-apertures "clear" (i.e., no prism or beam-splitter). I'm still experimenting with DIMM and would like to continue the tests this time using a prism. Does anyone have any idea where such a prism (roughly 2 to 3 inches square or circular) with a 30 arcsec "bending" can be found without paying an arm and a leg? For these purposes "an arm and a leg" can be defined to be no more than US$200. :-) I have always used Technical Optics (now CVI Technical Optics) for my "specials" as they have always been very competetive on pricing. I doubt if they can match your price target for a window as large as 7x7cm but they would still be my first port of call. CVI Technical Optics Ltd Second Avenue, Onchan, IOM IM3 4PA United Kingdom Europe http://www.cvi-tol.co.uk/ Tel.: +44 - (0)1624 - 647 000 Fax.: +44- (0)1624- 676 859 e-mail: Hope this helps you. ************************************************** ****** Dr Peter R Hobson Reader, Department of Electronic and Computer Engineering Brunel University, Uxbridge UB8 3PH G.B. |
#3
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DIMM seeing test, seeking prism source
A question came up in the STV group (SBIGSTV) at Yahoo regarding the DIMM measurement. DIMM = Differential Image Motion Monitor, and it appears to be the present de facto method to evaluate sites for astronomical use as I've found in 100s of papers and journals over the past 5 days. SNIP The prism must be custom made since I've found nothing like it in the Edmund Industrial Optics 2004 catalog 41B or elsewhere. Some have reported minor success with slight defocusing and with both sub-apertures "clear" (i.e., no prism or beam-splitter). I'm still experimenting with DIMM and would like to continue the tests this time using a prism. Does anyone have any idea where such a prism (roughly 2 to 3 inches square or circular) with a 30 arcsec "bending" can be found without paying an arm and a leg? For these purposes "an arm and a leg" can be defined to be no more than US$200. :-) I have always used Technical Optics (now CVI Technical Optics) for my "specials" as they have always been very competetive on pricing. I doubt if they can match your price target for a window as large as 7x7cm but they would still be my first port of call. CVI Technical Optics Ltd Second Avenue, Onchan, IOM IM3 4PA United Kingdom Europe http://www.cvi-tol.co.uk/ Tel.: +44 - (0)1624 - 647 000 Fax.: +44- (0)1624- 676 859 e-mail: Hope this helps you. ************************************************** ****** Dr Peter R Hobson Reader, Department of Electronic and Computer Engineering Brunel University, Uxbridge UB8 3PH G.B. |
#4
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DIMM seeing test, seeking prism source
"Thad Floryan" wrote in message om... A question came up in the STV group (SBIGSTV) at Yahoo regarding the DIMM measurement. DIMM = Differential Image Motion Monitor, and it appears to be the present de facto method to evaluate sites for astronomical use as I've found in 100s of papers and journals over the past 5 days. The SBIG STV device has a DIMM operational mode, but it seems everyone who has attempted it has run into some difficulties, and it doesn't appear to be due to the algorithms but, instead, to the aperture mask and whether one should have a prism over one of the sub-apertures or not (with "not" being a slightly defocused image). In brief, the mask is a Hartmann mask and the goal is to have 2 images of the same star appear on the CCD chip after passing through the mask whose baseline is typically 6 to 10 inches depending on the scope used. After a lot of web surfing and document search, the consensus is that a prism "should" be used in one of the sub-apertures of the DIMM mask for optimal results. SBIG's docs make no mention of a prism being used or needed for this application. Everywhere I've read about the prism, a 30 arcsecond deviation is what's being used whether with the SBIG STV or a custom, home-brew imaging station at some of the world's major observatories. The prism must be custom made since I've found nothing like it in the Edmund Industrial Optics 2004 catalog 41B or elsewhere. Some have reported minor success with slight defocusing and with both sub-apertures "clear" (i.e., no prism or beam-splitter). I'm still experimenting with DIMM and would like to continue the tests this time using a prism. Does anyone have any idea where such a prism (roughly 2 to 3 inches square or circular) with a 30 arcsec "bending" can be found without paying an arm and a leg? For these purposes "an arm and a leg" can be defined to be no more than US$200. :-) If anyone's interested in further reading, some of the better references site testing and DIMM usage are in the following list; most of the scopes used are SCTs from 8" to 12". Prisms used for DIMM in mask's sub-apertu http://www.ctio.noao.edu/telescopes/dimm/dimm.html http://www.ing.iac.es/Astronomy/development/hap/dimm.html http://www.astro.virginia.edu/~cp8h/dimm/ http://www.astro.washington.edu/rest/dimm/dimmtext.htm http://www.astro.caltech.edu/~george/celt/schoeck.ps http://www-astro.unice.fr/Concordia/sitetesting.html http://www-astro.unice.fr/Concordia/Documents/dimm.pdf The above PDF document (finally!) describes the prism: 7cm x 7cm with 30 arcsec deviation angle on one of the sub-apertures. Note this is being done in Antarctica and also referenced in the paper below: http://sait.oat.ts.astro.it/MSAIS/2/PDF/146.pdf http://galaxy.ps.uci.edu/~celtsite/equipment.html Also at the above site ("uci.edu"), click on the link for "Sarazin & Roddier 1990" for the original definitive article about DIMM. Defocused DIMM: http://www.iac.es/proyect/sitesting/PDF/defocus.pdf Defocused and with prism: http://www.tug.tubitak.gov.tr/gozlemler/SDIMM/ Hartmann-DIMM (without prism): http://astro.uchicago.edu/cara/research/papers/bob/ it appears that this is simply a 2 hole Hartmann mask . The new twist is having a 30 arcsecond permanent image shift by using the wedge in one of the 2 holes , which allows the 2 images to be permanently separated at the focal plane . The same effect of having the 2 images separated is accomplished by a slight defocus . I'd try first without any prism/wedge . I'd just make a cardboard Hartmann mask and see if the defocus that's required for a few arcseconds of star image separation makes the 2 star images too fuzzy/dim . In my C-11 scope for example I know this is ok, as I usually use a similar mask for focusing . If you get an optical window, most have non parallel back to front , the amount of wedge being more than enough for your purposes .. Try it first with any filter you may already own by simply placing it in front of one of your Harmann mask holes after focusing and see if you get enough separation. Many beamsplitters and cold or hot mirrors have a built in wedge to eliminate internal ghosting so getting one of them might work too . Check Thorlabs for broadband beamsplitters with a built in wedge angle of 30 arcseconds. best regards, matt tudor |
#5
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DIMM seeing test, seeking prism source
"Thad Floryan" wrote in message om... A question came up in the STV group (SBIGSTV) at Yahoo regarding the DIMM measurement. DIMM = Differential Image Motion Monitor, and it appears to be the present de facto method to evaluate sites for astronomical use as I've found in 100s of papers and journals over the past 5 days. The SBIG STV device has a DIMM operational mode, but it seems everyone who has attempted it has run into some difficulties, and it doesn't appear to be due to the algorithms but, instead, to the aperture mask and whether one should have a prism over one of the sub-apertures or not (with "not" being a slightly defocused image). In brief, the mask is a Hartmann mask and the goal is to have 2 images of the same star appear on the CCD chip after passing through the mask whose baseline is typically 6 to 10 inches depending on the scope used. After a lot of web surfing and document search, the consensus is that a prism "should" be used in one of the sub-apertures of the DIMM mask for optimal results. SBIG's docs make no mention of a prism being used or needed for this application. Everywhere I've read about the prism, a 30 arcsecond deviation is what's being used whether with the SBIG STV or a custom, home-brew imaging station at some of the world's major observatories. The prism must be custom made since I've found nothing like it in the Edmund Industrial Optics 2004 catalog 41B or elsewhere. Some have reported minor success with slight defocusing and with both sub-apertures "clear" (i.e., no prism or beam-splitter). I'm still experimenting with DIMM and would like to continue the tests this time using a prism. Does anyone have any idea where such a prism (roughly 2 to 3 inches square or circular) with a 30 arcsec "bending" can be found without paying an arm and a leg? For these purposes "an arm and a leg" can be defined to be no more than US$200. :-) If anyone's interested in further reading, some of the better references site testing and DIMM usage are in the following list; most of the scopes used are SCTs from 8" to 12". Prisms used for DIMM in mask's sub-apertu http://www.ctio.noao.edu/telescopes/dimm/dimm.html http://www.ing.iac.es/Astronomy/development/hap/dimm.html http://www.astro.virginia.edu/~cp8h/dimm/ http://www.astro.washington.edu/rest/dimm/dimmtext.htm http://www.astro.caltech.edu/~george/celt/schoeck.ps http://www-astro.unice.fr/Concordia/sitetesting.html http://www-astro.unice.fr/Concordia/Documents/dimm.pdf The above PDF document (finally!) describes the prism: 7cm x 7cm with 30 arcsec deviation angle on one of the sub-apertures. Note this is being done in Antarctica and also referenced in the paper below: http://sait.oat.ts.astro.it/MSAIS/2/PDF/146.pdf http://galaxy.ps.uci.edu/~celtsite/equipment.html Also at the above site ("uci.edu"), click on the link for "Sarazin & Roddier 1990" for the original definitive article about DIMM. Defocused DIMM: http://www.iac.es/proyect/sitesting/PDF/defocus.pdf Defocused and with prism: http://www.tug.tubitak.gov.tr/gozlemler/SDIMM/ Hartmann-DIMM (without prism): http://astro.uchicago.edu/cara/research/papers/bob/ it appears that this is simply a 2 hole Hartmann mask . The new twist is having a 30 arcsecond permanent image shift by using the wedge in one of the 2 holes , which allows the 2 images to be permanently separated at the focal plane . The same effect of having the 2 images separated is accomplished by a slight defocus . I'd try first without any prism/wedge . I'd just make a cardboard Hartmann mask and see if the defocus that's required for a few arcseconds of star image separation makes the 2 star images too fuzzy/dim . In my C-11 scope for example I know this is ok, as I usually use a similar mask for focusing . If you get an optical window, most have non parallel back to front , the amount of wedge being more than enough for your purposes .. Try it first with any filter you may already own by simply placing it in front of one of your Harmann mask holes after focusing and see if you get enough separation. Many beamsplitters and cold or hot mirrors have a built in wedge to eliminate internal ghosting so getting one of them might work too . Check Thorlabs for broadband beamsplitters with a built in wedge angle of 30 arcseconds. best regards, matt tudor |
#6
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DIMM seeing test, seeking prism source
Peter R Hobson wrote in message ...
[...] I'm still experimenting with DIMM and would like to continue the tests this time using a prism. Does anyone have any idea where such a prism (roughly 2 to 3 inches square or circular) with a 30 arcsec "bending" can be found without paying an arm and a leg? For these purposes "an arm and a leg" can be defined to be no more than US$200. :-) I have always used Technical Optics (now CVI Technical Optics) for my "specials" as they have always been very competetive on pricing. I doubt if they can match your price target for a window as large as 7x7cm but they would still be my first port of call. CVI Technical Optics Ltd Second Avenue, Onchan, IOM IM3 4PA United Kingdom Europe http://www.cvi-tol.co.uk/ Tel.: +44 - (0)1624 - 647 000 Fax.: +44- (0)1624- 676 859 e-mail: Thank you VERY much for that reference! I will contact them to see what can be arranged. I'm also sending email to several authors of the various DIMM papers since one person was thanked (in a postscript) for having secured the prism for use in the tests, and with the increasing usage of DIMM there should (soon?) be a larger market for such prisms. |
#7
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DIMM seeing test, seeking prism source
Peter R Hobson wrote in message ...
[...] I'm still experimenting with DIMM and would like to continue the tests this time using a prism. Does anyone have any idea where such a prism (roughly 2 to 3 inches square or circular) with a 30 arcsec "bending" can be found without paying an arm and a leg? For these purposes "an arm and a leg" can be defined to be no more than US$200. :-) I have always used Technical Optics (now CVI Technical Optics) for my "specials" as they have always been very competetive on pricing. I doubt if they can match your price target for a window as large as 7x7cm but they would still be my first port of call. CVI Technical Optics Ltd Second Avenue, Onchan, IOM IM3 4PA United Kingdom Europe http://www.cvi-tol.co.uk/ Tel.: +44 - (0)1624 - 647 000 Fax.: +44- (0)1624- 676 859 e-mail: Thank you VERY much for that reference! I will contact them to see what can be arranged. I'm also sending email to several authors of the various DIMM papers since one person was thanked (in a postscript) for having secured the prism for use in the tests, and with the increasing usage of DIMM there should (soon?) be a larger market for such prisms. |
#8
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DIMM seeing test, seeking prism source
"matt" wrote in message ...
[...] it appears that this is simply a 2 hole Hartmann mask . The new twist is having a 30 arcsecond permanent image shift by using the wedge in one of the 2 holes , which allows the 2 images to be permanently separated at the focal plane . The same effect of having the 2 images separated is accomplished by a slight defocus . I'd try first without any prism/wedge . I'd just make a cardboard Hartmann mask and see if the defocus that's required for a few arcseconds of star image separation makes the 2 star images too fuzzy/dim . That's what several of us have already attempted, and that's where the problem seems to lie. The slightly-defocused images aren't "good enough" for the DIMM and that's why the wedge prisms have been used as stated in most of the reports and papers I've found the past week. A second issue arises with the sub-apertures' baseline and diameters; There is disagreement as to the correct baseline percentage of clear aperture, with values ranging from 70% to 80% (and other) and sub-aperture diameter percentage of 30% to 20% (and other). I'm tabulating all the dimensions from the several 100s of reports I've found and need to re-read the 1990 paper for some more insight. In my C-11 scope for example I know this is ok, as I usually use a similar mask for focusing . If you get an optical window, most have non parallel back to front , the amount of wedge being more than enough for your purposes . Try it first with any filter you may already own by simply placing it in front of one of your Harmann mask holes after focusing and see if you get enough separation. Many beamsplitters and cold or hot mirrors have a built in wedge to eliminate internal ghosting so getting one of them might work too . Check Thorlabs for broadband beamsplitters with a built in wedge angle of 30 arcseconds. Thank you very much for the reference to Thorlabs (www.thorlabs.com); I've already noted they have 2 degree wedge prisms at a VERY reasonable price, so it should be possible to get some made with 30 arcsecond deviation within my original budget constraint. |
#9
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DIMM seeing test, seeking prism source
"matt" wrote in message ...
[...] it appears that this is simply a 2 hole Hartmann mask . The new twist is having a 30 arcsecond permanent image shift by using the wedge in one of the 2 holes , which allows the 2 images to be permanently separated at the focal plane . The same effect of having the 2 images separated is accomplished by a slight defocus . I'd try first without any prism/wedge . I'd just make a cardboard Hartmann mask and see if the defocus that's required for a few arcseconds of star image separation makes the 2 star images too fuzzy/dim . That's what several of us have already attempted, and that's where the problem seems to lie. The slightly-defocused images aren't "good enough" for the DIMM and that's why the wedge prisms have been used as stated in most of the reports and papers I've found the past week. A second issue arises with the sub-apertures' baseline and diameters; There is disagreement as to the correct baseline percentage of clear aperture, with values ranging from 70% to 80% (and other) and sub-aperture diameter percentage of 30% to 20% (and other). I'm tabulating all the dimensions from the several 100s of reports I've found and need to re-read the 1990 paper for some more insight. In my C-11 scope for example I know this is ok, as I usually use a similar mask for focusing . If you get an optical window, most have non parallel back to front , the amount of wedge being more than enough for your purposes . Try it first with any filter you may already own by simply placing it in front of one of your Harmann mask holes after focusing and see if you get enough separation. Many beamsplitters and cold or hot mirrors have a built in wedge to eliminate internal ghosting so getting one of them might work too . Check Thorlabs for broadband beamsplitters with a built in wedge angle of 30 arcseconds. Thank you very much for the reference to Thorlabs (www.thorlabs.com); I've already noted they have 2 degree wedge prisms at a VERY reasonable price, so it should be possible to get some made with 30 arcsecond deviation within my original budget constraint. |
#10
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DIMM seeing test, seeking prism source
"Thad Floryan" wrote in message om... "matt" wrote in message ... [...] it appears that this is simply a 2 hole Hartmann mask . The new twist is having a 30 arcsecond permanent image shift by using the wedge in one of the 2 holes , which allows the 2 images to be permanently separated at the focal plane . The same effect of having the 2 images separated is accomplished by a slight defocus . I'd try first without any prism/wedge . I'd just make a cardboard Hartmann mask and see if the defocus that's required for a few arcseconds of star image separation makes the 2 star images too fuzzy/dim . That's what several of us have already attempted, and that's where the problem seems to lie. The slightly-defocused images aren't "good enough" for the DIMM and that's why the wedge prisms have been used as stated in most of the reports and papers I've found the past week. A second issue arises with the sub-apertures' baseline and diameters; There is disagreement as to the correct baseline percentage of clear aperture, with values ranging from 70% to 80% (and other) and sub-aperture diameter percentage of 30% to 20% (and other). I'm tabulating all the dimensions from the several 100s of reports I've found and need to re-read the 1990 paper for some more insight. In my C-11 scope for example I know this is ok, as I usually use a similar mask for focusing . If you get an optical window, most have non parallel back to front , the amount of wedge being more than enough for your purposes . Try it first with any filter you may already own by simply placing it in front of one of your Harmann mask holes after focusing and see if you get enough separation. Many beamsplitters and cold or hot mirrors have a built in wedge to eliminate internal ghosting so getting one of them might work too . Check Thorlabs for broadband beamsplitters with a built in wedge angle of 30 arcseconds. Thank you very much for the reference to Thorlabs (www.thorlabs.com); I've already noted they have 2 degree wedge prisms at a VERY reasonable price, so it should be possible to get some made with 30 arcsecond deviation within my original budget constraint. They also have some plate beamsplitters which are broadband antireflection coated on one side and have a built in wedge between front and back of 30 arcmin to eliminate reflections . They come in a BK7 and a quartz version, both 1/10lambda surface . They go up to 2" in diameter . Part numbers are BSW13 to BSW18 depending on the beamsplitter coatings. Probably selecting the "wrong" coating would yield very little reflectivity at 0 deg . An even better fit would be the Thorlabs beamsamplers, which are optical windows made of BK7 or quartz, 1/10lambda , with an uncoated front and a broadband antireflection coated back . They too come in 2" diameter and have the built in 30 arcmin wedge between front and back surfaces . They're intended to reflect between 1% - 10% at 45 deg angle of incidence . Part numbers are BSP20-A1 and BSF20-A1 . If the 30 arcmin wedge is too much, although this is 4 times smaller than the wedge prisms, you may try to combine 2 of these beam sampler wedges . Depending on their orientation you can steer incident light and by rotating one wedge with respect to the other you can change the steering angle continuously from 0 . Simply rotating the 2 wedges will allow you to experiment with the angle value . best regards, matt tudor |
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