XLS to Calculate Etalon and BF Distances for PST Mods
XLS to Calculate Etalon and BF Distances for PST Mods
Hi everyone,
I've found a nice Excel sheet that can help you calculate where is the best position to place the etalon and the optimal distance between the etalon and the BF when you plan to do your PST mods.
It can be found here:
http://www.astroshot.com/PSTCak/PST-Eta ... Design.xls
I hope you'll find it useful.
Andrea
I've found a nice Excel sheet that can help you calculate where is the best position to place the etalon and the optimal distance between the etalon and the BF when you plan to do your PST mods.
It can be found here:
http://www.astroshot.com/PSTCak/PST-Eta ... Design.xls
I hope you'll find it useful.
Andrea
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Re: XLS to Calculate Etalon and BF Distances for PST Mods
Andrea,
That's NOT a good PST etalon spreadsheet.
The design parameters are that the front barlow lens on the PST etalon assembly MUST be 200 mm inside prime focus.
That's NOT a good PST etalon spreadsheet.
The design parameters are that the front barlow lens on the PST etalon assembly MUST be 200 mm inside prime focus.
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Re: XLS to Calculate Etalon and BF Distances for PST Mods
I know but maybe we can use that as a start to develop a correct one that can be used by anyone interested in modding the PST
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Re: XLS to Calculate Etalon and BF Distances for PST Mods
There are "correct" spreadsheets available...when I developed the "magic" 200mm spacing in 2008 we prepared spreadsheets...I'll find a copy and upload.
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Re: XLS to Calculate Etalon and BF Distances for PST Mods
Found!
This is from you from another forum
PSTmodcalcs.zip
This is from you from another forum
PSTmodcalcs.zip
Re: XLS to Calculate Etalon and BF Distances for PST Mods
I measured where the prime focus of the Vixen A80M lies and it's 25cm from the end of the tube (without the focuser block). So the distance of the etalon from the end of the tube must be 5cm and it's exactly what's now after I shifted it back a couple of cm yesterday.
The improvement of yesterdays images must be due to that "simple" shift.
Thanks for the very precious info!
Andrea
The improvement of yesterdays images must be due to that "simple" shift.
Thanks for the very precious info!
Andrea
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Re: XLS to Calculate Etalon and BF Distances for PST Mods
Always glad to be able to help get a successful PST Mod.
The Modded PST performance is highly underated.
The Modded PST performance is highly underated.
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Re: XLS to Calculate Etalon and BF Distances for PST Mods
Can't be more in agreement with you.
I loooove my modded PST!
I loooove my modded PST!
Re: XLS to Calculate Etalon and BF Distances for PST Mods
I've always assumed that Coronado had done the brainwork and that their parameters for etalon/BF spacings were optimal. In this context I have always used a correctly focused standar PST with either the objective or the whole "gold" tube removed to set the etalon position on the donor telescope. Using parfocal eyepieces such as TV plossls avoids refocusing.
Re: XLS to Calculate Etalon and BF Distances for PST Mods
That's what I've done. But in the case you want to remove the blackbox and use PST etalon and BF only, you have to do some homework as well
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Re: XLS to Calculate Etalon and BF Distances for PST Mods
Thank you all
for that valuable information
for that valuable information
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Re: XLS to Calculate Etalon and BF Distances for PST Mods
Andrea. Yes, I agree, my post was to draw attention to the correct placement of the PST etalon, this seems to me to be the more important of the two relevant spacings and should be maintained regardless of what happens to the rest of the system. The etalon to BF spacing seems to have a reasonable latitde of error(?)
Re: XLS to Calculate Etalon and BF Distances for PST Mods
I absolutely agree with you and I have the proof of that. Simply by shifting back the etalon of less than 2cm took to a great improvement of the details in my images with the modifies PST.
So the "magical" 200mm from prime focus rule seems to be the crucial one for best performances.
Maybe tomorrow I'll be able to test the "phase 2" version, so take out the blackbox and use only etalon/BF. Hopefully I'll be able to find again the right "settings".
So the "magical" 200mm from prime focus rule seems to be the crucial one for best performances.
Maybe tomorrow I'll be able to test the "phase 2" version, so take out the blackbox and use only etalon/BF. Hopefully I'll be able to find again the right "settings".
Re: XLS to Calculate Etalon and BF Distances for PST Mods
I've done a few PST mods so far, but I always struggle to find the exact focal plane of the objective (using a black piece of cardboard). When focusing the sun's image there is a distance of a few (2-3mm) where the image looks focused. But of course I want to know the EXACT position of the focal plane down to the mm. Is there a way to do this more precisely than by using the cardboard technique?
Re: XLS to Calculate Etalon and BF Distances for PST Mods
I don't think that 2/3 mm makes a great difference.
I've found the "right" distance with the cardboard as well and I don't think I'd be able to see any real difference moving the etalon 23 mm back or forth
Consider that my first try was wrong of 2cm! And images looked good already
I've found the "right" distance with the cardboard as well and I don't think I'd be able to see any real difference moving the etalon 23 mm back or forth
Consider that my first try was wrong of 2cm! And images looked good already
Re: XLS to Calculate Etalon and BF Distances for PST Mods
200mm inside focus in a f10 light cone will always give the best performance. We sorted it out in the UK Astroimaging forum solar equipment section.
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Re: XLS to Calculate Etalon and BF Distances for PST Mods
Andrea,
That's an *OLD* spreadsheet I put together several years ago.
Was never fuully finished. Well outdated at this stage.
Although I had an interest in the PST-mod, I never bothered with it too much as I had a daystar filter.
There are more up-to-date spreadsheets than that.
It does raise one query at the time though, which has probably been clarified since then:
For a pin-point star, placing a 20mm etalon midway in the focal length of 400mm f10 will be fine.
However, allowing for the sun's disc, does a slight amount of vignetting occur by default with the PST?
The *thinking* at the time on the UKAI boards was to allow for this issue in the spreadsheet.
That's an *OLD* spreadsheet I put together several years ago.
Was never fuully finished. Well outdated at this stage.
Although I had an interest in the PST-mod, I never bothered with it too much as I had a daystar filter.
There are more up-to-date spreadsheets than that.
It does raise one query at the time though, which has probably been clarified since then:
For a pin-point star, placing a 20mm etalon midway in the focal length of 400mm f10 will be fine.
However, allowing for the sun's disc, does a slight amount of vignetting occur by default with the PST?
The *thinking* at the time on the UKAI boards was to allow for this issue in the spreadsheet.
Michael in Kildare, Ireland.
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Re: XLS to Calculate Etalon and BF Distances for PST Mods
yes, there is some vignetting, even on the original PST- the 200mm demands a beam width of 20mm at the etalon, then you have the solar disk @ 1/100 the focal length...
The main issue identified was the -200mm focal length of the barlow used in the etalon assembly.
Roel, the best double check on the etalon position is the back focus distance. This should be exactly 200mm. Easy to measure in a stage 2 mod.
The main issue identified was the -200mm focal length of the barlow used in the etalon assembly.
Roel, the best double check on the etalon position is the back focus distance. This should be exactly 200mm. Easy to measure in a stage 2 mod.
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Re: XLS to Calculate Etalon and BF Distances for PST Mods
I've just spent a couple of minutes going back over my old spreadsheet...
Just to clarify, the spreadsheet assumes 200mm between chip and etalon as standard.
The main variable was the distance between the objective lens and the etalon, which brings up the point in my previous post.
Just to clarify, the spreadsheet assumes 200mm between chip and etalon as standard.
The main variable was the distance between the objective lens and the etalon, which brings up the point in my previous post.
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Re: XLS to Calculate Etalon and BF Distances for PST Mods
So, if the PST objective was pushed forward of the etalon by 35mm (i.e. 235mm from objective to etalon), it would avoid vignetting?
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Re: XLS to Calculate Etalon and BF Distances for PST Mods
Below is a link to an updated version of the spreadsheet.
http://www.astroshot.com/Solar/PST-Etal ... ign-V2.xls
I'd appreciate if it could be checked and I can update as necessary.
Thanks.
http://www.astroshot.com/Solar/PST-Etal ... ign-V2.xls
I'd appreciate if it could be checked and I can update as necessary.
Thanks.
Michael in Kildare, Ireland.
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Re: XLS to Calculate Etalon and BF Distances for PST Mods
The distance from the objective to the etalon is really not critical..
there's only one important distance and that is the distance from the prime focus to the etalon. For optimum performance this distance nust be 200mm.
That's it. Simple.
there's only one important distance and that is the distance from the prime focus to the etalon. For optimum performance this distance nust be 200mm.
That's it. Simple.
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Re: XLS to Calculate Etalon and BF Distances for PST Mods
Soooo, my method of calculating the magic 200mm for my 100mm and 127mm scopes isn't necessarily accurate: Assuming the quoated focal lengths of the scopes is correct I put a rigid tape measure in the ep end of the ota until it touched the objective, then, read 200mm back on the quoated fl. I guess the 2 assumptions in this method are the quoated focal length is accurate and the 'focal length' starts at the back of the lens; neither of which are potentially true :whistle:
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Re: XLS to Calculate Etalon and BF Distances for PST Mods
Mark,
As Roel has done..image the sun on a card and measure back 200mm towards the objective; that's where the PST etalon needs to be placed.
As Roel has done..image the sun on a card and measure back 200mm towards the objective; that's where the PST etalon needs to be placed.
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Re: XLS to Calculate Etalon and BF Distances for PST Mods
That's what I've done for the Vixen A80M and for the Omni 120 as well. I think is the only way to have a quite exact prime focus point. Anyway... as it should, is very near to the declared focal length, so you can use it to have an idea of where the prime focus lies even without using the card method
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Re: XLS to Calculate Etalon and BF Distances for PST Mods
Mark,
As Roel has done..image the sun on a card and measure back 200mm towards the objective; that's where the PST etalon needs to be placed.
Yup, that's what i'm going to do ultimately, just need the sun to come out :whistle:
As Roel has done..image the sun on a card and measure back 200mm towards the objective; that's where the PST etalon needs to be placed.
Yup, that's what i'm going to do ultimately, just need the sun to come out :whistle:
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Re: XLS to Calculate Etalon and BF Distances for PST Mods
I've updated the spreadsheet.
I'd appreciate a check.
http://www.astroshot.com/Solar/PST-Etal ... ign-V3.xls
*** Just made a quick edit to include "Actual Distance" over "Optimum Distance" ***
I'd appreciate a check.
http://www.astroshot.com/Solar/PST-Etal ... ign-V3.xls
*** Just made a quick edit to include "Actual Distance" over "Optimum Distance" ***
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Re: XLS to Calculate Etalon and BF Distances for PST Mods
Mark,
As Roel has done..image the sun on a card and measure back 200mm towards the objective; that's where the PST etalon needs to be placed.
That of course raises another point.
Just because a telescope is advertised with a focal length of say 1000mm, it's actual focal length may differ slightly.
Therefore, by measuring it via the method you have mentioned, it gets the etalon in the optimum position.
As Roel has done..image the sun on a card and measure back 200mm towards the objective; that's where the PST etalon needs to be placed.
That of course raises another point.
Just because a telescope is advertised with a focal length of say 1000mm, it's actual focal length may differ slightly.
Therefore, by measuring it via the method you have mentioned, it gets the etalon in the optimum position.
Michael in Kildare, Ireland.
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Re: XLS to Calculate Etalon and BF Distances for PST Mods
Well i've had a look and seems to be in working order to me.
I wonder, could somebody explain what the significance of the 'actual distance' and 'optimum distance' are for please? The only bit that has me pondering whats what...
I wonder, could somebody explain what the significance of the 'actual distance' and 'optimum distance' are for please? The only bit that has me pondering whats what...
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Re: XLS to Calculate Etalon and BF Distances for PST Mods
As I understand it....
The sun has an angular size as opposed to a star which is in theory a pinpoint of light.
To allow for the angular size, the etalon should be slightly further back from the objective.
In practice, it will most likely make very little difference as the effect of the vignetting will be very slight.
As it happens, the spreadhseet worked for f/10 scopes but didn't work for other ones. Error corrected below:
www.astroshot.com/Solar/PST-Etalon-Tele ... ign-V4.xls
The sun has an angular size as opposed to a star which is in theory a pinpoint of light.
To allow for the angular size, the etalon should be slightly further back from the objective.
In practice, it will most likely make very little difference as the effect of the vignetting will be very slight.
As it happens, the spreadhseet worked for f/10 scopes but didn't work for other ones. Error corrected below:
www.astroshot.com/Solar/PST-Etalon-Tele ... ign-V4.xls
Michael in Kildare, Ireland.
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Re: XLS to Calculate Etalon and BF Distances for PST Mods
The 200mm distance is a result of the first lens in the PST etalon being a -200mm focal length. The only way to have collimation is to have the lens placed at fl1+fl2=collimated beam. Since fl2 is negative we subtract.
Re: XLS to Calculate Etalon and BF Distances for PST Mods
I think the experience has been that cell f12=f21-200 and does not matter what the CCD diameter is. I have not done anything but disassemble the PST and make measurements of its properties. I have not done a mod.
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Re: XLS to Calculate Etalon and BF Distances for PST Mods
Does the design of the PST incorporate a slight amount of vignetting at the etalon?
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Re: XLS to Calculate Etalon and BF Distances for PST Mods
Michael,
Yes it does...the "optimum" set-up would have been a 35mm f10 system.
Yes it does...the "optimum" set-up would have been a 35mm f10 system.
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Re: XLS to Calculate Etalon and BF Distances for PST Mods
Just to come back to this topic... Regards the distance of the blocking filter. If I am using a BF10, is this to the front of the ITF (objective side) or the actual blocking filter itself (eyepeice side)?
Thanks,
Mark
Thanks,
Mark
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Re: XLS to Calculate Etalon and BF Distances for PST Mods
mark,
The ONLY critical dimension in a PST mod is the "majic 200mm"
The BF filter (or an upgrade BF10/ BF15) should be placed directly infront of the eyepiece or CCD...simples!
The ONLY critical dimension in a PST mod is the "majic 200mm"
The BF filter (or an upgrade BF10/ BF15) should be placed directly infront of the eyepiece or CCD...simples!
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Re: XLS to Calculate Etalon and BF Distances for PST Mods
mark,
The ONLY critical dimension in a PST mod is the "majic 200mm"
The BF filter (or an upgrade BF10/ BF15) should be placed directly infront of the eyepiece or CCD...simples!
Cheers Ken. All the pieces are starting to fall into place now; I have the donor OTA, all plumbing required to join together is present, got a spare SCT focuser to use, I now have 2 boa clamps, D-ERF is currently somewhere in transit from Germany, gonna make the cell for the ERF at work this week - have the work PST to test things out, then will buy a dedicated PST (well its etalon) at the end of this month to make a permanent installation! I'm looking forward to it!
The ONLY critical dimension in a PST mod is the "majic 200mm"
The BF filter (or an upgrade BF10/ BF15) should be placed directly infront of the eyepiece or CCD...simples!
Cheers Ken. All the pieces are starting to fall into place now; I have the donor OTA, all plumbing required to join together is present, got a spare SCT focuser to use, I now have 2 boa clamps, D-ERF is currently somewhere in transit from Germany, gonna make the cell for the ERF at work this week - have the work PST to test things out, then will buy a dedicated PST (well its etalon) at the end of this month to make a permanent installation! I'm looking forward to it!
http://brierleyhillsolar.blogspot.co.uk/
Solar images, a collection of all the most up to date live solar data on the web, imaging & processing tutorials - please take a look!