F11 PST scope

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F11 PST scope

Post by Moriniboy » Wed May 11, 2016 6:15 am

Hi

I have a Vixen 80mm f11 scope which I would like to use for a PST mod, as it is not the recommended F10 what differences in view can I expect?

Thanks Nigel
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Re: F11 PST scope

Post by marktownley » Wed May 11, 2016 8:40 am

Will be absolutely fine! Go for it :)
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Re: F11 PST scope

Post by Merlin66 » Thu May 12, 2016 8:22 pm

Yes, any donor OTA with f10 or greater is great for the PST mod.
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Re: F11 PST scope

Post by Valery » Fri May 13, 2016 12:07 am

Merlin66 wrote:Yes, any donor OTA with f10 or greater is great for the PST mod.
No! Not all OTAs are good. Only F/9-F/11. The best is F/10.

At F/9 one will loose some aperture size (10% of it) and at F/11 one will loose 10% of a useful sweet spot size. At F/15 one will loose half of possible sweet spot!


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Re: F11 PST scope

Post by Merlin66 » Fri May 13, 2016 7:49 pm

Agreed!
f10 is the optimum.
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Re: F11 PST scope

Post by antonello » Thu May 19, 2016 10:21 am

Perhaps it is better to be more specific

At the same focal, closing the opening to the objective, all extrassial rays, inside a collimated etalon, continuing to have the same angle. So the sweet spot remains the same.
In other words: A 80 mm vixen F: 11 has the same sweet spot of 40 mm F.22.

The Swet spot changes (decreases) if you increase the focal length to diameter equal.

A 80 mm F: 22 has a smaller sweet spot of a 80 mm F: 11

Or when increases the focal at equal F number

A 160 mm F: 11 has a seet spot smaller of a 80 mm F: 11

Do you agree?

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Re: F11 PST scope

Post by Merlin66 » Thu May 19, 2016 11:35 am

Antonella,
I'm sorry I can't comment on the possible changes to the "Sweet Spot".
All the PST mods I've done have made use of f10 or f11 donors.
I've successfully used apertures from 80mm to 102mm with no obvious change to the Jaquinot spot.
My understanding is that the spot can be influenced by the change in the solar field angle.
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Re: F11 PST scope

Post by antonello » Thu May 19, 2016 3:25 pm

... What I have written is a result of the analysis with an optical design software .... (OSLO)

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Re: F11 PST scope

Post by Merlin66 » Thu May 19, 2016 3:27 pm

Antonella,
Interesting....
How does the OSLO software determine the Jaquinot spot??
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Re: F11 PST scope

Post by antonello » Thu May 19, 2016 3:38 pm

OSLO (as zemax) does not determine the sweet spot...
Oslo allows you to calculate the angle of the collimated rays inside the etalon. Many larger is this angle, respect to optical axis, and many small is the sweet spot. Elementary.

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Re: F11 PST scope

Post by Merlin66 » Thu May 19, 2016 3:45 pm

Antonello,
My understanding of the Jaquinot spot is based on a 1/4 wave difference to the first interference ring.....if it is within this allowance that defines the size of the spot.
What field angle are you considering?
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Re: F11 PST scope

Post by antonello » Thu May 19, 2016 4:06 pm

My English is not good to understand what you're thinking. My approach is more experimental: in my collimated system with etalon (a Lunt etalon 60 mm) inside my F:10 telescope, 100 mm f:1000 there is an image area (0.5 °) in which I see the sun in H-alpha. Outside this circle (sweet spot) the image is out H-alpha. If I change the lens, increasing the focal length decreases the circle in H-alpha (< 0.5°). I hope to have write well in English my thoughts.
What do you mean for Jaquinot spot? Can you tell me a literature about it? Thank you.

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Re: F11 PST scope

Post by Merlin66 » Thu May 19, 2016 4:51 pm

Antonello,
I don't have access to my computer to supply the links I have...
If you search this forum on "sweet spot" and look for this discussions in early 2015 ( Bob Y. Etc) you should find the formula and discussions on the Jaquinot spot.
"Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before
https://groups.yahoo.com/neo/groups/ast ... scopy/info
"Astronomical Spectroscopy for Amateurs" and
"Imaging Sunlight - using a digital spectroheliograph" - Springer

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Re: F11 PST scope

Post by antonello » Thu May 19, 2016 5:00 pm

Thank you Merlin, it is always nice to learn.
Thank you for references. I will look.

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Re: F11 PST scope

Post by Merlin66 » Fri May 20, 2016 2:20 pm

http://www.astrosurf.com/viladrich/astr ... o-tele.htm
This is an excellent reference for those interested in etalons and their performance.
"Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before
https://groups.yahoo.com/neo/groups/ast ... scopy/info
"Astronomical Spectroscopy for Amateurs" and
"Imaging Sunlight - using a digital spectroheliograph" - Springer

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Re: F11 PST scope

Post by Valery » Tue May 24, 2016 3:32 am

antonello wrote:My English is not good to understand what you're thinking. My approach is more experimental: in my collimated system with etalon (a Lunt etalon 60 mm) inside my F:10 telescope, 100 mm f:1000 there is an image area (0.5 °) in which I see the sun in H-alpha. Outside this circle (sweet spot) the image is out H-alpha. If I change the lens, increasing the focal length decreases the circle in H-alpha (< 0.5°). I hope to have write well in English my thoughts.
What do you mean for Jaquinot spot? Can you tell me a literature about it? Thank you.
Hi Antonello,

In a collimated scheme a size of a sweet spot depends of the ratio De / Do. De states for etalon diameter and Do
states for telescope objective diameter.
It also depends of an etalon bandwidth. The narrower the bandwidth, the smaller the sweet spot.

For a 0,7A bandwidth the sweet spot in degree (on the sky) will be De / Do.


Valery
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Largest full size 185 - 356mm Dielectric Energy Rejection Filters (D-ERF) by ARIES Instruments.

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Re: F11 PST scope

Post by antonello » Tue May 24, 2016 11:16 am

Merlin66 wrote:Antonello,
I don't have access to my computer to supply the links I have...
If you search this forum on "sweet spot" and look for this discussions in early 2015 ( Bob Y. Etc) you should find the formula and discussions on the Jaquinot spot.
Merlin
Thank you,
but I already know of these documents. What I want to know is what is "Jaquinot spot", which I not find in this documents
Valery wrote: Hi Antonello, In a collimated scheme a size of a sweet spot depends of the ratio De / Do...
Valery

Yes, I am in agreement with you, but what I've written is related to experiments with a group collimated fixed Etalon that I have tried with varying objectives. For this condition are valid my statements, that do not conflict with the rule De / Do.

I have experienced these problems and also I thought, like you, that in a collimated system the Sweet Spot is "teorically" De / Do ...(is a mathematical deduction).

Unfortunately, I have experienced that the sweet spot in degrees of the sky, for me with 100% H-alpha, it is not exactly De / Do.

In my telescope of 100 mm f:10 with collimated etalon homebuilt, I see the sun's prominences, but the quality at 0.25° from the center (on the edge of the sun) is lower than I have with my telescope of 60 mm placing the etalon Lunt in front to the objective lens. I think it's important for avoid saying different things (and maybe find out after we're saying the same thing) define precisely the Sweet Spot. What is the correct definition of Sweet Spot? Is it the spot of 100% H-alpha or not?

I hope do not write wrong things, but I have experienced that the Sweet Spot has no defined limit, but it is degrading. There is a circle in which one can see everything in H-alpha, then a ring in which the quality deteriorates and the rest is no longer in H-alpha. In my system with 100 mm lens Ff:10 with etalon Lunt 60 mm collimated (0.7 A), the diameter in degrees sweet spot of 100% H-alpha not is 60/100 = 0.6 degrees (all the Sun), but smaller. My telescope 100 mm f:10 is now in the workshop and I can not precisely control, but I think the good circle of 100% Sweet Spot is about 0.3 degrees.

Thank you

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Re: F11 PST scope

Post by antonello » Tue May 24, 2016 6:33 pm

Merlin66 wrote:http://www.astrosurf.com/viladrich/astr ... o-tele.htm
This is an excellent reference for those interested in etalons and their performance.
OK Merlin, I have found the reference to Jacquinot spot:
http://www.cloudynights.com/topic/53713 ... ng-etalon/
Now I study this communication.

Meanwhile I have had confirmation that the Sweet Spot is not the circle of the points that have 100% H-alpha, but the spot where Δλ = √2 x FWHM

So if the Sweet Spot it is about 1 °, the maximum angle of the paraxial rays inside the etalon so that the image points are in the sweet spot must be 0.5 °.
To obtain the highest quality of H-alpha I think we need to take a smaller angle inside the etalon, about 0.25 ° -0.3 °.
This means That the Sweet Spot for a good quality of the entire image it could be "0.5xDe/Do"
What do you think?

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Re: F11 PST scope

Post by antonello » Mon May 30, 2016 10:32 am

Merlin

I read the messages of BYoesle:
http://www.cloudynights.com/topic/53713 ... ng-etalon/
with great interest.

Therefore "Jaquinot spot" (that I did not know before) it is only synonymous of Sweet spot ...

Valery

I think it is useful to point out that "Sweet spot = De / Do", for a etalon 0.7A, is theoretically, with good approximation, the field in degrees of Sweet Spot. But the real value will be a little smaller (0.6?) for many problems that all know.
I think it is useful to point out for those not familiar with the theory that in "Seet spot=De/Do", that "De" is NOT the physical diameter of the Etalon, but the diameter by the beam rays of a single point image (for example, for one point in center of the sun). This diameter is obviously less of etalon diameter...
In my previous post I wrote that in a telescope 80 mm f: 11 and in a telescope 40 mm f: 22 the Seet Spot is the same ... This is obvious...
Who wants use the PST for modding a telescope, should know that at equal focal the Sweet spot is the same, while at the same diameter, the telescope with a larger focal will have a smaller sweet spot, as you have rightly wrote.
This is why, by the modding of a PST is convenient to use a telescope with max f: 10 (f:10 because, as we all know, the collimator lenses are calculated for f: 10), but if we accept a small sweet spot, we can use any telescope with smaller number of 10, as it wants to (f: 15, f: 20, etc.).

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