Barlow F30?

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Galosimo
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Barlow F30?

Post by Galosimo » Tue Apr 28, 2015 12:32 pm

I was wondering if anyone has done an experiment like this: I would try to work my pst to F30, I mean. removing the front lens of the pst classical 25-200 focal length and mounting a barlow (in my case 5x) telescope 152/900 individual.... adapted to the thread what would come out? I ask this because thinking of what to change the daystar still working to f 30.

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Re: Barlow F30?

Post by marktownley » Tue Apr 28, 2015 1:39 pm

Needs to be telecentric at f30, not barlow at f30, you will get a shift in centre wavelength and a change in bandwidth across the field of view - basically a big sweet spot.
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Re: Barlow F30?

Post by antonello » Wed Apr 29, 2015 9:25 am

Yes, you have to use a telecentric Barlow and you have to remove both lenses from PST, also the positive rear ...

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Re: Barlow F30?

Post by marktownley » Wed Apr 29, 2015 10:18 am

Yes, if it's a telecentric.
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Re: Barlow F30?

Post by Galosimo » Wed Apr 29, 2015 2:05 pm

but the image will be improved or worsened? I thought of removing only the front lens and the rear riconverge beam, or not?

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Re: Barlow F30?

Post by mdwmark » Wed Apr 29, 2015 11:31 pm

There is nothing magical about F/30. An air spaced etalon will not improve by just making it F/30. I would keep it the same.
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Re: Barlow F30?

Post by antonello » Thu Apr 30, 2015 10:22 am

Galosimo,

If I understand correctly, you want to find a solution to use the Etalon PST in your telescope 152/900 f: 6 (which is different from the original PST telescope 40/400 f: 10).
As you know you can remove the front lens of 40/400 original PST and put another telescope f: 10 with a very large lens. Everything works perfectly, but with the disadvantage of a very small field, because the rays that originate from a field larger (for example, the edges of the sun) do not enter inside the Etalon of PST.
As you know, by directly using the PST on the telescope 150/900 you would use only 90 mm of the diameter of the lens, being the diameter of the Etalon PST it is only 20 mm in diameter, and being designed to f: 10.
If I understand correctly, you would like to replace the negative lens -200 mm focal length of the PST with a negative lens of shorter focal length (-40 mm, for example) so that the PST collects all the light lens, also the one that comes from the edges of the lens, to use the entire opening by 152 mm.
This is theoretically possible, in fact in this way all the axial parallel rays enter the objective lens in the negative -40 mm, will come out parallel, parallel pass inside the etalon and then will be brought to the fire to 200 mm from the positive lens behind the etalon ( 200 mm focal length).
The problem is that the negative lens and the positive lens of the PST are calculated to compensate their aberrations. Place a new negative lens in front of the PST, replacing the original (with a smaller focal, also) certainly it will produce a penalty of quality and this beyond the problems of sweet spot.
To work around this problem will need to design and to build a new lens or lens system that works well in combination with the rear positive original lens of the PST. But this solution, however, is not good because it works only for a very small angle.

We analyze the problem better.

Figure 1 shows a simulation of the original PST.
The green rays are the rays axial parallel, blue ones are the off-axis. The Etalon PST NOT is a telecentric system, but it is a "system collimated." The PST works perfectly because the angle between the assial rays and the axial out center inside the Etalon is a maximum of 0.61°, and you can see properly in H-alpha whole field. Remember that in order for an etalon functions well, all the beams must be parallel within 2 °! (f:30).
Now we change the lens -200 mm of the PST with a -40 mm lens and put the new group PST at the right place, in your telescope 152/900. (See Figure 2)

As you can see now the difference between parallel and axial estrassiali rays entering the Etalon is no longer 0,61 °, but about 5.6 °, and you will not see in H-alpha the entire disc of the sun. With this system (if the negative lens is well designed) you can see the H-alpha in a limited field of a few minutes of arc.
For all these reasons, this system is not good (for me).

To get a good result you have to remove not only the negative lens in front of the PST, but also the lens back positive. After this,you will put the PST after a telecentric Barlow as in Figure 3.

So that the system functions perfectly, cones output rays from the telecentric Barlow must be f: 30 and then you need a telecentric lens of 5X.

Using this system you do not have problem of the field of the Sun. All that enters in the CCD or within the iris eyepiece will be in H-alpha. The limit is only given by the diameter of the Blocking Filter.
Using a telecentric Barlow you will have a good quality, the highest possible, especially if you use a good barlw telecentric lens as Televue powermate 5X.
A good alternative is to build a self-made a telecentric Barlow from a normal Barlow. This is convenient if you have a normal good Barlow, for example a Celestron Ultima apo Barlow, that is one of the Balow with the shorter focal length (-60 mm) . The result you get (as is clear from my test) is excellent. If you can remove even the positive lens from the PST I can explain to you how do.

If you can not remove the positive lens from the PS, you could use it in another way (complete of the two lenses, but without 40/400 lens) planning with Zemax or Oslo a auxiliary negative lens (or lenses system) that transform the 152/900 in a 152/1520 (f: 10) and then use the etalon PST complete its two lenses. This solution is however very expensive.

Summarizing, if you can remove even the positive lens of the Etalon uses the solution of Figure 3 and everything is solved.
Sorry for my bad English
Galosimo wrote:I was wondering if anyone has done an experiment like this: I would try to work my pst to F30, I mean. removing the front lens of the pst classical 25-200 focal length and mounting a barlow (in my case 5x) telescope 152/900 individual.... adapted to the thread what would come out? I ask this because thinking of what to change the daystar still working to f 30.
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Last edited by antonello on Thu May 07, 2015 5:34 am, edited 3 times in total.

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Re: Barlow F30?

Post by Galosimo » Thu Apr 30, 2015 4:18 pm

Thanks for everything and I summarize. I or a 152/900 individual mod pst BF10 but BF led to 11 mm with change the front lens 25-120 modified , erf 75 lunt whithin focouser and focuser Secondary SC. Everything works well and turret is that it is a show, very beautiful images from fear to 90/100 x. but I was thinking of getting a better result and from what I understand I have to take a telecentric 5x TeleVue and remove all 2 lenses pst front and back that is the lens 25-200 and the 25 + 200 right ? I have to adapt to the telecentric lens thread blocks pst ? thanks antonello if you're Italian'll give you my cell so we feel I'm on facebook "galelli simone".
Last edited by Galosimo on Thu Apr 30, 2015 4:41 pm, edited 1 time in total.

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Re: Barlow F30?

Post by Galosimo » Thu Apr 30, 2015 4:39 pm

Ok
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Re: Barlow F30?

Post by mdwmark » Thu Apr 30, 2015 8:30 pm

I see you can do an ray trace for the telecentric. There is no way you can make an 5X telecentric with an -40mm singlet and keep the aberrations under control. I can see you want to use the small etalon for a full disk. But,please show what the aberrations look like ( spot size ,OPD,MTF)from your example.

If you had custom lens made and you where starting with at least an -100mm you might be able to use two lens. With standard lens that you can buy off the shelf . You would need at least 3 lens. Two of then, neg focal length and have them spaced out. The last an positive lens of about 200mm focal length or longer.
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Re: Barlow F30?

Post by Galosimo » Fri May 01, 2015 5:39 pm

I use a lens 25-120

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Re: Barlow F30?

Post by marktownley » Fri May 01, 2015 8:43 pm

Galosimo wrote:I use a lens 25-120

A collimating lens is most definitely different than a telecentric system...
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Re: Barlow F30?

Post by antonello » Sat May 02, 2015 7:19 am

Well said, marktownley

me too have concluded my message suggesting to Galosimo the use of a telecentric 5x .

As I have shown, under certain conditions it is possible to use a collimated system (such as Lunt telescopes with internal Etalon and like the CORONADO PST), but it is not possible or advisable in the case of galosimo, who wrote of being able to remove only the front lens of the PST (and not the positive lens of PST).

Galosimo now uses a single lens negativa Optosigma 25/120, but the system is not f:30, as he had requested in your first message and as I had understood that he wanted to (perhaps for reasons of correct pixel sampling).

However, it is difficult (though he writes of seeing well with the lens 25/120) that this 25/120 negative lens exploits all quality of his telescope 152/900.
If we had the parameters of the lens Optosigma 25/120 and of the positive lens of the PST would be easy to check (with Zemax or Oslo) that the quality is not the best possible.

To get the best possible quality, I can design (as I have already written in my previous post) "a new negative lens or a negative lens system" .

Without removing the positive lens of the PST, it will probably be necessary to design a negative lens consisting of two lenses (you must know the parameters of positive lens of the PST).

If, however, it is possible to remove the positive lens of the PST, I can design a new system of a single negative lens and a single positive lens (a new system of collimation, no more 1:1, as the original PST, but for example 3x or 5x), with perfectly controlled aberrations (for h-alpha only).

As I have already written, this is a waste of time: this solution costs a lot of money (more 40 $ for two lens with diametr 25 mm) and has a very small field.

The solution of the telecentric Barlow 5X is the most obvious and most economical.

I report to Galosimo that with the lens 25/120, it is used the entire diameter of 152 mm. only on the axis. With a telecentric system 5X will use all diameter of the telescope in all field that you see in the eyepiece.

Mdwmark,

In my message you do not see an rays trace for the telecentric (or for collimating) because I not have done an optical design. I just used Oslo to determine the geometric characteristics of the system and not for the aberrations.

Concerning the aberrations I have already above.
mdwmark wrote:... I can see you want to use the small etalon for a full disk....
No, No one asked this! Where did you read? ;) Galosimo not have request this (only in figure 2, the field is accidentally of 30 'because I used another project Oslo, leaving the value of 30', but no one wants to get all the solar diameter with this system or with a telecentric system).

I have also wrote in my previous post that the of use of the PST (without Objective lens) with a great new big telescope It has the disadvantage that you can not see the whole solar diameter: "the disadvantage of a very small field, because the rays that originate from a field larger (for example, the edges of the sun) do not enter inside the Etalon of PST".

Galosimo wants exploit all the diameter of his telescope, not to have all the field of the Sun (read well the first message).

Finally, that of Figure 2, in which I put the lens -40 mm (as requested by galosimo for to have f: 30), NOT is a telecentric system, but collimated.

Antonello
Last edited by antonello on Tue May 05, 2015 8:40 am, edited 3 times in total.

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Re: Barlow F30?

Post by mdwmark » Sat May 02, 2015 9:34 pm

I got it from the diagram where all the rays are going through the etalon from the objective lens.
That is what I thought you where going for.
I have tried using short negative focal length lens but it works out better if you use longer ones.
I attached and diagram for an Sky 90 to push it out to F/34. These where all stock lens.
The whole thing is about 250mm long with a 200mm back focus.
I only have the ray trace on file in this computer.
I'm always amaze that you can have the same focal length lens and depending on the lens design can make a big difference in the final results.
Mark W.
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WAGNER%20TAK%2090%20TELECENTRIC%20ATTACHMENT%20032406.docx
Shy 90 telecentric
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Re: Barlow F30?

Post by antonello » Sun May 03, 2015 3:27 pm

Thank you mdwmark

I saw your ZEMAX project.
Really many compliments for the excellent design with commercial lenses. From what I have seen everything has to work perfectly.
Your telecentric barlow transforms in f: 32 an f: 5.56, and seems suitable for a specific telescope, in fact is 5.75X.

Who needs a telecentric system 5X, can take this as a model is slightly adjust the project for to have exactly 5X (or use your project as is) .

Thank you for sharing your project. When I have a free moment I will check my projects and I will share what I will find useful for this post.

Many thank you

Antonello

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Re: Barlow F30?

Post by Galosimo » Sun May 03, 2015 7:03 pm

if you can get to the 152/900 so as to have a line with the specific lens. For me it is very interesting this discussion.

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Re: Barlow F30?

Post by mdwmark » Sun May 03, 2015 11:57 pm

I'll find the specs for this telecentric and post them.
If you ever seen any of Pat Stoker's images they where shot with this telecentric design on his Sky 90 stopped to 85mm.
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Re: Barlow F30?

Post by Galosimo » Thu May 07, 2015 7:44 pm

"The problem is that the final beam of telecentric system is 50 mm and the Etalon of PST it is 20 mm ... We were thinking, with Antonello, to try a normal Barlow to get 5X ... with the etalon very close to Barlow."

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