PST mod is it worth it?

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Re: PST mod is it worth it?

Post by Rusted »

The PST etalons are indeed tuned by straight compression.
Between a Teflon O-ring and quite a soft sponge ring.

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P1460764 rsz.JPG
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There is normally a textured rubber band over a shell type of ring.
The inner and outer ring are joined by a simple screw.
I have removed the rubber band and shell ring to allow direct access to the inner ring.
The inner ring is a protrusion on the edge of a machined, "top hat" component which houses the etalon.


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Re: PST mod is it worth it?

Post by christian viladrich »

Great ! Thanks for sharing.

I was wondering how much pressure is applied to the etalon.
Is it possible to assess how much the orange ring is compressed (in mm) when the etalon is tuned ? Rough figures would be fine.
Then, it would be interesting to see how much weight you need to put on the ring to compress it. Or put on it a given weigth and see how much compression there is (in mm).
Then we could calculate the pressure on the four triangular spacers separating the two plates of the etalon.


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Re: PST mod is it worth it?

Post by Rusted »

christian viladrich wrote: Sun Jul 18, 2021 3:00 pm Great ! Thanks for sharing.

I was wondering how much pressure is applied to the etalon.
Is it possible to assess how much the orange ring is compressed (in mm) when the etalon is tuned ? Rough figures would be fine.
Then, it would be interesting to see how much weight you need to put on the ring to compress it. Or put on it a given weigth and see how much compression there is (in mm).
Then we could calculate the pressure on the four triangular spacers separating the two plates of the etalon.
It feels as if it goes completely solid when adjusted too far. Even a coarse screw thread can put tremendous pressure on any object.
So I'd need to make an identical sized "plunger" to avoid stressing the etalon with hefty weights placed on top. :D
I'll turn up the end of a bit of brass rod in the lathe and measure the load required to flatten the sponge ring.
Or I cold use a fisherman's spring balance with a hooked plunger.
It may simply be that the the nested components are designed to contact each other internally before the etalon is crushed.
I can check for that without the etalon element in place.


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Re: PST mod is it worth it?

Post by Rusted »

Well, that was interesting.

The etalon is 30mm Ø x 16mm thick.
The sponge rubber ring is 33mm OD x 22mm ID x 4.5mm thick.

I applied loads via a 30mm brass plunger to the sponge ring.
It showed no visible deformation with up to 6kg on top.
Which was the only useful weight I had easily to hand.

How do I judge the required degree of deformation?
Do I keep adding more weight until the desk collapses? :D
I cannot noticeably deform the rubber ring with a 30mm disk on the edge of the desk using all my grip strength.

NB. There is no physical contact between the two metal components of the etalon housing.
The inner ring can be screwed right down out of sight without the etalon present.
The thread is quite coarse but I haven't tried to measure the pitch.


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Re: PST mod is it worth it?

Post by christian viladrich »

That is very interesting ! This orange sponge is indeed very rigid.

With some rough calculation, we have :
delta lambda / lambda = delta gap / gap = pressure / Young module of the spacers

We can estimate the force in Newton to be applied to the spacers for a 0.5 A shift of CWL :
- let's assume 4 spacers, each spacer is a triangle of 3 mm on a side (I am not sure of that)=> total surface = 18 10-6 m2
- fused silica young module is 70 10^9 Pascal
so the pressure to be applied to the spacers is : 70 10 ^9 * 0.5 / 6563 = 5.3 10 ^6
and the force to be applied on the spacers : 5.3 x 18 = 95 Newton (=9.5 kg).

Given you saw no deformation at 6 kg, this 9.5 kg for 0.5 A delta CWL looks quite possible.


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Re: PST mod is it worth it?

Post by christian viladrich »

I've found some formulae giving the axial force applied by a screw for a given torque. It depends on a number of assumptions (thread / diameter / friction coefficient / etc.). Still, with a very small torque, it is possible to have a 95 Newton axial force.


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Re: PST mod is it worth it?

Post by marktownley »

You could try 'packing out' the pST etalon Chirs, it is 30mm diameter, the foam ring is 33mm, so, you might be getting some 'slop' of the etalon in its sleeve housing. This can cause the etalon to tilt in it's housing which can cause the banding you see.

Wrap the etalon in PTFE tape to get an interference fit in the sleeve and see if this gets rid of the banding you are experiencing. It's completely reversible and non destructive.

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Re: PST mod is it worth it?

Post by thesmiths »

I think Mark is on to something here. If it's true the tuning is by compression rather than tilt, then the annular sweet spot is an aberration caused unintended tilt (rather than an intentional tilt). Having taken apart my PST, the rubber was in rather poor shape after a decade (most of it not being used much) so I could imagine it would not provide a very stable base which would compress uniformly.


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Re: PST mod is it worth it?

Post by christian viladrich »

My guess is that the etalon "sits" on the teflon O-ring. In other words, this is the reference surface setting the position of the etalon. This being said, depending on the amount of play in the thread, the threaded "cover" could be a little bit tilted (or not).


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Re: PST mod is it worth it?

Post by Rusted »

Thanks for the suggestions. :bow

I have been wrapping the etalon element[s] in plumbers tape in varying degrees for some time.
There is lots of play between the etalon element[s]s and their housing.
I have also been deliberately tilting the etalon.
There is quite some play between the two threaded, housing components themselves.

There was time when I could simply tilt the inner ring and the entire field would darken.
Revealing the surface texture in fine detail: Sadly, this is no longer the case.

It is quite possible that I am responsible for this situation.
If the etalon is resting on a rigid Teflon O-ring.
Then it probably doesn't like being tilted.
It may even produce a shearing action at the etalon joint.
The original design incorporated an outer shell and textured rubber, adjustment ring.
The single screw connecting the inner and outer components would not allow tilting.

The rubber ring may be a protective compliance.
Not just for knocks and the PST being dropped.
But preventing the worst forces from desperate users being applied. :oops:


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Re: PST mod is it worth it?

Post by thesmiths »

christian viladrich wrote: Mon Jul 19, 2021 4:16 pm My guess is that the etalon "sits" on the teflon O-ring. In other words, this is the reference surface setting the position of the etalon.
Christian, you're quite right, the Teflon ring would provide a flat reference surface. Another way the incoming light could become tilted would be if the collimating lens were not aligned correctly.


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Re: PST mod is it worth it?

Post by Merlin66 »

This is the reply I got on the subject:

"""The PST uses compression tuning. Probably the worst way imaginable to tune an etalon. The Teflon ring in the chamber is the main point of pressure. The silicone ring is designed to smash the etalon until it comes into tune. Since it never compresses evenly the silicone ring is trimmed and cut so that it applies pressure as evenly as possible. This can only be done while applying pressure with the rotational compression ring and trimming parts of the silicone while looking through the chamber at a hydrogen spectrum light. It is not compressing the feet at all it is deforming the plates around the feet. That is why the silicone is trimmed to even out etalon while it is crushed. The compression itself is not capable of enough force to compress the feet. And compression the feet since they are fuse silica as well would in fact crack crush or powder the feet to a point of loss of contact. Thus destroying the etalon. """


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Re: PST mod is it worth it?

Post by Rusted »

And yet most PSTs continue to function for years or even decades. Rust permitting.
While my own, unmodified, rubber/silicone [?] ring appears absolutely pristine.
It cannot have been replaced because the whole PST was cemented with hard, thread locker.

It would be difficult to imagine how the etalon collimation lenses could tilt.
Both are screwed firmly into the same housing.
Any tilt of the inner, etalon housing would go completely unnoticed.

My use of larger and better quality fittings no longer allows the filter stack to droop.
Perhaps there lies the secret? It may perform better as a floppy assembly.
Though the etalon is very early in the stack and relies directly on the stiffness of the focuser.
I went from an indecently wobbly, vintage 2" Vixen to a 2.5" FT.

Previous sag may have provided exactly the correct degree of tilt?
Though that would surely have been rotationally critical. [?]
As the OTA progressed from east to west. With a meridian flip midway.

I should add that I have tried tuning over a wide range of [very] non-200mm inside focus, etalon positions.
Just in case there was something non-standard about the collimation lenses.
Such is my desperation to continue imaging with this PST etalon before giving up.

European prices for Lunt etalons are far higher [1.6x minimum] than USD on straight currency conversions.
Bresser has the sole LUNT monopoly. While the EU applies a minimum of 25% importation + purchase tax to all goods.


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Re: PST mod is it worth it?

Post by marktownley »

Have you still got the 'trimmings' on the silicone ring Chris?


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Re: PST mod is it worth it?

Post by Bob Yoesle »

This is the reply I got on the subject:

"""The PST uses compression tuning. Probably the worst way imaginable to tune an etalon. The Teflon ring in the chamber is the main point of pressure. The silicone ring is designed to smash the etalon until it comes into tune. Since it never compresses evenly the silicone ring is trimmed and cut so that it applies pressure as evenly as possible. This can only be done while applying pressure with the rotational compression ring and trimming parts of the silicone while looking through the chamber at a hydrogen spectrum light. It is not compressing the feet at all it is deforming the plates around the feet. That is why the silicone is trimmed to even out etalon while it is crushed. The compression itself is not capable of enough force to compress the feet. And compression the feet since they are fuse silica as well would in fact crack crush or powder the feet to a point of loss of contact. Thus destroying the etalon. """
Hi Ken,

The relevant Coronado patents, US 7,054,518 B2 and US 7,149,377 B2, state that the etalon is tuned by compressing the spacers (aka "feet") which thereby effects the change in the etalon cavity length; the foam ring makes the compression forces symmetrical. Some people find it hard to believe that the thin glass spacers can be compressed, but on the micro-scales involved it indeed apparently works as intended - and patented.


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Re: PST mod is it worth it?

Post by christian viladrich »

Merlin66 wrote: Tue Jul 20, 2021 2:45 am This is the reply I got on the subject:

"""The PST uses compression tuning. Probably the worst way imaginable to tune an etalon. The Teflon ring in the chamber is the main point of pressure. The silicone ring is designed to smash the etalon until it comes into tune. Since it never compresses evenly the silicone ring is trimmed and cut so that it applies pressure as evenly as possible. This can only be done while applying pressure with the rotational compression ring and trimming parts of the silicone while looking through the chamber at a hydrogen spectrum light. It is not compressing the feet at all it is deforming the plates around the feet. That is why the silicone is trimmed to even out etalon while it is crushed. The compression itself is not capable of enough force to compress the feet. And compression the feet since they are fuse silica as well would in fact crack crush or powder the feet to a point of loss of contact. Thus destroying the etalon. """
Hi Ken,
Very interesting. And still a bit strange.

Calculation shows we just need about 10 kg force to tune the etalon by 0.5 A. I can't imagine how 10 kg could crush the spacers. Spacers are stuck with molecular force.
If the plate was deformed between the spacers, it would be easily seen with the Ha lamp. I can't see anything like this. I will double check. For the record, the deformation of plate due to the central spacer is obvious in the SM 40 etalon.

If the teflon ring is indeed "trimmed", it means that a difference in rotation of the ring would change the uniformity of the etalon. I am not sure this is a case.
My guess is that Teflon is quite homogenous. Non even compression would come from non even shape. I don't know what are the tolerance in shape for this kind of ring.
Last edited by christian viladrich on Tue Jul 20, 2021 7:58 am, edited 1 time in total.


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Re: PST mod is it worth it?

Post by christian viladrich »

BTW, what was the source of information Ken ?


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Re: PST mod is it worth it?

Post by christian viladrich »

Just wondering ...Rusted can you measure the thickness of the O-ring in different locations (i.e. four different azimuths) ?
Maybe with 0.01 mm accuracy ?


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Re: PST mod is it worth it?

Post by Rusted »

christian viladrich wrote: Tue Jul 20, 2021 8:13 am Just wondering ...Rusted can you measure the thickness of the O-ring in different locations (i.e. four different azimuths) ?
Maybe with 0.01 mm accuracy ?
EDIT: Apologies and a correction to my earlier statement:

I have found my mislaid, metric micrometer.
Measuring the Teflon O-ring, in situ, in its housing, I find 0.02mm variation.
The high is in one spot only. Not two, diametrically opposite.
With 0.01mm at roughly the halfway points. So slightly tilted?

There is no sign of any modification, nor addition to the orange, rubber ring.
Some users have reported added spacer tabs on opposite sides. Mine has none of these.
The orange ring seems perfectly uniform.


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Re: PST mod is it worth it?

Post by Merlin66 »

The reply and subsequent discussions (!!! - I didn't believe it...) were "off the record" with Brian Stephens.
My initial message:

Quote
30 Mar 2020, 14:27

to BRIAN
Brian,

I'm hoping you may be able to assist me. I'd like to know how the etalon in the PST is tuned....
We've struggled with this for over ten years! If you can put me out of misery I'd be grateful.

The Lunt Patent from 2005 https://patents.google.com/patent/US7149377 and the referred Figs 13 and 14 infer some type of "pressure tuning" but the mechanism is unclear.

-The O ring and foam ring don't seem strong enough to support squeezing of the etalon spacers.
-The sealing arrangement around the etalon assembly doesn't seem robust enough to support atmospheric tuning.
We therefore infer that somehow the etalon is tilt tuned....aided by the front foam ring...
The options I think are:

1. The PST etalon tuning is achieved by tilting of the etalon
2. The PST etalon is tuned by pressure applied to the spacers in the etalon
3. The PST etalon is tuned by increasing/ decreasing the atmospheric pressure around the etalon
4. Another method????

Can you confirm which one is correct??
I really appreciate your help and assistance to clarify this 15 year old mystery.

Unquote

Subsequent discussion:

The distortion of the etalon plates due to the pressure point difference between the rear ring and the front foam is used for tuning.
The spacer flakes are NOT distorted/ squeezed/ made thinner. The offset between the pressure circle of the Teflon ring and the silicon ring causes the bending of the etalon plates and hence the tuning.

Don't shoot the messenger! I was as surprised as you to hear the "correct answer".
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Re: PST mod is it worth it?

Post by AndiesHandyHandies »

Hi

If the pressure is only applied round the edge it can only logically work by compessing the feet slightly.

The orange ring will compress over time I would think to even out the force.

Frank Hourigan made me a 1" plate F4 mirror for the Birmingam AS. Designed for the scope to fit in a family hatchback. I mounted it on a thin sheet of Rover panel damping foam on birch ply. The foam just adapted to the pressure. I had a lovely view of Mars polar caps with a Pretoria with it.

Surely the modulus of compression for glass is available.

I designed a big horseshoe mount, Nautilus, using 3/4 ply and was able to calculate how much it would deform under load. Small enough.

Cheers. Andrew.


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Re: PST mod is it worth it?

Post by Merlin66 »

Andrew,
“..the pressure is only applied round the edge….”
I’m now convinced that the “feet” act as support pivots allowing the outer section of the etalons to bend and provide tuning.


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Re: PST mod is it worth it?

Post by thesmiths »

It sounds to me that even the designers may not entirely understand the detailed mechanism how it works, just that given a particular setup, it will work. Having worked in engineering at several companies, that is often the way it happens. However, it you want to debug or improve something, it is much better (even essential) to understand the actual mechanism is before you change anything that may (or may not) improve the performance.


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Re: PST mod is it worth it?

Post by AndiesHandyHandies »

Merlin66 wrote: Wed Jul 21, 2021 9:03 am Andrew,
“..the pressure is only applied round the edge….”
I’m now convinced that the “feet” act as support pivots allowing the outer section of the etalons to bend and provide tuning.
Hi Merlin,

I assume the spacers are centred under the orange foam, is that so? I have not noticed a shadow or 'dust bunny' because the feet are in the field of view.

You idea would result in the two thick etalon plates bowing so the bandwidth would vary across the etalon.

With equal sized feet being identically compressed the etalon plates stay flat but move together enough. I thought that CV above had calculated the force reported on the etalon and so the feet, forced them to compress enough to give 0.5A of tuning due to the gap change.

Also if the force is not placed orthogonally on the feet the molecular bond would fail surely.

Cheers. Andrew.


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Re: PST mod is it worth it?

Post by AndiesHandyHandies »

Hi

I spent about £500 on a SH PST, paid £250 and a dirt bike to Rodastro for it, maybe some sellers regret.
Comparing with Daystar bandwidth example images looks like 0.7A.

A couple of Swiss adapters aprox £150.

1.5A Omega Bob solid etalon on hand for a blocker about £75.

Various filters on hand and or useful additions.

Rod kindly contributed bits out of his bits box and some lathe work for a nosepiece for the moved PST Collimator to T2 adapter.

DeepSky64 rates my visual observations with a Mak180 mod as the best.

Montana says my reported details match her 100mm Solarscope.

Tuning the blocker by tilting I see surface datails similar to Stardusts DS images.

I can see more detail in prominences than posted images.

And more realistic details, like granulation in bright areas and fractal borders on them, rather than over-processed lines.

Quiet happy.

Cheers. Andrew.


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Re: PST mod is it worth it?

Post by minhlead »

AndiesHandyHandies wrote: Fri Jul 23, 2021 4:21 pm Hi

I spent about £500 on a SH PST, paid £250 and a dirt bike to Rodastro for it, maybe some sellers regret.
Comparing with Daystar bandwidth example images looks like 0.7A.

A couple of Swiss adapters aprox £150.

1.5A Omega Bob solid etalon on hand for a blocker about £75.

Various filters on hand and or useful additions.

Rod kindly contributed bits out of his bits box and some lathe work for a nosepiece for the moved PST Collimator to T2 adapter.

DeepSky64 rates my visual observations with a Mak180 mod as the best.

Montana says my reported details match her 100mm Solarscope.

Tuning the blocker by tilting I see surface datails similar to Stardusts DS images.

I can see more detail in prominences than posted images.

And more realistic details, like granulation in bright areas and fractal borders on them, rather than over-processed lines.

Quiet happy.

Cheers. Andrew.
I am thinking about double stacking my Quark with a 1.5A filter (or 1.7A filter from Apollo). Do you have any experience with using the 1.xA filter as contrast booster (and perhaps double limb suppressor) for Quark?
I'd happy to see some comparison image if you have one. Thanks!


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Re: PST mod is it worth it?

Post by Merlin66 »

Minh,
I've successfully modded many PST's over the years but it's becoming more and more expensive.....
I would recommend a Quark today, or a MiniSHG (gives you better resolution across many wavelengths)


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Re: PST mod is it worth it?

Post by AndiesHandyHandies »

Hi

I am not doing imaging.

Only 2 quick visual views.

Cheers. Andrew.


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Re: PST mod is it worth it?

Post by AndiesHandyHandies »

Hi Merlin,

Are mini SHGs cheap then?

Cheers. Andrew.


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Re: PST mod is it worth it?

Post by minhlead »

I am interesred in MiniSGH too. Naver heard of them. Would be nice to have their out of pocket price and availability(ie. How tk buy them)


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Re: PST mod is it worth it?

Post by Merlin66 »

Guys,
The MiniSHG would probably cost around us$750 in components.
It's a DIY solution. Buil has his 3D printed version, if you have that capability.
Mini_SHG.pdf
(1.33 MiB) Downloaded 136 times


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Re: PST mod is it worth it?

Post by minhlead »

Merlin66 wrote: Sat Dec 11, 2021 3:17 pm Guys,
The MiniSHG would probably cost around us$750 in components.
It's a DIY solution. Buil has his 3D printed version, if you have that capability.
Mini_SHG.pdf
That is some serious resolution. I find this very interesting. Should not too hard to build, judging by the look of it. But you wont be able to shoot multiple frame and stacking them to beat the seeing right? This is gonna be a problem for hi res imaging. I guess it's limited to FD for this reason? Almost every image I see with a Spectroscope out there is a FD.


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Re: PST mod is it worth it?

Post by Merlin66 »

Minh,
With the current scanning rates of a few seconds, it’s pretty easy to take multiple SER files and then combine the images.
I use an ED80 with a focal length of 600mm to give me a FD, but there’s no reason if your conditions are good enough why you could not use longer focal length and get “closer to the action”


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Re: PST mod is it worth it?

Post by minhlead »

Merlin66 wrote: Sun Dec 12, 2021 1:27 am Minh,
With the current scanning rates of a few seconds, it’s pretty easy to take multiple SER files and then combine the images.
I use an ED80 with a focal length of 600mm to give me a FD, but there’s no reason if your conditions are good enough why you could not use longer focal length and get “closer to the action”
Thanks, Ken.
I am curious about post processing and scanning. Are you slew the mount around to scan the sun manually or there is an app for that?
Also how is processing and joining the strips together? If it's not too much of a trouble could you please share a raw ser?


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Re: PST mod is it worth it?

Post by Merlin66 »

Minh,
The "weapon of choice" for SHG processing software at the moment is Valerie's INTI software.
This will process a 16 bit SER video file to a solar image (including a prom image if full disk) in a few seconds.

I use a SW HEQ5 mount which is polar aligned and scan the solar disk with the Synscan slew buttons - usually x3 slew in Declination which gives a complete solar scan in less than 9 sec.
If you use a slit length less than the solar disk size you'll end up with slices which can be joined using ICE or similar.
Each scan is usually around 3Gb with my ASI 178 camera, about 2400 frames 2400 x 300 pixel.
I have a sample SER on my Dropbox.
https://www.dropbox.com/s/20u1oyae1ewwj ... 9.ser?dl=0

There's a couple of English versions of the INTI software on GitHub:
https://github.com/mattconsidine/Solex_ser_recon_AVI
https://github.com/thelondonsmiths/Solex_ser_recon_EN
You can use either of these to process the sample SER video - see how long it takes and the quality of the image produced.

You can use the SimSpec_SHG to enter the parameters of you telescope/ spectrograph/ camera and see the probable results.
SimSpec SHG V1_3a.xls
(60.5 KiB) Downloaded 86 times
Hope that helps.
Ken


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Re: PST mod is it worth it?

Post by minhlead »

Thanks, Ken. I think I am gonna give this a spin


Minh.
Loves from Viet Nam <3
My gear:
Scope: SkyRover (Kunming Optics) 152mm F/6 Doublets
Mount: JTW Tridents GTR
Camera:
PlayerOne Apollo M-Max
Quark Chromosphere Doublestacked with PST etalon
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