PST Mod - Band is very "thin"

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gbealnz
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PST Mod - Band is very "thin"

Post by gbealnz »

Hi All,
after trying various Ha systems, and also while continuing with the loaner SM60/BF10, I have acquired a used etalon and blocking filter from an old PST. I have also had made adaptors to attach to the front and rear, plus a Borg helical focuser.
I'm all set you could say. I wish.
The plan was to connect the mods to the rear of my current donor scope (the one I use the SM60/BF10 on), an old Zeiss AS63/840 achromat. It has a long sliding draw-tube, plus a helical, and I have at least/more than the required 200mm "in-focus". The etalon attaches via the Zeiss M44 thread and the etalon M50 thread, solid as. On the rear of the etalon the same M50 thread becomes a T thread, and from there I can use T extensions etc, plus the Borg helical, to get the rear spacing spot on.
Given a decent day yesterday I tinkered, measured it all up, and went to view. On the objective I have an RG630 filter, and on the eyepiece I have a UV/IR cut filter.
Views? Well, there was certainly surface detail to be seen, and of course a couple of stunning proms, but the band or sweet-spot was thin, oh boy really thin. If I tuned it, the band moved across, and you could see the thin swathe move across, but at no stage could I see the detail/proms on the whole disk, only a thin slice. This was with the standard blocking filter (BF5??), as well as my standby the BF10.
Is this normal? What have I done wrong, or is this to be expected, and as good as it will get? I thought of shooing an image, but it would look awful, so I haven't bothered. Do I dump the etalon and sell off the fittings? Certainly the plug and play simplicity of the SM60/BF10 is hard to deny.
Any ideas?
Gary


peter drew

Re: PST Mod - Band is very "thin"

Post by peter drew »

Gary.

To a certain extent my experience mirrors yours. I have a range of PST mods on which I'm able to try 5,10 and 15mm BF's. The sweet spot on my etalons seems to cover the small FOV afforded by the 5mm BF when used on a PST. During tuning the spot on the larger mods it gradually forms a ring. The larger BF's, having a wider field makes this more noticeable and show probably 30% of the field on band with the rest tapering offband. The good 30% on my 150mm F10 is as good as a 152mm Lunt so on balance I'm pretty happy with it.


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Re: PST Mod - Band is very "thin"

Post by gbealnz »

Hmmm, thank you Peter, seems I have done it all correctly then. Not sure what I'll do from here, persevere a bit perhaps. Good fun tinkering anyway.
Thanks again,
Gary


colinsk

Re: PST Mod - Band is very "thin"

Post by colinsk »

The interference fringes from two parallel plates when illuminated by a diverging spherical wavefront will make a bulls eye. If you don't have a bulls eye you have to consider tilt of either the etalon assembly or the etalon plates relative to each other. We know you have the primary. Perhaps you would like to try changing the parameters of the etalon to be able to make it more flat?

You can effect tilt, pressure and parrallelism.


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Re: PST Mod - Band is very "thin"

Post by gbealnz »

Colin,
I am indebted to you for this, but mate, I have no idea what you mean, sorry.
I'm keen to try it, anything, but you will need to nurture my basic brain a bit, help me out, what do you suggest I do please.
Gary


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Re: PST Mod - Band is very "thin"

Post by marktownley »

Hey Gary!

Sounds to me like your etalon is tilted excessively to the optical train. Fear not tho mate, there are things that can be done: Firstly, try rotating the pst etalon assembly with respect to the optical axis - you will find there is an optimum position where detail is greatest (tightest bandpass) and also the sweetspot is at it's largest.

Is there any sag in the sliding draw tube with all the extra mass of the etalon assembly, blocker, helical etc? You can confirm this with a cheshire ep / laser collimator.

Also, are you viewing full disk or zoomed in at a higher magnification? PST mods work better (optimum sweet spotting) at higher magnifications....

My gut feeling is the band is a result of sag in the optical train, which may be able to compensate for by rotating the etalon assembly. If not (and sag is the cause!) look at being able to 'raise' the sliding draw tube...

Hope this helps,

Mark :)


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Re: PST Mod - Band is very "thin"

Post by sullij1 »

Colin,
I am indebted to you for this, but mate, I have no idea what you mean, sorry.
I'm keen to try it, anything, but you will need to nurture my basic brain a bit, help me out, what do you suggest I do please.
Gary


:D :D Colin always does that :D :D ROFLOL, glad it aint just me!!!!


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Re: PST Mod - Band is very "thin"

Post by sullij1 »

All kidding aside, what you are describing sounds common to the PST and is one of the reasons Mark has worked so diligently to come up with a solution. You need to look up Marks PST mod video here on SC and watch.

I took these images last sunday and I think they exemplify what you are talking about for a full disk; Here:


Post for PST conversation by Sullij1, on Flickr

What Mark is saying is seen here in the long focal length shot:


Post for PST conversation by Sullij1, on Flickr

So in short, you are good to go for long focal length and may have to live with the uneven field illumination on the full disk.

Mark, did the correcting ring help with full disks?


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Re: PST Mod - Band is very "thin"

Post by gbealnz »

Thanks all, especially Sullij1, yes, you seem to have hit it on the head. Long shots should be OK, full disks, maybe not.
Mark, appreciate the reply, I tried briefly today rotating the items, etc, I think there is some minor mis-alignment, there is a vignetted side in the FOV.
In the end I figure I might just stash the parts, or if someone advertises looking for them, sell.
It means returning to the SM60, and or whatever else I can find, but the wimp in me says I would rather be looking/imaging than tinkering with this right now.
Appreciate all the help though fellas.
Gary


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Re: PST Mod - Band is very "thin"

Post by marktownley »

Mark, did the correcting ring help with full disks?

In terms of aligning the sweetspot, yes. Your pics show the effect really well; in the full disk it is offset to the bottom left of the disk - and this offset is something I see pretty much common to >90% of images taken with PSTs. The ring just brings this sweet spot more towards the dead centre of the fov.

In terms of imaging a full disk, I have found that if I image at the native f10 of my scope (~1000mm) then I would around the periphery of the fov see the effects of the bandpass changing, getting gradually broader pass and less detailed towards the edges. This is a natural behaviour of the etalon and is something we have to accept with this type of design. However, if I increase the f-ratio to f16 (~1600mm in my case) then I am imaging in the centre of the sweet spot, and across the resultant (narrower) fov then what I see is even in terms of detail and bandpass and using this method have been able to get some pretty darn big mosaics.

However, this is not the only factor at play. There seems to be a trend at the moment for imagers to want to get the camera with the largest chip area possible. This is not the way to go when imaging with a PST mod: The larger the area of the chip on a camera only reveals more of the off band sweet spot aroumd the edges of the fov. I did some back of an envelope calculations based on my experience of cameras; If I were to use a f10 1000mm fl pst mod I could use this at native focal length with no adverse sweet spot issues using a DMK21. If I use the DMK31 I need to be upping the f-ratio to the region of f16, I only see acceptable results with the DMK41 when were at >f20, ideally f25. Problem I find is that the longer the f-ratio being imaged at, the fewer the occassions this can be used due to poor seeing - there is an inverse ratio.


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Re: PST Mod - Band is very "thin"

Post by marktownley »

I tried briefly today rotating the items, etc, I think there is some minor mis-alignment, there is a vignetted side in the FOV.

Was thinking more about this...

The retractable draw tube on the frac; I presume with the PST mod kit in place it is not retracted much and so most of it remains in the ota? If this is the case this is most certainly the problem and the cause of the vignetting. Normally I would be quick to suggest a hack-saw, but seeing as this is a zeiss frac we're talking about, on this occassion I with refrain from my usual hacking recommendation :P


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Re: PST Mod - Band is very "thin"

Post by gbealnz »

Mark,
ah, bingo, maybe you are right. You certainly are when you say we shouldn't hack this particular scope. Never thought of the vignetting from the tube, darn.
Gary


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Re: PST Mod - Band is very "thin"

Post by marktownley »

Is it a 2" draw tube Gary?


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Re: PST Mod - Band is very "thin"

Post by gbealnz »

Mark,
Ummmm, sort of. Zeiss is a weird thing, the output on the very end is M44, and I have used that. Having said that, the inside of the tube would be close to 2" yes. BUT............ The tube is long, about 200+mm and fully in as you can imagine, I mean how else do I get that magic 200mm inside focus?
I reckon you have nailed it.
Gary


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Re: PST Mod - Band is very "thin"

Post by marktownley »

The front collimating lens assembly on the PST etalon unscrews. This is the crucial bit that needs to be at the magic 200mm. If you uncrew it you could mount this higher up in the the draw tube on the zeiss, thus you could then have the draw tube fully retracted and avoid the vignetting. I did this with my 5" frac as had taken some considerable effort to get the moonlite focusser on it fully collimated with respect to the OTA and didn't want to cut the tube down and have to go through the re-collimating process all again...

I talk about removing the front collimating lens in my youtube vid http://www.youtube.com/watch?v=llSJy99Om9E about a minute in...


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Re: PST Mod - Band is very "thin"

Post by gbealnz »

Cheers Mark.
The fitting of that lens is in the too hard basket I'm afraid, getting inside the Zeiss is something I will avoid.
I have viewed your vid, many times, love the roadworks even.
Gary


colinsk

Re: PST Mod - Band is very "thin"

Post by colinsk »

In terms of imaging a full disk, I have found that if I image at the native f10 of my scope (~1000mm) then I would around the periphery of the fov see the effects of the bandpass changing, getting gradually broader pass and less detailed towards the edges. This is a natural behaviour of the etalon and is something we have to accept with this type of design. However, if I increase the f-ratio to f16 (~1600mm in my case) then I am imaging in the centre of the sweet spot, and across the resultant (narrower) fov then what I see is even in terms of detail and bandpass and using this method have been able to get some pretty darn big mosaics.

I tried to post a reply yesterday but I must have not hit post. To digress a little, if you make Michelson Interferometer (see wikipedia) you will see a bullseye pattern only if all of the optics are aligned. This is the best case scenario for an etalon. If the mirrors are mis-aligned the bullseye is out of center and you see a curved band. My PST had a band. My Lunt has a bullseye. I have attributed this to spacer thickness variations creating tilt in the etalon gap.


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Re: PST Mod - Band is very "thin"

Post by sullij1 »

Mark,

Thanks for the detailed response. Despite the nuances of chip size and focal length it sounds like the ring is a winner and does the best job possible for this design.

Thank you for your efforts.


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Re: PST Mod - Band is very "thin"

Post by gbealnz »

Again, a huge thank you for the assistance, I am deeply indebted. Colin, for the average dipstick like me, your replies are way over my head, I'm sorry. Unfortunately, all I can do is actually look through it, and also image with it. If the results from looking or imaging are great then I'm happy, if not, then I move on. I do appreciate the help though, please don't take this comment in any other way.
OK, got a little time yesterday arvo, sun is less "spectacular" though.
Shot with the PST mod first, and then slipped the "ordinary" rig on, (F/T, SM60-BF10) and redid the image. Attached they show slight, but not very much difference, encouraging.
Thanks all.
Gary







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Re: PST Mod - Band is very "thin"

Post by marktownley »

Nice results from both there Gary! If you can get results like this from yoour PST mod slap that SM60 on the front and get double stacking, you wont regret it ;)


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Re: PST Mod - Band is very "thin"

Post by gbealnz »

Cheers Mark, it was all the positive encouragement from here that did that.
SM60 on the front, Hmmmmm, now you have me thinking.
Gary


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Re: PST Mod - Band is very "thin"

Post by marktownley »

Do it! :evil: ;)

It will blow you away the difference...


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Re: PST Mod - Band is very "thin"

Post by colinsk »

Hi Gary,

I keep trying to figure out better ways to explain some of the conclusions I have come to. Your feedback helps me go back and remember to write fundamentals. I have spent 10 years studying and writing about holography and holography labs. On a lab bench you get to create novel optical systems at will. Trying to explain results with the assumption that someone does not have that experience is a challenge for me.

Cheers!


colinsk

Re: PST Mod - Band is very "thin"

Post by colinsk »

Ok, perhaps this helps. The on band section looks like a zone plate:



If you have tilt then instead of looking like the center it looks like an edge.

The larger the ratio of the objective to the etalon diameters are the more of this pattern will be visible. My LS60 shows 70% of the solar disk on band and looks like a bulls eye. This is the fundamental reason the PST mods are better for small fields of view.

When I double stack it gets more complicated.


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Re: PST Mod - Band is very "thin"

Post by gbealnz »

Thank you again Colin, and I am glad you took my comments as they were meant. Your knowledge here is immense obviously, and most likely (hopefully) understood by most, but in my case a fair chunk of it went clean over the top. Sorry.
Keep answering, even if it is bashing the head sometimes.
Regards
Gary


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Re: PST Mod - Band is very "thin"

Post by colinsk »

The blue circle shows a good etalon bandpass. The red circle shows a tilted bandpass. :silly: I'll keep trying...



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