AP155 and LS80 DSII

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AP155 and LS80 DSII

Post by george9 »

I thought I would report on my LS80 DSII to AP155EDF f/7 mod. It's pretty straightforward. Remove the Lunt objective and its optical tube, remove the internal ERF, and attach that to an AP155EDF f/7 with a 160mm DERF in front. I removed the AP155's focuser so the pressure tuner would be close enough to the objective.

The LS80 DSII is my 9-year-old LS80 that I love with the circular polarizer and brightened blocking filter.

I positioned it through trial and error for the best view. I found that if I positioned it such that the focal point behind the focuser on the AP155 was identical to the focal point where it would be on the original LS80 DSII, then I got the best high-power view. Super sharp, super contrast, dark background.

The sweet spot is a little smaller than the Sun, as predicted for this 33% etalon diameter versus objective lens. If I push the etalon toward the objective by 20 or 30mm, then I get a nice full disk view and a beautiful medium-power view, but a soft high-power view. When I really want a full-disk view, I just pull the LS80 DSII off the AP155 and put its ERF and objective back on.

You can see the setup in the photo.

I previously have used my DSII unit to double stack a Quark or a Solar Spectrum on my 155. I have two DSII units, so I tried using them double stack, but unlike Brian, I never got a beautiful view. It always looked soft, presumably because the second one (further from the objective) was not operating at its nominal focal ratio. I considered adding a corrector lens to fix that, but just putting the LS80 DSII on it worked better.

George
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Re: AP155 and LS80 DSII

Post by Rusted »

Thanks for sharing your mods.

You seem to be varying your etalon[s] position[s] by trial and error.

Whereas Marty suggests 264mm inside focus.
A figure to which I myself adhered with my iStar 150/10.

I am only interested in optimum position for hi-res imaging.

May one presume it would be worth further experimentation in etalon positioning?


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Re: AP155 and LS80 DSII

Post by pupak »

Backfocus 264 mm only applies to LUNT LS60MT. Other models have different values. The LS 80TH is 275mm. It should be noted that collimated lenses in the LS80 TH-a etalon will shift the focus back by 1.071x. For the LS80, it will thus change from 560 mm to 600 mm. In the design for larger diameters, this shift must be taken into account. So far, I have only measured the primary Etalon LS80 TH-a, I have to measure how the focus of the LS80 DSII moves when the sun comes out.
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Re: AP155 and LS80 DSII

Post by christian viladrich »

Very nice setup George. Thanks for sharing !


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Re: AP155 and LS80 DSII

Post by george9 »

Thanks, all, and thanks for the diagram.

I am thinking that maintaining the same distance between the etalons and the focus is equivalent to measuring the 275+40 in the above diagram and duplicating that in the new setup.

I don't know if using a bigger objective makes you want to shift the focus with respect to the etalons. That is, does that focus shift of 40mm in the above diagram grow or shrink when you move from 80mm to 155mm, or does it stay the same if they are both f/7?

But then yes, I do trial and error for the view to confirm what I got. Keeping the focal point the same resulted in the best high-power view, but there is a wide range of equivalent views (i.e., can shift by 5 or 10mm without a noticeable difference). Pushing the etalon further inward presumably vignettes the objective until you get to the point that the sweet spot is about as large as the Sun, and the low power view looks very nice.

George


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Re: AP155 and LS80 DSII

Post by Bob Yoesle »

Hi George,

I have found the same thing - 10+ mm shifts in the position of the collimator/etalon assembly do not produce much of a noticeable effect. That being said, I still leave mine at the theoretical optimum position with the coincident collimator and objective focal points.

I would think in this ideal position of coincident objective/collimator focal points, and therefore truly collimated light passing through the first etalon, that all the subsequent downstream optics will have the same separations and focal distances, and that this is irrespective for the objective focal ratio. A differing focal ratio between the objective and collimator can affect whether or not the image is 1) vignetted, or 2) restricts the effective aperture of the objective.

You might find this thread of interest from when I was designing another solar telescope with the help of Mike Jones and Vladimir Sacek.

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Re: AP155 and LS80 DSII

Post by marktownley »

Great setup and interesting thread!


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Re: AP155 and LS80 DSII

Post by Rusted »

Bob Yoesle wrote: Mon Sep 12, 2022 3:59 pm Hi George,

I have found the same thing - 10+ mm shifts in the position of the collimator/etalon assembly do not produce much of a noticeable effect. That being said, I still leave mine at the theoretical optimum position with the coincident collimator and objective focal points.

I would think in this ideal position of coincident objective/collimator focal points, and therefore truly collimated light passing through the first etalon, that all the subsequent downstream optics will have the same separations and focal distances, and that this is irrespective for the objective focal ratio. A differing focal ratio between the objective and collimator can affect whether or not the image is 1) vignetted, or 2) restricts the effective aperture of the objective.

You might find this thread of interest from when I was designing another solar telescope with the help of Mike Jones and Vladimir Sacek.

Bob
Thank you Bob.

It is many years since I wrote my own ray tracing software. [In Basic!] :D

I am using a 60MT PT etalon as per Marty's project.


http://fullerscopes.blogspot.dk/

H-alpha: Baader 160mm D-ERF, iStar 150/10 H-alpha objective, 2" Baader 35nm H-a, 2" Beloptik KG3, Lunt 60MT etalon, Lunt B1200S2 BF, Assorted T-S GPCs or 2x "Shorty" Barlow, ZWO ASI174.
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Re: AP155 and LS80 DSII

Post by Rusted »

george9 wrote: Mon Sep 12, 2022 12:44 pm Thanks, all, and thanks for the diagram.

I am thinking that maintaining the same distance between the etalons and the focus is equivalent to measuring the 275+40 in the above diagram and duplicating that in the new setup.

I don't know if using a bigger objective makes you want to shift the focus with respect to the etalons. That is, does that focus shift of 40mm in the above diagram grow or shrink when you move from 80mm to 155mm, or does it stay the same if they are both f/7?

But then yes, I do trial and error for the view to confirm what I got. Keeping the focal point the same resulted in the best high-power view, but there is a wide range of equivalent views (i.e., can shift by 5 or 10mm without a noticeable difference). Pushing the etalon further inward presumably vignettes the objective until you get to the point that the sweet spot is about as large as the Sun, and the low power view looks very nice.

George
Thank you George.

I kept the FT focuser by shortening my main tube on my 150/10.
So I can play with etalon:focus distance to my heart's content. :D


http://fullerscopes.blogspot.dk/

H-alpha: Baader 160mm D-ERF, iStar 150/10 H-alpha objective, 2" Baader 35nm H-a, 2" Beloptik KG3, Lunt 60MT etalon, Lunt B1200S2 BF, Assorted T-S GPCs or 2x "Shorty" Barlow, ZWO ASI174.
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Re: AP155 and LS80 DSII

Post by george9 »

Thanks, Bob. I remember that thread. Good point, I agree f-ratio should affect vignetting or waste but not distances. As long as the stock configuration is correct, I should be good.

Yes, Rusted, I went into it hoping to use my 4" AP focuser for tweaking, and I could have shoved most etalons up there, but the PT got in the way and I was not prepared to reroute the air pressure cylinder. Nor could I bear to shorten an AP155.

George


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Re: AP155 and LS80 DSII

Post by MapleRidge »

Hi George...

Your DSII mod looks great...and kinda familiar too ;)

Well done!

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Re: AP155 and LS80 DSII

Post by OlegLviv »

This is all good but where is sun photo?


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Re: AP155 and LS80 DSII

Post by george9 »

I am hoping to get some photos when I bring the setup to Black Forest (how I spend the day when I am waiting for the dark sky). I am out of the season for getting the Sun out the window. I am 100% visual except for CaK and the corona shots, but I need to get a good photo through this setup.

For the record, in the stock LS80 setup, I measure the focus being 338.5mm behind the rear of the rear collimation lens of my primary LS80 etalon. I think that correlates with the 275+40=315mm in the figure, as the collimation lens is about 24mm in front of the primary etalon's red shoulder (so 314.5mm). Half a mm is well within my measurement error. I set the AP155 so the focus is at the same point.

George

(EDIT: first version had an arithmetic error. I got 314.5, not 318.5.)


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Re: AP155 and LS80 DSII

Post by Rusted »

George,

Did you remove the red Lunt filter on the nose of the etalon?

Or isn't there one on the LS80 etalon?

I replaced the red filter with the 2" 35nM H-a and the KG3.

I can well understand your desire NOT to cut into that exquisite instrument! :D


http://fullerscopes.blogspot.dk/

H-alpha: Baader 160mm D-ERF, iStar 150/10 H-alpha objective, 2" Baader 35nm H-a, 2" Beloptik KG3, Lunt 60MT etalon, Lunt B1200S2 BF, Assorted T-S GPCs or 2x "Shorty" Barlow, ZWO ASI174.
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Re: AP155 and LS80 DSII

Post by george9 »

Yes the LS80 has a red filter in front of the primary etalon that unscrews. I am using a Baader 160mm DERF in front of the AP155, so that covers me without additional filters.

Interesting question though. Is that red filter just an RG6**, or also a KG3. I don’t know. But the B1200 should cover long IR anyway.

George


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Re: AP155 and LS80 DSII

Post by Rusted »

george9 wrote: Wed Sep 14, 2022 2:24 pm Yes the LS80 has a red filter in front of the primary etalon that unscrews. I am using a Baader 160mm DERF in front of the AP155, so that covers me without additional filters.

Interesting question though. Is that red filter just an RG6**, or also a KG3. I don’t know. But the B1200 should cover long IR anyway.

George
Thank you George.

I too have a 160mm Baader D-ERF and was shocked at how much heat passes through it.
Easily enough to cause a painful, skin burn near focus. Or produce instant smoke from black foam insulation.
I use the Baader 35nM H-a and Beloptic KG3 to enure that heat doesn't reach the ASI174 camera.
Though I may be an extreme case in regularly tracking the sun for many hours at a time.

I have no idea what the Lunt red filter is. No doubt one of our experts can provide the necessary data.

Chris


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Re: AP155 and LS80 DSII

Post by george9 »

Thanks, Chris. This is an important point and worth discussing.

Through the f/7 AP155 and DERF, I can keep my hand at the focus for a few seconds, but not more. And the AP155 puts through four times the heat as the LS80 objective.

My first surface after the objective is the LS80's primary etalon. I have a lot of experience using the DSII etalon as the first surface (when I double stack), and so far at least, no damage or obvious effect.

The DERF is blocked well from 300 to 1400nm other than the 85nm around H-alpha, where it is 95%. I am thinking that not much heat is getting through from 1400nm-2500nm. Important for eye safety, yes, but for protecting the etalon, not sure. (Bob Y's graphs show the heat trailing off as you get that long.)

KG3 protects long IR well but drops 30% of the H-alpha brightness. That's really noticeable in the eyepiece, so I don't use that unless it is the only long-UR protection in the train. The B1800 should cover that for eye safety.

The 35nm H-alpha filter would drop half the heat compared to the DERF (35 vs 85). But it comes in sizes up to 50.8mm unmounted, so say 47mm mounted, and that drops too much from my 52mm etalon.

If the etalons are resilient, then the next risk is the blocking filter. I suspect there's not too much heat by the time you get through 2 etalons and a circular polarizer. Plus the B1800's first element is either blue glass or KG3, either of which are as cheap as the 35nm filter.

So for now, I am just using the DERF, but I will report back if I see any hint of problems with heat management.

George
Last edited by george9 on Thu Sep 15, 2022 10:37 am, edited 3 times in total.


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Re: AP155 and LS80 DSII

Post by Rusted »

Thank you for your very thorough response George.

I cooked my first ZWO camera. That was their more affordable 120MC.
Back then I was using a 90mm Baader D-ERF internally. Thinking I was safe.
I wasn't. Not remotely! Any claims of a cool optical train is false advertising.
I can share the instant smoke on YouTube if Baader argues with my own findings.

Cooking my ASI174 would probably end my H-alpha imaging. Hence the default 35nM and KG3.
I stopped doing any visual observation after discovering the severe heat problem.
There should be a clear health warning on every Baader D-ERF. IMHO.


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Re: AP155 and LS80 DSII

Post by pupak »

I basically use two ERF filters for the 150mm diameter. The first 160mm D-ERF on the lens and the second 55mm ERF from Lunt in front of the etalon. I have UV/IR cut on the camera, so thermal contraception is perfect.
Behind the etalon, the beam temperature is minimal. D-ERF alone will not prevent the burning of the plastic cap in the focus, so temperatures above 150 degrees Celsius.


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Re: AP155 and LS80 DSII

Post by george9 »

Wow. Thanks, Chris and pupak.

Chris, was that using the DERF as a photosphere filter (white-light but in red) or with some kind of H-alpha filter?

I certainly would rather not cook my etalon, but my only real concern is vision. The vision seems fine with everything else in the path after the DERF. I have run a spectrogaph on my LS80 although it only went to about 1000nm, not 2500nm. It was well-blocked in the available range.

I guess "cool" was just relative to the older RG630 ERFs (or new hotter DayStar yellow ones), which pass more heat in IR.

Since these ERFs pass 95% of the light in their open band, stacking them shouldn't help that much if they overlap. In other words, two Baader DERFs in series let about the same amount of heat through. A Baader DEF plus a 35nm H-alpha filter does remove extra heat (85nm wide versus 35nm wide).

The DERF plus the Lunt ERF should cut some heat, but not that much. I see the Lunt is RG630. The DERF seems to cut around 615nm, so the RG630, which cuts at 630, may get rid of additional heat in the 615-630 range, which is about 17% of the heat in the 615-700 range that the DERF passes. The UV-IR cut filter should not add much to the DERF, which already has those wavelengths covered. Most UV-IR cut filters don't go below 1100nm or so (I think). The etalon and blocking filter are doing the most work in reducing heat after the DERF in that setup.

So compared to someone who has multiple ERFs, I am letting through either twice the heat (35nm H-alpha) or roughly the same amount (second ERF, CCD red filter, etc.).

I do see a benefit from redundancy in keeping everything safe. Having an internal ERF means if you forget the front ERF, you don't instantly cook everything behind it. Lunt makes sure that if you forget the blocking filter or if you forget the telescope and have just the blocking filter, you will get something very bright but won't go blind. For fully home-built filters, I usually put a BelOptik KG3-UV-IR (plus usually an ND1 or even ND3 at first look) in series just in case I made a mistake setting up.

George


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Re: AP155 and LS80 DSII

Post by pupak »

I admit I haven't measured anything, but two ERFs in tandem will give a heat in the focus that I can barely feel on my hand, where only the D-ERF burns me instantly. This is authoritative for me.


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Re: AP155 and LS80 DSII

Post by george9 »

Thanks. This is worth looking into more. I like both testing it out and understanding it theoretically. Then I feel safe.

Just to confirm, pupak, you are comparing DERF in front of a 150mm refractor versus DERF in front and Lunt ERF in the middle and no other filters anywhere.

I can test that out. I have the ERF that comes with the LS80, but not sure it is identical to yours. I also have a separate RG630.

Could be that the DERF lets through more light than it is spec'd for.

Could be the Lunt ERF is not just an RG630 but also has some other filtering. Or that it is not near 100% transmission around H-alpha for some reason.

Note that the Lunt ERF that I have is absorptive, not reflective, so if the DERF is sending through a ton of heat and the Lunt is internal and absorbing it, be good to check its temperature in operation. It may not be a problem because the heat is spread over a wide area, not just a focus point.

George


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Re: AP155 and LS80 DSII

Post by george9 »

I tried it out and here is what I get. I used a Pronto, which is 70mm and f/6.8, so the heat per area of skin should be the same as my AP155 f/7.

No filter - very hot, need to move right off

DERF - warm, I can leave it there for seconds, but I can tell it is heating up

DERF + Lunt ERF - noticeably less warm, heats up maybe half as fast

Then I looked through my hand spectroscope. The Lunt ERF cuts at 630, as predicted. The DERF cuts not at 615nm but at about 600nm. I found a spectrum for it online that matched the 600 (previously I found the 615 one). That means the RG630 will be helping even more, cutting 600-630.

So it seems for mine, it cuts the heat about in half. If I remember Bob's graph, in terms of solar radiation dropping off, 600-630 may be similar to 630-700 in amount of heat. So it could all fit.

George


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Re: AP155 and LS80 DSII

Post by Rusted »

Is there a suitable target which could measure the temperature at certain points in the focused beam of our telescopes?
I'm thinking of using the handheld "pistol" {remote or laser guided thermometers.] Now inexpensive thanks to widespread use on human foreheads.
These tools can't measure the air temperature directly. So would need a suitable, non combustible target to read local heat intensity.
That target would probably have a black finish, be metallic and needs to have a low thermal capacity. Allowing it to heat up and cool rapidly.


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Re: AP155 and LS80 DSII

Post by george9 »

Chris, your wish is my command.

I have an IR thermometer but the Pronto (70mm f/7) solar image is so small that it mostly counted the area around it. I switched to a quick-read meat thermometer and used heat grease to thermally connect it to a small black metal clip that was clipped to a wood bar. (Forgot to take a photo.) I timed the temperature rise in 30 seconds at focus. Here are the results.

Pronto by itself: 85 deg to 325 deg = 240
DERF to Pronto: 89 deg to 135 deg = 46
DERF to Pronto to Lunt ERF: 85 deg to 110 deg = 25

So more or less a ratio of 10:2:1. The DERF passes 20% of the heat (which sounds familiar) and the Lunt drops half the heat from the DERF (which matches my last post).

George


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