AP155 and LS80 DSII

Frankenscope? Let's see it!***be advised that NOTHING in this forum has been safety tested and you are reading and using these posts at your own peril. blah, blah, blah... dont mess around with your eyesight when it comes to solar astronomy. Use appropriate filtration at all times...
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Rusted
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Re: AP155 and LS80 DSII

Post by Rusted »

Thank you George! :bow

You have obviously gone into the thermal issues with some rigour.
I thought it only myself who obsessed over such details. :D

I keep an eye on SharpCap's monitoring of camera temperature.
The high 40sC seem quite normal after some hours of solar tracking with a {supposedly] D-ERF protected 150mm.
Though even these readings may have nothing [whatsoever] to do with heat infall on the sensor.
Which is my own greatest worry.


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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 pupak »

Rusted wrote: Sat Sep 17, 2022 5:50 am Thank you George! :bow

You have obviously gone into the thermal issues with some rigour.
I thought it only myself who obsessed over such details. :D

I keep an eye on SharpCap's monitoring of camera temperature.
The high 40sC seem quite normal after some hours of solar tracking with a {supposedly] D-ERF protected 150mm.
Though even these readings may have nothing [whatsoever] to do with heat infall on the sensor.
Which is my own greatest worry.
If you use cameras from ZWO, then 42-45 degrees is normal. It is evident that the ZWO camera does not have a thermal bridge between the chip and the body. The active cooler will reduce the temperature of the chip from 42.6 to 35.1 degrees. Celsius at an ambient temperature of 26 degrees. Celsius. This is important for chip protection. It should not exceed 50 degrees. With high-quality filtration, the influence of heat and sun is negligible. The electronics of the camera heats up more.


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Re: AP155 and LS80 DSII

Post by george9 »

Thanks. I defer to you two on cameras. I do have a ZWO ASI1600 pro, but since I am using it mostly on my coronagraph, there is not too much heat getting to the camera, and I have not really used its cooler very much. I ought to, though, because the corona is so dim that I need long exposures, up to 2 seconds.

This is a little off the original topic but relevant to this part: Temperature is important to me because my corona filter on my coronagraph (i.e., my other scope) is very temperature sensitive. I made my own oven, but my attempts at a Peltier cooler didn't go so well (the challenge is mainly insufficient back focus to fit much instrumentation and a desire to keep it lightweight due to the very long optical train). So we picked a filter temperature, 32C, that we thought would be below ambient most of the time. We are usually on a mountain, and that helps. I have experimented with ice packs, but haven't had to use them yet. If you pick too high a temperature, then it takes forever to heat up to that level in the cooler months and you get bigger gradients across the filter or filters.

But in designing the coronagraph, I was measuring the temperature in the back end of my 155mm H-alpha filter, and in the summer (up to 34C outside) I saw temperatures up to 46C around the inside of the diagonal! That's without direct sunlight, neither inside nor outside. It was after most of my H-alpha filter, so there could not have been much solar energy left. My Solar Spectrum ran at 50C, and so heat probably transferred back when I used that filter, but I don't think that was the only hot setup. It may also have been external sunlight hitting the binoviewer, transmitting heat back to the shaded diagonal. A better shade would help. But my point is that I was surprised, and measuring was useful.

(Also, by the way, my temperature measurements corroborate pupak's experience with the ERFs. 135C feels too hot to touch, and 110C feels comfortably warm. I am sure the temps are different between the two, but the point is that amount of heat difference can swing it quite a bit.)

George


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Re: AP155 and LS80 DSII

Post by george9 »

Ah ha. For the record, it just dawned on me why pushing my etalon in 20mm too far would solve the low-power sweet-spot problem. I had thought perhaps vignetting the objective, but at f/7, that's only 3mm, and I would need to vignette 50mm.

I suspect when I push it in 20mm, the beam out of the front colllimating lens is no longer collimated, which would broaden the bandwidth of the etalon with some shifted blue-ward by the tilting of the rays. So parts of the image that should be out of band now have some H-alpha signal in them. It is hard to predict exactly what it would look like by theory alone, but in practice, by shifting the tuning of the etalons and by refocusing the scope, I get something that looks pretty good at low power. I don't remember seeing a double limb even in this configuration. And sharp enough for low power.

At high power, it was definitely soft, probably mostly due to the optical aberrations of using the collimating lenses out of their proper place.

George


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Re: AP155 and LS80 DSII

Post by pupak »

Shifting the etalon does not change the angle of the beam, but only the area hit by the cone of light from the lens. The primary focus F1 should be the same as the focus F2 of the negative coimating lens. This is the optimal position of the etalon in the lineup.
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Re: AP155 and LS80 DSII

Post by george9 »

Sorry, I should have been more clear. My negative collimating lens is attached to my etalon. When I say I moved the etalon 20mm closer, I really meant I moved the negative collimating lens 20mm closer. That changes the effective focal ratio seen by that lens. It looks like a longer focal ratio because F1 is now further back behind the collimating lens (put another way, the collimating lens is only seeing the middle of the objective's beam, which means a higher f-ratio). But F2 didn't change with respect to the collimating lens, so there is a mismatch, and the beam coming out is no longer collimated. I think it is a little divergent. (But please double check.)

George


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Re: AP155 and LS80 DSII

Post by pupak »

You are only interested in the angle of the beam with respect to the Etalon, and it does not change with displacement. The area of the etalon should be used efficiently.


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Re: AP155 and LS80 DSII

Post by george9 »

I just drew a diagram to check my work. If you push the negative collimation lens toward the objective, then you no longer have a collimated system.

The top ray trace is collimated. The distance, x, is the same from the collimation lens to F1 and to F2. Their focal ratios are about the same, around f/4.

If you move the collimation lens by distance x toward the objective, then the collimation lens sees f/8, as the distance from the lens to F1 is now 2x, or about f/8. But the lens stayed the same, about f/4. If the objective was a 100mm f/4, then the negative collimation lens sees only the middle 50mm, which again looks like an f/8.

As a result, the rays coming in diverge at the other side. (Sorry about the crossed-out lines; they looked too much like the result of a parallel beam coming in.)

As you say, it is important that the distance from the lens to F1 and to F2 be the same. When you push it in, the two distances are no longer the same.

George
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