11 replies to this topic

[ecorm]

It wasn't clear which subforum to post this in, so I'm trying here. I was looking at a diagram of the Zwicky Transient Facility imaging system (the one what detected Comet 2022/E3 ZTF), and I can't understand how the heck it manages its 46 degree field of view. That diagram can be found on page 4 of this academic article (pdf).

 

I can't figure out how light rays from 46 degrees (edit: this was incorrect) off-axis can hit the primary mirror. Is it what they call the "Aspheric trim plate" that does the magic of bending those off-axis light rays?

 

Edited by ecorm, 26 January 2023 - 09:02 PM.

[carolinaskies]

IF you look down that paper it actually states the camera's FOV: 
7.323◦ (E-W) × 7.504◦ (N-S), measured on sky

Edited by carolinaskies, 26 January 2023 - 08:24 PM.

[ecorm]

IF you look down that paper it actually states the camera's FOV: 
7.323◦ (E-W) × 7.504◦ (N-S), measured on sky

Oh, now I understand my confusion. The 46 figure I saw is in square degrees. Amateurs never talk about square degrees for field of view.

 

It's still an impressive system. It seems like the days of amateurs discovering comets are long gone now.

Edited by ecorm, 26 January 2023 - 08:44 PM.

[luxo II]

OK it's a conventional Schmidt, with a triplet corrector - and they evacuate the interior. What's really interesting on page 6 is how dew/frost is managed:

 

"A conductive indium-tin-oxide (ITO) coating is employed as one of the layers of the anti-reflection coating on the interior surface of the window, to act as an electrical heater that replaces energy lost through black body radiation into the cryostat. Without such heating the center of the window would equilibrate to a temperature well below freezing causing condensation and cold convection cells which would degrade image quality. With 150 Ω/square sheet resistance, about 316 mA at 47 VDC is required to dissipate 15 W typically required to offset the radiative cooling. Electrical contact is provided by a thin copper foil sandwiched between the window and Viton support gaskets running most of the width of the window at North and South edges. Power is scaled according to the Stephan Boltzmann law for radiative transfer using measured tube and CCD temperatures, with scale factor calibrated using a thermal imaging camera in the lab."

 

Now that's what I call neat. With a little thought that could be applied to SCT's, if Synta bothered to do a little product improvement - for amateur sized scopes it would scale perfectly to work at 12VDC with a PWM controller.

 

Could even be laid out as a complex path, like that used for car rear window heaters. 

Edited by luxo II, 27 January 2023 - 01:26 AM.

[ecorm]

"A conductive indium-tin-oxide (ITO) coating is employed as one of the layers of the anti-reflection coating on the interior surface of the window,

 

Now that's what I call neat. With a little thought that could be applied to SCT's, if Synta bothered to do a little product improvement, and it would scale perfectly to work at 12VDC for amateur sized scopes.

Wouldn't need as much power as dew heater straps that wrap around SCT tubes, that's for sure, since the heat would presumably be distributed evenly across the corrector plate's surface.

[luxo II]

Could equally be applied to the front element of refractors.

 

Now ... why aren't you already doing it ? Takahashi ? LZOS ?

Edited by luxo II, 27 January 2023 - 01:34 AM.

[BucketDave]

I was messing around with conductive sheet heating elements 20 years ago. If you make the sheet resistance high enough, it will effectively become transparent at the EM wavelength of interest. The stuff i was using had an impedance of 900 ohms/square, so the figure quoted sounds reasonable.

If you applied power via two conductor rails, one around the inner and outer perimeters, you end up with more heating per unit area at the hub than at the tip. The bigger the ratio of hub to tip perimeter, the bigger the ratio of hub to tip heating density. Which means the centre will be hotter and may distort the figure of the corrector lens.

Ways to solve this? Devise a way of making the current density more uniform. Or minimise power input (sounds like that happens here). Or the expensive way.... design the figure of the corrector lens to account for the non-uniform heating and consequent differential thermal expansion. Not sure i like the last idea... very inflexible if environmental conditions change.

[luxo II]

It’s sufficient to keep the glass above the dew point - we’re not talking “toasty” - so I really doubt there’s a thermal expansion issue. As the paper points out, without heating the glass cools well below zero which is a greater thermal change - if you’re worried about thermal expansion that’s a bigger issue right now on most scopes.

What I was thinking was instead of a simple continuous sheet, create a pattern in it which forms a long complex path to heat the glass evenly. A gap of a few microns would suffice.

Edited by luxo II, 27 January 2023 - 08:26 PM.

[freestar8n]

Thanks to the OP for pointing to the article - it's very interesting.  Note that the 'scope isn't just any Schmidt - it's a retrofit of the 48" Palomar Schmidt - now the Samuel Oschin Schmidt.

 

I found the section on flat fielding very interesting and will aim to discuss it in the experienced imaging section.

 

As for conductive coatings - the main complication is that you would need additional coatings to keep them anti-reflective - and if the coating has some kind of pattern to it - I imagine it would be hard to keep it invisible in long exposures.  Either way - it's likely to be a fairly expensive thing to develop, and then add to the cost.  Particularly since people already complain about the cost.

 

And just recently they *did* provide a new way to prevent dew - with the heater ring that mounts flush to the corrector.  I plan to give one a try.

 

Also note that the heated element in the Schmidt is inside the 'scope and part of the CCD cryostat.  So it isn't preventing dew per se - but frost on the cooled camera cryostat window.  I haven't found mention of how they handle dew on the front corrector of the scope - but I know that for some observatories they just don't open the dome or try to observe when conditions for dew are likely.  For those of us with recent cmos cameras - there is often a built in heater near the ccd window to prevent frost, and it doesn't need to be fancy and transparent.

 

Frank

[RichA]

It wasn't clear which subforum to post this in, so I'm trying here. I was looking at a diagram of the Zwicky Transient Facility imaging system (the one what detected Comet 2022/E3 ZTF), and I can't understand how the heck it manages its 46 degree field of view. That diagram can be found on page 4 of this academic article (pdf).

 

I can't figure out how light rays from 46 degrees (edit: this was incorrect) off-axis can hit the primary mirror. Is it what they call the "Aspheric trim plate" that does the magic of bending those off-axis light rays?

 

Screenshot from 2023-01-26 21-10-07.png

 

Like most professional Schmidt cameras, the primary mirror must be a fair bit larger than the corrector assembly and the CCD mosaic is large.

What's interesting is amateur astronomers fret over spider vane thickness but here it is obviously less important than stability. 

[azure1961p]

Yeah amateur comet discoveries might be gone but the better knowledge of what's coming inbound by earth is far a better place. Pete

[gnowellsct]

Point of order.  Amateur discovery capabilities have increased as well.  But it's likely that you'll need to be imaging down to 20th magnitude or deeper and looking for things that move.  Not the old eyeball and eyepiece sweeps of yore.  Greg N