Sorry for the long delay for some of these questions – I was traveling.
As for the discussion around the brightness of the Corona, I can contribute the following:
– I have seen scientific papers where the result is that the E Corona is about .1% to 1% of the total brightness of the Corona. If the total Corona is about 2-5 * 10-6, then the E Corona is about is about 2-5 * 10-9 to 2-5 * 10-8.
– When the old Sacramento peak coronagraph site was still up, they gave absolute brightness in millionths per Angstrom, i.e. if all the light of the green or the red line would be confined into a rectangular sliver of one Angstrom width, the height would be the stated value. These values ranged from 0 to 200 at the very most, very rarely exceeding 100.
- I have really only attempted a direct measurement once, and if I recall correctly the stray light on that day was about 20 millionth and the Coronal brightness about 100 millionth. However, the filter profiles need to be taken into account and I am not even totally sure whether had one or two filters at that time. However, I’m in the ballpark of the Sac Peak measurements.
So the conclusion is that the E Corona is very faint, but if you filter out the right slice of wavelength, it is about 10 to 100 times brighter than the K/F Corona. That is why one uses a filter.
> What type of Polaroid are you using? A regular low cost one may not do so well out at 700+ nm for the K-corona
Just the cheapest of the cheapest Amazon filters. However, I do not go above 700 nm, and I’m pretty sure I tested these Polaroids with filter combination I’m using (i.e. crossed polarizer blocking) and they were okay.
> Hi, with the imx462 mono you should get at least twice the signal for this wavelength (660 nm to 690 nm) compared to the imx249.
Yes, I have considered this camera. I am, however, not really limited with exposure time for the K Corona because of the broad band of the filter. The IMX462 is a great IR camera though, and I have considered using it with the appropriate filters (at 800nm-900nm) because the scattering of the Earth’s atmosphere is much lower there. But then again, I am mainly a visual observer, I am just taking these images because otherwise nobody would believe me what I’m seeing
>>>> I’m going to have to field test this- i don't see how having an objective mounted erf will "eliminate" the corona while onband transmission is maintained at 95% on the occulting cone with all the infrared light and uv removed. Baader bessel series filters are very high end and are designed to prevent defects like scatter, reflections and halos. I understand how the high cirrus cloud scatter affect happens, but diminishing them entirely because a modern filter is mounted on the objective does not seem correct. I am assuming you are using two traditional soft coated custom order Andover filters which each consist of two glass plates (sometimes 3), epoxied together (four total in the double stack)- Seemingly, four plates of glass with 2 epoxy barriers of scatter did not have much of an affect on your ability to capture corona with just 20% transmission.
>>>
As George said, filters behind the Lyot stop are not the problem, but before they are. Now I have seen prominences with just a red filter through a double glass window, and as George says one can use a cemented achromat to see the Corona, so maybe these modern filters would work. I have never tried one myself.
However our systems work just fine as demonstrated by all the images so I’m not sure what I would gain by adding an ERF. For larger apertures that may be different.
> Can you share the actual spec sheets Andover shipped you for your custom filters? Usually its on a usb stick with a .pdf file attached
They stopped sending USB sticks for cost reasons, just a printed curve now. However there’s nothing special. It is just a one cavity filter with an FWHM of 2 Å.
> Not sure what aperture you are using but you could get that 65 x 65mm square to potentially boost your contrast significantly via a darker background sky- The bessel V is od5.5 blocked out to 1100nm and od3 blocked to 1250nm
The Andover filters should have OD 5 blocking into the far infrared. However, even that is not enough: if the sun is out of focus for other wavelengths you compete with the photosphere which is 10 ^5 times brighter than the Corona, and that over the whole spectrum, therefore much outshining the E Corona even at OD5. So you are correct: when I used a single 2 Angstrom filter, I added another green filter (Meade CCD green) to improve the out of band blocking. Now the second 2 Angstrom filter is taking care of that since they add up to a OD10 out of band blocking performance.
> ill be on the coronagraph team in a couple weeks to test this because im Pretty confident this isnt going to be an issue.
So you are planning to build one? That is exciting. Please report on the progress here and let us know if you need any suggestions.
Klaus
Edited by Klaus_160, 10 November 2024 - 02:50 PM.
