Jump to content


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Herschel + ordinary H-alpha?

Solar Refractor Video Astronomy Equipment Filters Imaging
  • Please log in to reply
3 replies to this topic

#1 cancian123


    Lift Off

  • -----
  • topic starter
  • Posts: 6
  • Joined: 29 Nov 2021

Posted 29 November 2021 - 03:38 PM

Hi everyone.


I was looking for a cheap way to image the chromosphere of the sun, and I came up with an idea:

Is it possible to use a herschel wedge in conjunction with an ordinary H-alpha filter attached to a DSLR?

I think that if the image coming out of the herschel wedge is broad band and has the full spectrum of visible light, we could simply add  an H-alpha filter to isolate this specific wave length. If this works, we would be able to image prominances with a simple herschel + H-alpha filter, which is way cheaper than a daystar filter.


Ps.:By "ordinary H-alpha filter" I mean a filter that is usually used for imaging DSOs (ZWO, optolong, etc...). 


Thanks a lot

#2 MalVeauX



  • *****
  • Posts: 9,809
  • Joined: 25 Feb 2016
  • Loc: Florida

Posted 29 November 2021 - 04:02 PM



Unfortunately no. Herschel wedges reduce transmission to 5% or lower, and that's just too low for transmission through an etalon of any kind as an etalon gobbles up transmission. The net result is a black image.


An "ordinary" HA filter will also not do at all for chromosphere imaging. You need at bare minimum around 0.8~0.9A (0.08 to 0.09 nm) and even then, that's only going to be ok for prominences on the limb, if you want the surface, it will need to be closer to 0.7A (0.07nm). Ordinary HA filters are commonly 3nm at the most narrow, to 7nm~12nm and up to 35nm and none of these are remotely close to below sub-angstrom to be able to image the chromosphere of our sun. You'll just see the overwhelming brightness of the photosphere.


There's no cheap way to HA chromosphere in solar. It requires an etalon essentially for the cheapest approach. So, lowest way to do this is a dedicated HA etalon based scope or rear mounted etalon filter with appropriate blocking filter.


Very best,

  • BYoesle, Ernesto.Nicola and AllanDystrup like this

#3 BYoesle


    Fly Me to the Moon

  • *****
  • Posts: 7,374
  • Joined: 12 Jun 2004
  • Loc: Washington USA

Posted 30 November 2021 - 07:15 AM

Hi cancain123, as Marty notes, there is a reason why it takes a much more sophisticated and expensive filtering system to observe the chromosphere directly.


In my presentations on solar filters, I use these graphic representations to demonstrate why you can not use a white light solar filter with a nighttime H alpha filter of any bandpass to see features of the chromosphere such as prominences, filaments, flares, etc. 


Image 5a crop.png

The H alpha absorption line is only ~ 0.12 nm (1.2 Angstroms) wide.


In pane 1 below, a bright white painted exterior wall represents the intensely bright photosphere, and the window represents the broad dark H alpha absorption line in the continuous spectrum of the photosphere. This can be thought of as looking into a dark room with no other windows, in which a candle glows. This candle represents the H alpha emission line from the chromosphere, ~ 100,000 times dimmer than the photosphere.


In pane 2 we use a standard white light filter to dim the photosphere to a safe level for observation. This has required a 100,000 times reduction in brightness, but we have also dimmed the brightness of the emission line (the candle) from the chromosphere 100,000 times as well.


Adding a nighttime H alpha filter (pane 3) reduces the brightness of the photosphere a bit by eliminating all but the red wavelenghts around the H alpha absorption line, but cannot do anything to bring back and reveal the H alpha emission (the candle), which has been greatly reduced in brightness by the white light filter.


The only way to see the H alpha emission (the candle) from the chromosphere is to completely eliminate light coming from the photosphere (the wall), while leaving the window un-dimmed. We can then peer into the window (the absorption line), where the chromosphere - the candle - can now be seen in pane 4.


Ha nighttime for solar filtering.jpg


So you must completely block all wavelengths from the UV (200 nm) to the far IR (3000 +nm) to OD5 (1/10,000), while simultaneously passing an extremely narrow slice of the spectrum (0.1 nm or less) essentially unimpeded. You cannot do this easily, or with any filter designed for observing the photosphere. It takes multiple filters, and an Fabry Perot etalon filter with very high manufacturing tolerances, to be able to do this.


Hope this helps to understand why you can't really implement your idea with any real hope of success.

  • techmgr, germana1, Ernesto.Nicola and 4 others like this

#4 cancian123


    Lift Off

  • -----
  • topic starter
  • Posts: 6
  • Joined: 29 Nov 2021

Posted 30 November 2021 - 11:55 PM

Hi Guys.

Thank you for the answers.

BYoesle, that was very clarifying. Thank you very much.

I think the only option is to save some money and buy an appropriate filter.

  • BYoesle and hamers like this

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Recent Topics

Also tagged with one or more of these keywords: Solar, Refractor, Video Astronomy, Equipment, Filters, Imaging

Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics