Improved full-disk SHG H-alpha imaging at 700mm FL

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thesmiths
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Improved full-disk SHG H-alpha imaging at 700mm FL

Post by thesmiths »

We made a few minor improvements to the previous setup described here: viewtopic.php?t=36537.

To aid with focusing, an M42 helical focuser was inserted between the collimator lens and the slit holder. This finally allowed good focus to be achieved after much playing around (there are three focuses that must all be set correctly -- the telescope, the collimator and the camera lens).

To help with heat dissipation of the solar energy deposited on the "blue oxided" chrome (see ealier post viewtopic.php?t=36508), the Thorlabs optical tube -- which holds the chrome/quartz chip mounted on a copper disk -- is now heat sunk to the big piece of metal that encompasses the collimator lens and telescope focuser. The heat deposited is only around 8W, which is easily dissipated with proper heat sinking. The chrome/quartz chip itself seems impervious to intensive optical radiation.

Both the collimator and camera lenses were 100mm f4 vintage SLR-type lenses. This means the width of the camera sensor needs to be at least 7mm, so a 178MM (with a width of 7.4MM) would be (just barely) acceptable. To give room to maneuverer, we decided to employ a 183MM sensor, whose 13.2MM width gives plenty of room for error. Both the 178MM and 183MM have 2.4 micron pixel size and are very similar in most respects.

Since there is no attenuation of the solar radiation (via ND filter or Herschel wedge), a short shutter speed could be used (1.0 ms) with a very low gain (46 or 10%). With an ROI of 3200 x 180 pixels, an FPS of 255 was reached. This meant a 1:1 scan ratio could be achieved with a scan speed of 12X sidereal rate (i.e. a scan time of around 10 sec).

This is one of the first attempts with the new setup, taken this afternoon from central London:

H-alpha SHG image, 700mm FL, 106mm aperture, 9 micron wide chrome-on-quartz slit, single scan at 12x sidereal, 2400 l/mm grating, 183MM sensor.
H-alpha SHG image, 700mm FL, 106mm aperture, 9 micron wide chrome-on-quartz slit, single scan at 12x sidereal, 2400 l/mm grating, 183MM sensor.
144606_clahe_small.jpg (223.16 KiB) Viewed 693 times

It is worth comparing this to a highly quality H-alpha etalon image taken on the same day (June 14): viewtopic.php?t=36600.

For the processing of this data, I decided to try the newest version of Valerie Desnoux's software Solex_ser_recon v3.3.0 (which we had a hand in developing). The Python code in her GitHub (https://github.com/Vdesnoux/Solex_ser_recon) is not the easiest to get working if you are not very familiar with setting up a Python environment. An easier route is to download the "Windows executable" version http://valerie.desnoux.free.fr/inti/. For the most part, processing went smoothly (although not on every SER file).

The software first produces a "raw" image, which does not have any geometric correction applied (hence the image looks slightly elliptical), any noise removal (hence some "transversallium"), and no contrast enhancement. The raw image below is for the final SHG image above. In the final image, there are some dark vertical lines on the east and west edges that are processing artefacts not seen in the raw image. The artefacts are from the top and bottom of the raw data where the image is darker and the software may have confused protuberances in the neighbourhood with noise.

Raw SHG data from Solex v3.3.0
Raw SHG data from Solex v3.3.0
144606_raw.jpg (80.59 KiB) Viewed 693 times

The program then does the image processing steps mentioned (geometry, noise and contrast) and a series of corrected images is outputted. Here is a "high contrast" default image from a different SER file than the one above. This one seems to have a few missing lines of data near the right edge (probably a FireCapture issue, due to pushing the data rate to the max).

High contrast, geometrical correction, noise removal using Solex v3.3.0
High contrast, geometrical correction, noise removal using Solex v3.3.0
144623_diskHC.jpg (102.69 KiB) Viewed 693 times

From one of "processed" image files, further filtering via Photoshop, etc can be done (and of course rotation, since the scanning was done in DEC). It's worth mentioning that with a 106mm aperture and the large 183MM sensor, there is a lot of data in these SHG images (binning was 1x1). Below is a 100% crop from the previous image in the area of AR 3032.

Area around AR 3032, 100% crop from the previous image.
Area around AR 3032, 100% crop from the previous image.
144623_diskHC_closeup.jpg (365.65 KiB) Viewed 693 times
Last edited by thesmiths on Thu Jun 16, 2022 5:57 pm, edited 1 time in total.


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Re: Improved full-disk SHG H-alpha imaging at 700mm FL

Post by Montana »

WOW!! it is so clear and detailed :bow :bow :bow

Alexandra


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Re: Improved full-disk SHG H-alpha imaging at 700mm FL

Post by p_zetner »

That’s a very nice result.
Well done.
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Re: Improved full-disk SHG H-alpha imaging at 700mm FL

Post by christian viladrich »

Hello,
Could you post a picture of the M42 focuser arrangement for the collimating lens ? Currently, I just use a spacer to change focus between Ha and Ca II. Still, it is not very practical.
BTW, the quartz slit is a great idea !


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Re: Improved full-disk SHG H-alpha imaging at 700mm FL

Post by torsinadoc »

These are really spectacular.

Well done


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Re: Improved full-disk SHG H-alpha imaging at 700mm FL

Post by thesmiths »

christian viladrich wrote: Wed Jun 15, 2022 12:21 pm Could you post a picture of the M42 focuser arrangement for the collimating lens?
There are of course two types of M42 (I have one of each): the "Pentax" camera thread is 1.0mm pitch and the "T-mount", which has a 0.75mm thread pitch. My SHG setup uses the Pentax type lenses and extensions tubes (these "macro extension tubes" are very cheaply available from eBay and usually come in a set of various lengths -- try to buy the "made in Japan" ones, by the way). The helical focuser is the part with the white tape on it (which is a marker for positioning purposes). By the way, the collimator lens is a Pentax 100mm f4 bellows lens which does not have a built-in focuser. The camera lens is a Japanese-made vintage 100mm f4 lens which has a built-in focuser. The second photo shows the inside view of the helical focuser (officially called a "variable close-up ring", meant to be used for macro photography) where you can also see the chrome/quartz chip, attached to the copper plate.

I have also used a "T-mount" helical focuser from Borg, the "Oasis M42 helicoid T (7839)". The issue with this is it is not as strong as the Asahi Pentax version and struggles supporting anything too heavy. So I would not recommend it for this application. The Pentax focuser is meant to support a camera lens while the Borg seems to be designed to support an eyepiece.

M42 helical focuser and extension tubes
M42 helical focuser and extension tubes
1396.jpg (145.47 KiB) Viewed 623 times
Inside view of the "variable close-up ring" from Asahi Pentax, showing the chrome/quartz chip mounted on the copper plate.
Inside view of the "variable close-up ring" from Asahi Pentax, showing the chrome/quartz chip mounted on the copper plate.
1399.jpg (172.9 KiB) Viewed 623 times


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Re: Improved full-disk SHG H-alpha imaging at 700mm FL

Post by christian viladrich »

Thanks !
Indeed, my concern with the usual M42 focusers we can find in astro (Borg, TS or others), is that they are not strong enough to support the weight of the SHG (in my case Sol'Ex, which is not very heavy, but still ...).
Maybe I have some old macro focusing tubes somewhere.


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Re: Improved full-disk SHG H-alpha imaging at 700mm FL

Post by thesmiths »

christian viladrich wrote: Wed Jun 15, 2022 6:50 pm my concern with the usual M42 focusers we can find in astro (Borg, TS or others), is that they are not strong enough to support the weight of the SHG
The Asahi Pentax version (also called the "Pentax helicoid extension tube") is considered the king of helical focusers. They are a little hard to find these days but well worth looking around for one. I should mention it is very stable when static, but it is not easy to turn if there is significant sideways torque on it, which would be the case if it is attached to a telescope in a non-vertical orientation. To tune when "live" on a telescope, you need to support the SHG a bit with one hand and turn the ring with the other. It has an extremely smooth movement and is "nicely stiff".


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