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H-alpha solar imaging with QHY163 and ADC


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This post and the one further below discusses solutions to problems I encountered when I recently added a QHY163 mono camera to the 80mm f/7 Lunt LS80THA in our observatory. Using an ADC I managed to get rid of Newton-rings and centre the Sun in the Lunt's sweet-spot. In addition I managed to get rid of the chessboard-pattern caused by the Panasonic chip that is used in the QHYCCD QHY163 (and ZWO ASI1600) and remove the vertical banding caused by the Unsharp-Mask routine.

How it all began: A check with Stellarium learned that the combination of a Lunt LS80THA solar scope and QHY163 camera could theoretically produce full-disc images of the Sun when used in combination with a 2x Barlow. So day before yesterday I mounted a barlow (TeleVue 2x PowerMate) and the QHY163 camera behind the scope and took an image of the Sun (seeing was rather poor, about 8"):

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Needless to say that the Newton rings were rather disappointing. I never experienced them when using a ZWO ASI174MM, so it must be due to the camera. I had read about them before and knew this could be solved by applying a tilt-adapter. This, however, was not at hand and before ordering one I first checked if Newton-rings could be solved at all with this camera. The answer I found, although not immediately concerning this camera type, on SolarChat forum: instead of using a tilt-adapter one could also use a dispersion-filter (optical wedge) to eliminate Newton rings. The good point of that is that I do own two of them, which together form an ADC. And we do not need even need to take the ADC apart, but simply apply the whole ADC to the imaging train:

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In above image the levers of the ADC are at perpendicular orientation to each other. In that way the two wedges have a perpendicular orientation to each other, resulting in enough optical tilt to get rid of the Newton rings. In below image the above shown solar-image (i.e. without ADC) is shown at the left. At the centre another solar-image is shown with ADC and with levers in a zero degree orientation, while at the right a solar-image is shown with the levers at a perpendicular mutual orientation.

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Clearly the Newton-rings have vanished completely in the right-hand image. Adding the ADC has another advantage as is shown by the left two images: with ADC the solar disc is shown 2.5% larger than without. Of course there is also a downside to the used of the ADC: exposure time dropped by 33% from 0.73ms to 0.97ms, but that still is acceptable.

Yesterday seeing was around 4", so I gave it another try with significant better result. This time I noticed that the camera results in a chessboard-pattern, ouch!

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I had seen this pattern in a milder form when using a ZWO ASI1600MM Pro Cool (same Panasonic MN34230 chip). Another Internet-search resulted in a post by Qiu Hongyun, founder and CEO of QHYCCD, in which he explained that it was "... due to the channel difference between R,G,G,B channel" even though this is a mono-camera. Simply because the Panasonic chip was originally designed for a RGGB-Bayer-pattern, the parameters for the four photosites of the chip vary. Qiu Hongyun did not specify whether it was due to variations in gain or offset, but stated that they could be solved by either using a flat-frame or dark-frame. Problem is that with the current capturing software this requires to take full-frame images, which is rather undesirable as it significantly affects frame-rate.

Solution was given directly below that post: applying a 1.5px pre-blur in AutoStakkert! (can be found under experimental features) to the original data during processing. Indeed this greatly reduces the chessboard-pattern without significantly affecting detail as can be seen when comparing two images (left without pre-blur, the pattern can be seen in the solar surface, centre with pre-blur and significant less pattern):

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Both the left and centre images also show vertical banding (noticeable in the prominence). This is caused by the Unsharp-Mask algorithm in PaintShop-Pro that I use. To get rid of that I first resized the whole image by 200%, then applied Unsharp-Mask, followed by a 50% resize. The result is shown at the right in above set of images.

So with that solved as well, I could finally produce full-disc image of the Sun using the QHY163 mono (image taken on 9 May 2022 @ 08:27UTC):

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With a diameter of about 2840px this has a significantly higher resolution than the 900px diameter I managed with the ZWO ASI174. Now the waiting is for sunny weather with better seeing.

Above image still has an issue with the sweet-spot, as a result of which the illumination is uneven. This can be solved with the ADC as explained in the second part of this post below.

Hope this post is of use and inspiration to others!

Nicolàs

Edited by inFINNity Deck
Added second part of post and introduction to solutions found.
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  • 2 weeks later...

Found another advantage of the ADC today: centring the so-called Sweet-Spot. H-alpha viewers such as the Lunt have an internal etalon, two glass plates that are very close to each other and therefore act as a filter. The light passing through an etalon is reflected between the two glass panes and by varying the distance from the light path, it is possible to amplify or extinguish certain frequencies. There are two ways to vary that distance: by forcing air between them (pressure tuner) or by tilting the etalon (tilt tuner).

Regardless of the type, the etalon has the property that it has an area, approximately one degree in diameter, within which the damping/amplification is more or less uniform, the so-called sweet spot. Now the Sun is about half a degree in diameter, so it fits perfectly within that area. Since the damping/amplification within the area is just not quite the same, it is best to centre the Sun as well as possible around the sweet spot. Failing to do so will result in a solar-image that is not equally bright everywhere, something that can be clearly seen in the photo above (top right it is too dark, bottom centre too light).

Today spent a few hours puzzling to get the Sun centred around the sweet spot. To do so, I first mounted the QHY163 without ADC and without Barlow behind the Lunt and shifted the sun over the image by slewing the scope until the sweet spot was centred in the Sun. Whether it is properly centred can be seen when the image is slightly overexposed. If the image of the Sun is not in the sweet spot, there is an overexposed area on the Sun's surface that is not centred within it's limb. It is then a matter of slewing the telescope until the overexposed area is neatly visible in the centre of the Sun.

In my combination (Lunt and QHY163), the sweet spot turned out to be approximately at (2680, 1580) pixels. The camera chip has a resolution of 4656 x 3522 pixels and a size of 17.7mm x 13.4mm. Based on this, we can calculate that the sweet spot was projected approximately (1.34, 0.69) millimetres from the centre of the chip. Now, as this is just a small offset, I was wondering if the ADC could correct for it. So I mounted the 2x PowerMate and ADC and again checked the sweet spot again by slewing the scope and looking at the image. The sweet spot now turned out to be that far from the centre of the chip that the Sun didn't quite fit any more. Then played with the ADC (first only moved the levers, then turned the entire ADC a quarter of a turn and played with the levers again), until the Sun was neatly centred on the chip again. Since the telescope was still pointed at the Sun in the same way during this operation, the sweet spot was now also neatly centred (and without Newton rings :-)). Then at 15:48UTC attached image was taken (it was not the first of this afternoon ;-)). The seeing was not optimal with a mere 4″, but the important thing in this image is that the Sun is nicely and evenly exposed.

Nicolas

 

Sun220528_1548UTC_H-alpha.jpg

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