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Lunt 50 'sweet spot' 9 box analysis

Imaging Solar
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#1 BinoGuy

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Posted 16 April 2021 - 12:42 PM

Hullo everyone,

 

In a recent thread about the sweet spot I promised to do a 9 box study.  Rather than resurrecting/hijacking that thread I thought the topic would be interesting enough to many people that I should start a new thread.

 

I had 45 minutes during an early lunch (10am) on Tuesday to set up, align, test the ASI guide camera, and take some images in the new Lunt.  These are in no way, shape, or form good pictures and I'm under 90 minutes on the scope so far due to clouds and work so I'm obviously still learning focusing and tuning of both the filters.

 

Acquisition

No flats, refocusing, or changing any settings; just 1,000 raw images with the DS in-place for each frame.  Had I known I was going to be purchasing the new Player One camera I would have waited but since I did the work I figured I'd share.  When I do this task again I'll spend more time to make sure each image of the sequence is the best it can be and in exactly the same X or Y coordinates.

 

Processing

The sequence was performed over six minutes and all exposure settings and stacking were the same, best 50% of 1000 images, sharpening, auto grid @104, no drizzle. 

 

Analysis

I think there is clearly a darker area in the top right of these images. It is maybe a little darker towards the bottom left.  Because I was on a GEM the camera was tilted about 60 degrees to the left of 'upright' so this isn't exactly what one would see visually and I think the darker area would be near the top of the image.  I think my sweet spot is a circle about 70-80% of the image, centered in the middle of the image and maybe a little low.

 

gallery_230512_9047_2105753.png

 

 

The only processing was in Autostakkert:

gallery_230512_9047_6766.png

 

 

Clear skies  BG  °¿°

 

 


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#2 Jaimo!

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Posted 16 April 2021 - 12:58 PM

I don't want to be critical, but the only two consistently imaged out of the bunch are the center and bottom middle.  The disc is different in almost every other location.  Then again, I rarely know all of the variables involved.

 

Jaimo!



#3 BinoGuy

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Posted 16 April 2021 - 01:08 PM

Fair point, Jaimo!.  My intention was to reduce all possible variables by taking them in as brief a window as possibleand not changing any capture settings but you are correct, most are not properly exposed.  I could probably retitle the sequene "Seeing changes over 6 minutes" or "Vagaries in Lunt glass across the imaging surface" and the presentation would be equally useful (e.g. not that much).

 

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Clear skies  BG  °¿°



#4 Jaimo!

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Posted 16 April 2021 - 01:16 PM

Please do not take my criticism to heart, I appreciate you sharing and has got me thinking: What is the sweet spot on my scope?  Will it change with etalon adjustments?  How to collect a flat with a solar scope to subtract the "vignetting" (for lack of a better term)?


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#5 Tom M

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Posted 16 April 2021 - 01:26 PM

Another consideration is thermal equilibrium.

 

When I use my 60 and get on band/focused I can watch the shift for the first few minutes. Retuning is definitely required for the image in the same (as near as possible) spot until things settle down. Effectively, it seems that the sweet spot is not "fixed" in place until things stabilize from a thermal standpoint.

 

That said, I think mapping it out is a valuable lesson. But probably needs a bit more up front time to make sure everything is fairly stable.


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#6 Great Attractor

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Posted 16 April 2021 - 04:01 PM

Your sweet-spot appears to be off-center. In a pressure-tuned telescope it's either a circle (when the etalon is properly tuned) or a smaller circle + a larger annulus (when somewhat detuned) - as in your images. I marked its approximate location:

 

9box.jpg

 

Now, it would be best if you could also just post a single frame (with a lowered gamma, for better contrast) of each of these videos (so that any slight drift during recording doesn't show, as it would in a stacked image).

 

Note that it's the darker areas that are "on-band". You should always adjust the pressure in such a way that the dark area is circular and maximized in size (unless you want to specifically capture in blue or red wing of Hα, of course).

 

It if turns out that, after tuning as best you can, the sweet spot is too far off-center to capture the whole disc in the camera's FOV, you can 1) try to stiffen the focuser using the adjustment screw (...there is one, if I remember correctly), 2) get a better focuser.


Edited by Great Attractor, 16 April 2021 - 04:02 PM.

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#7 BinoGuy

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Posted 16 April 2021 - 04:12 PM

bow.gif    So much excellent information for me to absorb, Great Attractor,.  Thank you for the time and attention. I will perform this same test with the new Player One camera and much more attention to detail.

 

 

Clear skies  BG  °¿°

 



#8 Gregory Gross

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Posted 17 April 2021 - 01:10 PM

I look forward to your further analysis, BinoGuy. I remember my DS'ed 50mm Lunt having had a sweet spot that was just a bit smaller than the Sun's disk (I traded it in for my current DS'ed 60mm Lunt). It would be nice to see more systematic evidence of this at least for your particular example.



#9 Great Attractor

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Posted 17 April 2021 - 04:34 PM

I forgot to add that what I wrote pertains only to a single-stacked system. I imagine with DS you would get some combination of the rotationally symmetric shape described above, and a circular/oval (?) from the front tilt-tuned etalon.




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