Modded scope is rather underwhelming

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RaumHund
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Modded scope is rather underwhelming

Post by RaumHund »

Hi SolarChat board,

I built a scope but the performance is not great - I'd appreciate the input from some experienced scope modders.

The start:
Lunt LS50 PT with B600.
This is my scope for about a year now and I'm quite satisfied with the performance it delivers.
But I wanted more surface details in general. My plan was to use a 80mm refractor and use all the LS50 filtering.
I got the TS ED 80/560, I mounted the PT and the B600. And the scope worked kinda, but the overall performance was just slightly better than the LS50.
See image below. I made two comparison images just 5 minutes apart, both were processed equally and just scaled and rotated to fit.

I understand, that the etalon and BF600 are rather undersized, but I want to know if I should invest more time in the scope to dial out distances and positions of etalon, BF and camera. I think I positioned them ok, the image was ok in the upper half but without details and to bright in the lower part. (I'll try to attach an image). I would like to understand what limits my image performance, is it the rather small etalon, the 6mm BF or something else?
Any help would be appreciated!
Best regards from Heidelberg, Germany!

Comparison of 50mm LS50 and 80mm modded scope
Image

50mm
Image

80mm
Image


Dennis
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Re: Modded scope is rather underwhelming

Post by Dennis »

Moin,
what kind of camera are you using and what are your frames per second? How do you stack your images?

Apart of this my guess would be:
-For full disk images you dont need so much aperture in general
-The 80mm scope would play its resolution advantage out if you would use a 2x barlow or even higher magnifications - here is the difference.

VG
Dennis


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Re: Modded scope is rather underwhelming

Post by marktownley »

Good start, I just think your tuning is off, practice with this and you will get the detail / contrast you are after.


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RaumHund
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Re: Modded scope is rather underwhelming

Post by RaumHund »

Thank you for you replies! :bow2
I doubt that the tuning is of - I have a bit of experience with the LS50 and usually need less than a minute to get a decent view.
But I modified my mechanics today (had access to a lathe and reduced the diameter of the pre etalon filter housing, got rid of a 3d printed part) and will have another test as soon as the clouds allow.

For the imaging:
Scope is mounted on a SkyWatcher StarAdventurer
I use an ZWO ASI 178MM and take usually a 10s .ser (gain around 200 to get an integration time ~3-400µs), stacking 10-20% in PSS, wavelet sharpening, -> curve correction in gimp. To be fair, the seeing was not great when I did the comparison, but the images were taken less than 5 minutes apart.
This is an example picture I took with my LS50

Here are links to the high resolution images from by previous post:

50mm
80mm
comparison
Last edited by RaumHund on Wed Apr 12, 2023 8:57 pm, edited 1 time in total.


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Ivan
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Re: Modded scope is rather underwhelming

Post by Ivan »

Hi!

Maybe you know, I'll just remind you just in case.

Any etalon has a "sweet spot" - this is the place where the image has maximum contrast and details. Any solar telescope is designed so that the size of the full disk of the Sun and the size of the "sweet spot" coincide.

Finding the "sweet spot" is simple, take your Lunt50 and "move" the Sun around the frame (this can be done by looking into the eyepiece or at the laptop screen). Look carefully, and you will see that in different parts of the frame, the same details either disappear or lose a lot in contrast (or light up if you look at the monitor), then they are perfectly visible.
You have to find the place where the Sun is completely perfectly detailed. Remember that you have a pressure tuner and do not forget to use it.

When you take etalon from lunt 50 and put it on a telescope with a larger aperture, the image scale increases (the Sun's disk has become larger), but the size of the sweet spot has not changed. That is why part of the disk does not have details in your photos. This is normal, just use the Barlow lens, the full disk will not fit into the frame, but you will be able to do mosaic or close-ups.

Good luck!

Ivan


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Re: Modded scope is rather underwhelming

Post by Bob Yoesle »

The LS50 appears to have a rather small sweet spot to begin with, and the LS50 image appears to show it is centered, but slightly off-band, with the periphery of the sun's disc more on-band.

The real angular size of the sweet spot is dependent of the ratio of the focal length of the collimator lens to the focal length of the objective. If the etalon has a native sweet spot of 1 degree, placing that etalon in a collimator system with a collimator lens 1/2 the focal length of the objective will result in a field angle magnification of FLO/FLC = 2 > and the sweet spot will be 1/2 a degree. If the collimator has 1/4 the FL of the collimator, the filed angle magnification is 4, and the sweet spot will shrink to 1/4 a degree.

Therefore, even without knowing the FL of the collimator, we can deduce going from the LS50 FL of 350 mm to the TS ED 80 FL of 560 will also reduce the size of the sweet spot based on the ratio of the two focal lengths: 350/560 = 0.625. The sweet spot will be reduced to 62% of what it is with the LS50. So if the entire disc of the sun was in the sweet spot with the LS 50, with the TS ED80 only 62% of the disc will be in the sweet spot.

Additionally, the sweet spot appears decentered with the TS80 mod. This is likely due to a mechanical and/or optical alignment issues.

Best wishes,

Bob


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