135 mm issues. - Forums

Topic starter

Oct 20, 2023

I have set up a Rokinon 135mm lens with an astrodymium bracket, A nikon t2 filter drawer adapter and a 533mc or a 294 mc (image shown) with the 11mm spacer and
I get this terrible gradient.... I have a Radian triad filter in the drawer.
Flats I take look normal and flats and dark flats do not take this away at all.
The stars are uniform thoughout and look good.
Does anyone have an idea what is causing this?

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andrea tasselli

Frequent contributor

Oct 20, 2023

At what focal ratio was it taken?

tazdevil71

Oct 20, 2023

Not sure it is an material issue.
it looks like light pollution gradient. With a wide aperture like the samyang, light pollution is quickly visible.
is it the first Time you use it?

Dan Kearl

Topic starter

Oct 20, 2023

Thanks for the responses.
Andrea, I am not sure what you mean by focal ratio.
The image shown in 135mm w/ asi294 so it is 270mm effective focal length.
Tazdevil71, I have used it several times with asi533 and asi294 cameras.
Yes it is in bortle 8, so heavy light pollution but I have no problems with my other setups.
Maybe it is the 135mm lens, but no amount of PP gets rid of the gradient, it just shifts around with
background or dynamic background removal and calibration makes no difference.

andrea tasselli

Frequent contributor

Oct 20, 2023

Dan Kearl:
Andrea, I am not sure what you mean by focal ratio.

It's a lens, it should have a diaphragm and therefore a relative aperture setting. If it is a normal Samyang f/2, it could be either f/2, f/2.8, f/4, f/5.6 (and intermediates). It's on the barrel.

Daniel Carter

IOTD Staff

Oct 20, 2023

It looks like a light pollution gradient. It doesn't really look like anything is wrong with the lens itself. When shooting at such a fast focal ratio (f/2.0 if you are shooting wide open), any stray light show up. It could be a street light or even the moon showing such a gradient. If you are in a Bortle 8 then you have a ton of light pollution to contend with. When I shoot closer to the city Im from I tend to shoot in narrowband to avoid the strongest light pollution gradients.

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Dan Kearl

Topic starter

Oct 20, 2023 · Edited: Oct 20, 2023

Andreas, I am sorry for my understanding. I was shooting at f2.8.
I have shot it at f4 with the same result.
I shoot at f4.7 with my Williams scope at 337mm in my bortle 8 sky with no problem if you look
at my images. I just can't seem to make this 135mm work?

andrea tasselli

Frequent contributor

Oct 20, 2023 · Edited: Oct 20, 2023

Short lens (relative to our typical focal lengths 300 and above) do have tendency to pick up anything stray that might get your way, even more so if in heavily light polluted locations such as yours. What Triad you were using, by the way? I get significant gradients even at with filters such the ASKAR Duo-Magic and I am shooting from a marginally Bortle 6 location (most of the southern side is however 7). So the result doesn't surprise me. If anything check that the filter isn't somewhat tilted with respect to both the lens and the camera. By the look of it it could be easily flattened in PI and I have flattened far worst results than that, so there is hope…

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Sean Mc

Oct 20, 2023

It looks like light pollution to me ¯\_(ツ)_/¯

Have you tried wrapping the lens up to rule out light leaks?

andrea tasselli

Frequent contributor

Oct 20, 2023

Incidentally f/2.8 is a tad too fast for most normal tight NB filters and often marginal rays are shifted outside the pass-band, the bluer the more, at f/2.8.

Tim Ciasto

Oct 20, 2023

Yes, this could be a light pollution gradient. But if you took this image with the ASI294MC then it could also be a calibration issue. The 294MC is known for funky colour gradients when paired with dual/tri narrow band filters. I have the same camera, learned to manage this characteristic, and repeatedly calibrate and clean my images correctly.
The formula I use is the following:
Use a high-quality light panel with even colour distribution as a light source for my flats.
The exposure time for flats should be at least 3 seconds because the camera can produce inconsistent results with shorter exposure times.
Take darks with the same exposure time as your flats = dark flats.
Take darks with the same exposure times as your lights.
Ensure you take all the exposures with the same gain and offset settings as your lights.
I always take 20 flats, dark flats and darks as calibration files and use these to calibrate my files. I had so many issues with colour gradients before I started using this recipe. Now my files clean up like a miracle.
This may sound tedious but in my experience, it's the only way to make this camera work with narrow-band filters.
The Rokinon 135mm is a very fast lens, which can also contribute to issues with narrow band filters due to bandpass shift, so there are probably several factors responsible for your gradients.
I hope this helps.

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D. Jung

Oct 21, 2023 · Edited: Oct 21, 2023

Are you using the lens hood? That helps with reducing stray light. If you have a 3d printer you could print a longer lens hood (without cropping into the field of view).

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Dan Kearl

Topic starter

Oct 21, 2023

Thanks for all the responses. It was a light leak issue I think as I just took the set up apart and put back together.
I have used the 294 for years so I know the calibration issues and it was not the issue.
Her is a 2 hr. image I took last night of the Elephant Trunk.

churmey

Oct 21, 2023 · Edited: Oct 21, 2023

I am suspect that this isn't a light leak. This is typical LP gradient that is common with such a wide field that is dependent on your location and angle to the subject in the sky. I see this in my widefield images but it's also easily correctable with a processing tool designed for it, such as PI's DBE dynamic background extraction tool. Here is a quick single pass with DBE.

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Dan Kearl

Topic starter

Oct 21, 2023

Thanks churmey, but the subs were completely different so something was going on. I realize you can work and work to remove gradients but you should not have to.
The images I got last night after I reassembled the kit were 100% better so I do think there was a leak. The image I posted form last night has has NOTHING done to it, the Fit file converted to a jpeg is all.

ManuManu

Oct 22, 2023

Hello,
Sometimes the filter drawer is not free from leakage due to small play. If the direction of light pass through the leakage you may have this issue. So this depends to the orientation of the lens versus light source. Try to cover the drawer with adhesive tape to check any difference.
Clear skies

Tim Ciasto

Oct 22, 2023

I am glad you were able to find the problem. Clear skies!

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