degraded images from ASI120mm mini during guiding

Tech@ZWO

More like the dew problem on the scope, check the glass on the front of the guide scope first.
If no dew on it, add a UV IR cut on the camera to see if any improvement.

Mapleleaf

Thanks tech@zwo and Kevin_A.

The image I submitted was slightly out of focus as I grabbed while trying to adjust focus, trying to eliminate the halos. Racking focus had no apparent effect on the halos. I’ve subsequently got focus achieving 2” FWHM and halos are still prominent.

At the time, I did check the guide scope and the time; dew heater was on and scope glass was clear.

I am wondering whether there is a moisture within the camera (behind the AR window) or on the sensor. Visually, there is nothing obvious but might still be there.

I do not recall having noticeable halos during the first two years of using the 120mm and I’ve always had a dew heater on the guide scope.

Can the AR window be removed and the sensor chamber opened up for cleaning? Are there any instructions on this for the 120mm? I’m not thrilled about opening it up but I’m running out of ideas on how to remedy this.

Tech@ZWO

Mapleleaf The dew more like on the guide scope instead of the camera, the images should be a black shape in the center if the dew is on the camera.
Did you try a UV IR cut filter?

w7ay

Mapleleaf Here's a sample image shot at 1.5s @ gain 75 in a Bortle 4 location with heavy dew.

You said it yourself, it is dew on the objective of the guide scope.

Notice that your stars are in good focus; nice small points of light (so, it is not a gross focus problem). But each star has a halo, that is indicative of what dew near the optical axis does to the point spread function.

If the dew were even across the objective, the entire star field would be blurred, instead of looking like stars with halos. So, this is likely to be a case of an onset of dewing, where the outer parts of the objective (warmer) has not yet dewed.

Further, I agree with Tech@ZWO that this is not caused by dew on the camera window. Dew starts at the center of a round glass (coldest part of the glass). If the dew were near the sensor (for example, the camera window), you will have a circular central part of the image plane where the stars are degraded, while the stars are not degraded towards the corners of the guide frame. Your halos appear to be spread out evenly.

Use a longer dew shield and good dew heater, and the problem should go away. Keep your OTA objective over the dew point. Occam's Razor says it has nothing to do with the camera.

(I have no idea why tech@ZWO suggested using a UV-IR cut filter! )

Chen

Mapleleaf

Thanks Chen & tech@zwo

Will crank up the settings on the guide scope dew heater next time out. The Uniguide 50mm already has a long dew shield but I can cobble together a extension to test. Should be able to find an empty Pringles can here somewhere :-)

The night I took the image, I did check the guide scope; no droplets or visible fogging of the outer lense surface were under a red light but it is possible I missed a very light fogging I missed.

What is the possibility that the misting is happening on the camera side of the objective? If there is moisture trapped in the camera side, that might explain the sudden onset and why it is happening consistently since.

I've open the scope up a couple times removing the camera so it should have had a chance to dry out but I will double check from the other end as well.

w7ay

Mapleleaf

Was it foggy? I have seen even thin, high clouds produce such diffraction "halos" around stars.

What is the possibility that the misting is happening on the camera side of the objective?

Probably. You can try to air the 50mm guide scope out under sunlight by removing any reducer/flattener, filter, camera, and perhaps even sneak a small desiccant bag in there temporarily.

I stared at your image again, and there is an interesting phenomenon. Notice that all the halos have the same diameter, and each is very evenly illuminated across the entire disk of the halo (as you mentioned, not a defocused Airy disk). On dimmer stars, the halos are simply dimmer, but not smaller, and on brighter stars, the halos are brighter, until a couple of them saturates the sensor, without becoming larger.

Chen

Mapleleaf

Could it be internal reflection?

I took one of the stars from the middle and cropped down in Photoshop. The halo is roughly 40 pixels across and, applying the formula in Bracken's Deep Sky Imaging primer, the reflecting surface is only .29mm from the sensor: (40 pixels x 3.75 μm x 4 focal length) / 2 = .2875 mm gap.

There is some error creeping in as I try to accurately crop to the exact pixel boundary of the halo but I don't think it is material.

That seems really close but is there a protective layer on the sensor itself? If there is, what could suddenly cause that surface to become reflective?

The irony is the 120 has a protective AR window but it is an order of magnitude further away from the sensor.

w7ay

Mapleleaf The irony is the 120 has a protective AR window but it is an order of magnitude further away from the sensor.

The constant diameter of the halos does look like the halos from OIII internal glass reflections, don't it? You didn't recently added some cheap filter in your guide scope, did you?

Your stars appear perfectly centered on the halos even when they are near the corners of the image plane (I assume it is not a small crop of an original image). That should be another hint if it is internal reflections in some glass. With halos from internal reflections, the central star should be offset from the center of the halo when the star is located away from the optical axis.

Chen

Kevin_A

I had the same halos tonight while testing out a typical budget 50mm doublet guide scope and at perfect focus…I had big huge halos. I knew it was going to be hazy tonight but I was doing a few alignment tests. Once I noticed the halos I looked over at the moon and it too had a halo… my upper atmosphere had started to lightly cloud over with lots of moisture in the air too, and all my stars in the cheapish doublet had huge halos with pinpoint stars inside. Just most likely an atmospheric condition.

Mapleleaf

Kevin_A Good point. Certainly humidity has been high during my imaging sessions this summer; I hadn’t considered that the camera might be picking up atmospheric haze that I wasn’t seeing myself.

Mapleleaf

Nope, no filters on the guide scope.

The optical path is:

lense cell
AR protective window
sensor (and possibly a sensor cover)

On a 50mm aperture, would decentering be too small to be (easily) noticeable?

So, if not internal reflection, dew or other film return as the prime suspect.

Hoping to get out in the next couple nights to test again.

Kevin_A

Mapleleaf the doublets usually are glued sets so decentering can not be adjusted and it really just makes them slightly fuzzy and not crisp. Once the thin hazy atmosphere/ thin clouds disappeared last night the halos did too. Usually clouds dont do this but when the moisture in the sky is just right… halos. My humidity was 95% and temperature was 10c outside.