6 replies to this topic

[clint.ivy]

Back story: In January, due to error ID-10-T my camera suffered a ~5.5 foot fall. The brunt of the force was taken by the lens (now toast) and the interlevometer cable (also toast). The camera body seemed fine and a quick test with another lens (during daylight) didn't seem to show any problems. So, even though I wasn't really in the market for new glass -- off I went to Amazon and got a new-to-me Tamron SP 70-200mm F/2.8 DI VC G2. So, over the last month or so I've noticed some weird defects/artifacts in my integrated lights. (see first screenshot) that presents as a very dark circle in the center of the image with a lighter ring around it (that may repeat more faintly as you go towards the outer edges of the image.

 

Gear that we're concerned about:

• Nikon D7100 (nearly a decade old, suffered a 5+ foot drop)
• Tamron SP 70-200mm lens

 

First Image -- Defect in Lights:

(Note: I use PixInsight for post processing) In this screenshot, on the left are my lights (16 @ 45s each) registered and stacked -- without calibration frames followed by the same master with basic BGN, PCC and ABE applied. On the right, is the master light with calibration frames applied and basic BGN, PCC, and ABE applied. All frames have the same minimal crop applied to remove stacking artifacts.

 

Second Image -- Defect in Master Flat:

So, in trying to diagnose what this defect is, I applied basic post processing (BGN, ABE) to a copy of the master flat (after debayering it) and the defect is there!

 

Third Image -- 5 Min Dark Test:

At this point, I'm pretty suspicious that the defect is coming from the lens, or the sensor so I took some really long (for me) darks and while I was surprised to see this banding in my Nikon, it doesn't appear that heat noise is showing the same defect that we see in the other frames.

 

So, now I'm really worried that I've got a lemon of a lens. I've got a D7100 on the way from Borrow Lenses to see if the same defect shows up on a different camera body, but in the mean time, has anyone seen anything like this and can help me narrow down what issue I need to be solving for?

 

Masters and some sample subs are available on my dropbox if you want to download the data and take a hard look at it.

 

Thanks in advance for your help, and Clear Skies!

[TxStars]

I would start by taking new data sets

 

The darks look as tho there was some light leak,

Try again with caps on the lens and the camera in a box with lid in a dark room just to double check

The flats have uneven illumination across the rings which can be a problem.

Might try additional diffusion to help even this out when taking the flats.

 

It also looks like you may have had moisture or haze on the lens when taking the lights..

[clint.ivy]

Thx TxStars,

the circular defect is consistent across multiple imaging sessions and targets (low humidity, high humidity, dew strap, no dew strap, lens hood, no lens hood, etc) so unless the dew is condensing in exactly the same way, every time, despite disparate conditions...

 

The sample darks were just a test to see if heat glow created the same circular pattern when the exposure time was extreme (5 min, vs my typical 45s), but FWIW lens and viewfinder were capped and while I wasn’t in my dark room; I was in a room that was dark. In any case, potential light leaks aren’t really relevant to the issue at hand.

[TxStars]

With most lenses/scopes you will get some circular vignetting from illumination fall off.

Flats are used to correct for this

Depending on the camera you can get hot pixels and amp glow.

Darks are used to correct for this

 

There are other issues that can show up and most can be corrected in one way or another..

[Tapio]

I suspect you have some light leak in lights (the 3D effect hints that way).

Maybe more shielding ?

And try what comes after calibration.

[clint.ivy]

@TxStars, yes I know what calibration frames are used for. Let me pose the question a slightly different way: I suspect there is a physical defect in my imaging train that calibration frames don’t compensate for ... how should I go about determining if that is the case?

 

@Tapio, interesting thought ... these lights were taken without the lens hood so it’s possible that the embossed look is coming from light reflecting through the lens (the moon was pretty bright ~50% full) the only kicker there is that the images that look embossed are the calibrated master ... before and after BGN, PCC and ABE. My calibration frames were taken as normal, in a completely dark room with lens cap (for bias and darks) and viewfinder cap (always) in place.

[TxStars]

I was just double checking to make sure...

 

Yes without the lens hood and with the moon up is likely the issue.

It is best to do the flats and lights with the lens hood on..