Vertical line on really bright star

[mcbras]

Gents,

 

yesterday I was testing a diy dewheater band and I took the chance to image something. I pointed the setup to the horse head nebula, after doing M33 with success, and every single frame is kind of like this, in LRGB. The ones in here are in Lum 120s exposure. Should I reduce the exposure time in order not to saturate the adjacent pixels? Why it only happens in the short image direction?

The setup is a fast reflector with coma corrector along with an Apogee Astra 8300 CCD.

Thanks for any possible inputs.

NGC_2024_LIGHT_Lum_120s_BIN1_-15C_002_20201113_050723_642_PA29439_E_F41715.FIT NGC_2024_LIGHT_Lum_120s_BIN1_-15C_003_20201113_035152_301_PA29439_E_F41825.FIT

[vlaiv]

This is very very strange.

There are 3 possible causes that I can think of and I'm not sure it is anyone of those (I'll explain why):

1. No anti blooming on CCD

2. OAG prism diffraction

3. Some sort of wire over the telescope - like phone or power line that goes over your imaging location.

For first one - it's obvious - this happens on CCD sensors that don't have anti blooming. Your camera has it according to specs. Also, when this sort of electron spill happens - it looks different. It is also in vertical direction because of CCD structure - columns of pixels are used to read out pixels so there is "channel" that connects each vertical column of pixels.

Second one looks differently - it happens only on bright stars on one side of the image - where OAG prism sits and is usually perpendicular to closest edge (because we tend to orient OAG prism to be parallel to closest edge) - but in principle it is perpendicular to OAG prism.

Effect is very small if it happens at all and solution is simple - moving OAG prism a bit more away from sensor. There is additional telltale "signal" that it might happen - flats that show OAG prism shadow.

Third is straight forward - there is power line over your imaging site and it acts as secondary spider stalk - creates diffraction spike.

Problem with that is that you have diffraction spikes in your image - and you know what they look like - cable diffraction spike will look like that - straight beam of light that grows fainter as one moves away from bright star.

It will not show this:

This is what happens when you have:

moving bright star in frame and you shake telescope.

Only other explanation that I can offer, and I'm certain it can't be true as it would imply something very silly, but it might lead you on a right track to figure out what is happening would be:

You have a flip mirror that you flip during exposure for some strange reason and that causes shakes in the telescope. Action of flipping the mirror will move reflection of bright star across the image - in a straight line. There will be some shake and that will result in above wiggle line.

There will be some "bounce" action that will create line on other side of the star.

That is really my best guess - otherwise, I'm not sure I can come up with sensible answer.

[mcbras]

vlaiv,

thank you for your thoroughly explanation. 

In the place I'm at, no power lines close by, not even if I image close to the horizon, so I can put that aside, along with the anti-blooming, unless something is wrong with the camera.

I did a primary cleaning followed by a collimation a few days ago, so I'll check if the mirror is well secured. The funny thing, though, is that I took 4h of frames with same filters in M33 before this sequence and nothing like that showed up on camera. Only difference is that my guiding was really terrible for this target in Dec (I still fight with my NEQ6-Pro to get good Dec, even though I can get 0.5 RMS most nights). 

I will for sure image this target and play with different exposures to see what else is to become the next challenge (there's always one in this hobby) to be solved.

Once again, thanks for the valuable info.

Best

Miguel

[mcbras]

Hi, as a follow up on this issue, may others take advantage of it, seems like my mirror cell wasn't tight enough so I was getting those lines in there. I still have to do a final testing, but weather in this northern europe countries really present themselves a challenge for astrophotographers, geez! 😬

[kirkster501]

I have this issue too on very bright stars.  I need to tinker with the OAG prism. Remember you'll need to refocus the guidecam if you move the distance of the prism.