do this flat look right ?

[ribuck]

Hi all,

I'm just new to imaging and i've tried taking my first flat. I've calibrated my camera so that the flat background ADU value is approx 40% of the fully saturated value.

I used an EL display panel to take the flat for an evenly illuminated image, however the flat doesn't appear to be evenly illuminated as it seems to be focused on the left side of the image but the right side does appear to be properly illuminated.

The flat was 1.5 seconds as i have a 8300 based camera with a mechanical shutter. i've alsi uncluded the screen stretch info from maxim.

Rich.

[steppenwolf]

Very strange, it looks like just the top RH corner of a typical flat.

[ribuck]

I really dont understand it, seeing that i bought an EL display which is placed directly over the end of my OTA.

I'm wondering if it could be more frosting issues on my rubbish qhy9 camera, as the normal light image is the same.

[MikeD]

I guess it all depends what your light frames look like.

Do your light frames also have a dark right hand edge?

here is a light frame and a flat of mine from last night.

They have been opened in maxim and "View/ equalize screen stretch" applied (the screen stretch graph is from the flat).

My lights and flats have a darker left hand edge ( probably a light leak from the filter wheel) regardles of weather i am imaging with my 10" newt or 80Ed.

Mike.

[MikeD]

If they are the same then the flats should correct the imbalance, give it a go.

Mike.

[ribuck]

Hi mike,

I see what you mean, but mine seems a lot more extreme. the odd thing is that my background adu value seems about right @ approx 24,000 and my bias value is around the 970 level.

My flat image time was 1.3 seconds i'm not sure if it's worth trying doing some new flats tonight. So if i'm reading your right, the darks bits on the right are light leak and the image has become totally saturated on the right side of the image ?

Rich.

[MikeD]

I may not be light leak it could also be that you collimation is out.

In my case I am guessing there "may" be a light leak on the brighter side of the image(Right).

I your image if it was a light leak it would be from the left bottom.

I find flats Darks and Bias frames all work best if taken on the same night as the lights.

Mike.

[ribuck]

Hi Mike,

give me a jiffy and i'll double check the collomation. It should be fine as i just did it the other day with my shiny new hotech, but better safe than sorry.

will be back in 5 mins.

Rich.

[MikeD]

I would calibrate your lights with dark,bias and the flats you have and see if you get the desired effect.

Mike.

[FraserClarke]

Looks fine to me.

Just looks like the chip is not on the optical axis of the telescope (the brightest bit of the image). This could be collimation, or it could be that the chip is not physically centred in the camera body. The flat seems to match your light frame well, so that is fine. It's optimal to have the brightness distribution equal over the chip (i.e. brightest bit in the middle) -- but it's not absolutely necessary...

[ribuck]

Hi guys,

I went out an checked the collimation in my scope and it looks fine, teadwarf, you mentioned the optical axis. Do camera's need to be placed in the telescope at a certain orientation ?

[Euan]

Hi Rich

I noticed from you other posts that you were looking to upgrade your focuser? I have a 200p and I tried to put a QHY8 plus filter wheel on it and I was getting serious sagging going on.

I ended up buying a Moonlite, it was the only thing that could take the weight without taking the whole thing off axis, which is what it looks like from your picture.

The best way to analyse it is to download the trial of CCD Inspector and take a few star field shots. It will tell you the image tilt and if it's in the direction of towards the ground compared to the orientation of the scope, it's focuser sag.

Link here: Welcome to CCDWare - CCDInspector

[ribuck]

Hi euan,

I did do some more testing and i did notice some slack on the focuser so i'm toying with the idea of sending back the sywatcher and upgrading to the moonlite.

Everyone swears by moonlight, so i think it looks like more money to being spent. I'll speak to FLO when they get back from kelling.

Thanks for the ccd insepctor, that will be fantastic to see what's going on.

Rich.

[ribuck]

Hi Guys,

I ran ccd inspector over an image i took and these are the results. The tilt looks ok so the chip should be sitting square and the FWHM wasn't bad as it was a rough focus and seeing wasn't great.

My collomaiton seems fairly good, just not sure how to interpret the data to confirm my collimation is ok and rule it out as being an issues with the flats.

[ollypenrice]

The flat doesn't look at all right to me. I sometimes get similar effects with an EL panel and to avoid it I do two things;

- only ever shoot flats in the dark to avoid light leaks

- place a folded white T shirt over the dewsheild. I have the dewsheild fully extended. I don't know why the white fabric is necessary between scope and panel but it is, or I get a bright side and gradient, albeit less extreme than yours. I know plenty of other folks don't get this effect.

I doubt if this alone would be your answer but I pass it on anyway.

Olly

[FraserClarke]

You are measuring the collimation with a hotech; and therefore setting the collimation for that device. There is no guarantee that is the same for the camera (if the chip is not exactly central in the camera body, for example). How physically big is the chip? That will give you an idea of the amount of decentering. Say the chip is 10mm across, your images would imply it's ~2--3mm decentred relative to the hotech. You could try to adjust the collimation so that it is optomised for the camera if you want...

However, if the flats divide out the background structure in your light images, I really wouldn't worry about it. Flats are there to take out these uneven illumination patterns; so if they do their job it's all OK.

Another test would be to take some twilight flat frames (early evening, a bit after sunset when the sky is still bright) and see what they look like. If they look the same as your EL panel flats, all is good...

[BlueAstra]

Perhaps try rotating the panel to see if the shadow follows the rotation of the panel? I don't think it will since the vignetting is an angular effect, but might be worth trying. More likely to be collimation or focusser sag. Something is clippiing the light cones on the right.

[Beyond_Vision]

It's possibly due to the mechanical shutter or light leakage. I had similar issues with my Atik 383L. I had to resort to turning down the voltage to the EL panel as much as I could and covering the scope with 2 layers of T-shirt and setting the exposure to 5 secs. I figured that the longer exposure would swamp out the effects of the slow shutter

Regards

Kevin

[ribuck]

Hi Kevin,

I turned my el display down to 3v. I tried a 5 sec exposure with several sheets of paper between the scope and the EL panel and i see exactly the same results.

[Beyond_Vision]

Just been doing some experiments and it looks like I may be getting light leakage in through the SX filter wheel connector locations.

Regards

Kevin