CCD Camera issues - help needed

[bendiddley]

Hi all. I have a starlight xpress SXVF-M25C colour CCD camera which I bought second hand recently and I took it out for a spin for the first time last night. Unfortunately I experienced some issues which I'm hoping some folk might be able to help with...

The first problem I experienced was using the bahtinov mask. This was fine with my old canon dslr, but with the CCD I couldn't get a nice sharp rendering of the spikes, it was all blurry. See below photo which is the best I could get having focussed in and focussed out several times. Now, when I used my DSLR I was able to use live view and zoom into the star and then magnify (I am using APT by the way), but with my CCD camera in live view you can't zoom in, all I could do was magnify. Because I am magnifying something very small I am wondering if this is why the bahtinov spikes are so blurry, what do others think? And is there a way round this so I can get nice sharp spikes with my CCD?

The second problem is with elongating stars. Having managed to guess the focussing a bit I did manage to take a few images in the end and had some files to play with. However for some reason the stars are elongated towards the edges of the frame. I have used a coma corrector which I thought would have prevented this from happening but for some reason it doesn't seem to have worked. It maybe good to mention that  I had to position the CCD camera slightly out of the focussing tube in order to be able to use it as there wasn't enough back focus when it was all the way in, I am wondering if this might be causing the problem?

Hope you guys can help.

Cheers, Ben

[tomato]

HI Ben,

Did you try the mask back on the scope when you thought you had the camera pretty much at focus? 

If you have the scope centred on a bright star, you should be able to see the diffraction spikes without magnifying the star. There seems to be a lot of colour fringes on the magnified image, is your software debayering the image correctly before displaying, I’m not familiar with APT, but I assume you can configure it to use this colour CCD camera?

If the nose piece of the camera is someways out of the focuser tube to achieve focus, this could introduce tilt which could create elongated stars. Best to add some screw in extension pieces to the camera body to get full engagement, or better still get the correct length threaded adapter so the camera is attached to the scope by a fully screwed together assembly.

Keep persevering with the CCD, you should find it way more sensitive than your DSLR for AP.

Steve

[Ronclarke]

Debayering for this CCD is RGBG which is different from the DSLR (RGGB) hope this helps..

[bendiddley]

11 hours ago, Ronclarke said:

Debayering for this CCD is RGBG which is different from the DSLR (RGGB) hope this helps..

I've just checked and it was on RGGB. I can't see an option for RGBG though, the only ones available are GRBG, GBRG, BGGR and CMYG.

[bendiddley]

12 hours ago, tomato said:

HI Ben,

Did you try the mask back on the scope when you thought you had the camera pretty much at focus?

If you have the scope centred on a bright star, you should be able to see the diffraction spikes without magnifying the star. There seems to be a lot of colour fringes on the magnified image, is your software debayering the image correctly before displaying, I’m not familiar with APT, but I assume you can configure it to use this colour CCD camera?

If the nose piece of the camera is someways out of the focuser tube to achieve focus, this could introduce tilt which could create elongated stars. Best to add some screw in extension pieces to the camera body to get full engagement, or better still get the correct length threaded adapter so the camera is attached to the scope by a fully screwed together assembly.

Keep persevering with the CCD, you should find it way more sensitive than your DSLR for AP.

Steve

Thanks. Yep, tried the mask when I thought it was on focus and it didn't make any difference. Ron clarke has just pointed out the debayering setting might not be correct so I will look at that next time to see if it makes a difference.

Do you know what the correct length is for the threaded adapter or the size of the screw in extension piece?

[bendiddley]

I've just noticed that there are some screws on the camera which enable the nose piece to be tilted and some tilt has been applied, see below photo which show's this. Wondering if this might have something to do with the elongating stars? Not sure what the tilting function is for?

[Tomatobro]

As far as I know tilt able focus supports are to stop multiple reflections between the sensor cover and filters when imaging at certain wavelengths.

For normal use the tubes should be square and parallel .

[ollypenrice]

Firstly is your chip distance correct for the coma corrector? This is not connected with focus. The CC will have a specified chip distance, the distance from the back of the CC to the chip. This will be available from the CC manufacturer. You then need to look up the front of camera to chip distance on the SX website and subtract this from the CC chip distance, then add spacers to make up that distance. Only when this is done can you attempt to find focus with the B mask. (It is possible that you might then find you have insufficient movement in the focuser to find focus but cross that bridge if/when you get to it. The chip distance cannot be used to adjust for this, it must be respected.)

In my understanding the tilt adjustment is there to allow for the correction of tilt on the chip. None of my cameras has such an adjustment and I've never needed it so I would probably start by setting it square again, perhaps using feeler gauges if that's possible.

Olly

[alacant]

20 hours ago, bendiddley said:

This was fine with my old canon dslr,

Hi

The flange of the ccd will need to be over 2cm further out than the flange of your dslr when the latter is at focus. The ccd's sensor is 18mm from its flange. The dslr's, 44mm.

HTH and good luck.

Edited September 3, 2019 by alacant

[Ronclarke]

I understand it to be 55 mm, I used an adjustable spacer from Baader to get this distance.

[bendiddley]

44 minutes ago, Ronclarke said:

I understand it to be 55 mm, I used an adjustable spacer from Baader to get this distance.

Great thanks. Do you know what sized spacer you used?

[Ronclarke]

It was a variable spacer, 42 - 30 I think, check on Google.. Defo a Baader spacer.

https://www.amazon.co.uk/Planetarium-VariLock-Lockable-Extension-Connector/dp/B005D7TAC4?SubscriptionId=AKIAILSHYYTFIVPWUY6Q&tag=duc08-21&linkCode=xm2&camp=2025&creative=165953&creativeASIN=B005D7TAC4

Edited September 4, 2019 by Ronclarke
added text

[bendiddley]

On 04/09/2019 at 16:51, Ronclarke said:

It was a variable spacer, 42 - 30 I think, check on Google.. Defo a Baader spacer.

https://www.amazon.co.uk/Planetarium-VariLock-Lockable-Extension-Connector/dp/B005D7TAC4?SubscriptionId=AKIAILSHYYTFIVPWUY6Q&tag=duc08-21&linkCode=xm2&camp=2025&creative=165953&creativeASIN=B005D7TAC4

Yikes. Didn't really want to spend that much. Hoping for a cheaper solution if there is one.

[michael8554]

I would ditch the CC and tilt corrector etc and find focus with the camera on it own. 

If you can get focus then introduce the CC at the correct spacing without the tilt corrector, and see if the focuser has enough range. 

Michael 

Edited September 5, 2019 by michael8554

[newbie alert]

When you said elongated stars at the edges are the fanning out from the center or ok in the middle but wrong on the edges..

This should explain things better

Does sound like a spacing issue

 

[michael8554]

There is strong practical evidence from previous posters that these charts are wrongly captioned, they should be Too Far, then Too Close. 

Might be due to different designs of CC, so just watch the trend as you add or remove spacers. 

Michael