My imaging nightmare - time to throw in the towel?

[dmahon]

You attach the T- ring and camera directly to the coma corrector because the T-ring is 10mm and the distance from the front of the camera to the chip is 45mm. Or thereabouts.

If you use a CCD instead, you need to know the distance from the front of the camera to the CCD. Then subtract that from 55. Also subtract your filterwheel. What is left is how much you need to add in spacers (plus 1, to account for the optical effect of glass filters).

Measurements of chip distance and filterwheel size are usually available on the manufacturers websites.

[lukebl]

You attach the T- ring and camera directly to the coma corrector because the T-ring is 10mm and the distance from the front of the camera to the chip is 45mm. Or thereabouts.

If you use a CCD instead, you need to know the distance from the front of the camera to the CCD. Then subtract that from 55. Also subtract your filterwheel. What is left is how much you need to add in spacers (plus 1, to account for the optical effect of glass filters).

Measurements of chip distance and filterwheel size are usually available on the manufacturers websites.

Hi there. I'm aware of all that stuff, but just wondered where the 55mm is measured from. As I said, my CCD sensor is exactly 55mm from the lip of the Coma Corrector as I'm told it should be. This is achieved by various spacers as explained in the attached image. However, this produces oval stars which are reduced if I remove the 7mm spacer (creating a distance of just 48mm), so I must be measuring the distance from the wrong part of the coma corrector. Any ideas, or have I got the wrong sort of coma corrector (I have a Skywatcher one). Maybe I should have, say, a 5mm or 9mm spacer rather than a 7mm one.

[sulaco]

Hi,

I would be very happy with that film, it's great but the nature of my work makes me a bit of a perfectionist so I can see why you may want to get it tack sharp.

[lukebl]

Can anyone help me on this one?

I have my coma corrector at precisely 55mm from the sensor as specified, yet it produces blobby oval-shaped stars. this is the setup:

I know that collimation is fine, as images taken without the coma corrector show really nice crisp, round small stars at the centre of the frame.

[adamsp123]

Luke, are you sure the sensor is 7mm from the front of the camera -- oops read the drawing wrong - so ignore.

However if smaller spacing works better then that is the way to go.

[lukebl]

Luke, are you sure the sensor is 7mm from the front of the camera? seems a bit on the low side it is not 7mm to the shutter? Just a thought as you said the smaller spacer improves the image.

No, the sensor isn't 7mm from the front of the camera (it's 17.5mm), but the TOTAL distance from the lip of the coma corrector to the sensor (with all those adaptors and filter wheel) adds up to 55mm. So it should be correct.

[adamsp123]

No, the sensor isn't 7mm from the front of the camera (it's 17.5mm), but the TOTAL distance from the lip of the coma corrector to the sensor (with all those adaptors and filter wheel) adds up to 55mm. So it should be correct.

Luke, I had edited my reply but you must have seen it with the old text - sorry I stupidly mis-read your drawing.

[lukebl]

Definitely getting to towel-throwing time now.

Using spacers, I've tried various alternative spacings of the coma corrector to the sensor from 47mm to 57mm. All of them produce roughly the same oval stars. Compare the images below with one taken without the coma corrector. As you can see, the image without the coma corrector is much better. You may ask 'why bother with it then?' Well I really want crisp stars to the edge, like I used to get with my DSLR. In theory, with the smaller CCD sensor, the coma should be less of a problem.

So what the heck's going wrong? Should I try a Baader Coma Corrector? That would be a bit of an expensive trial if I was proved wrong.

Anyway, here's a larger image, taken without the coma corrector. Incidentally, it's M29, a really pretty open cluster. Looks a bit like a miniature Pleiades.

[Bizibilder]

Maybe the coma corrector itself is not set "square" within its mounting?

[lukebl]

Maybe the coma corrector itself is not set "square" within its mounting?

I've thought of that. It certainly looks square, and when you put it out of focus, the stars are nice and round with good airy discs:

[pwalsh61]

Do you know of anybody in your area/local AS with the same coma corrector? One way of proving whether it was/wasn't the CC would be to try someone else's. If you still get mis-shapen stars it's something to do with spacing. If you don't, however, then there's a problem with your corrector (in theory anyway)

[lukebl]

Do you know of anybody in your area/local AS with the same coma corrector? One way of proving whether it was/wasn't the CC would be to try someone else's. If you still get mis-shapen stars it's something to do with spacing. If you don't, however, then there's a problem with your corrector (in theory anyway)

I'm certain there's nothing wrong with the Coma Corrrector as it works fine with my DSLR. What I don't understand is that, despite experimenting with a variety of spacings either side of the correct one, the stars are misshapen whatever distance I have the corrector.

What I'd rather do is compare it with another type of Coma Corrector, so is there anyone in the Norfolk area prepared to lend me a Baader one to compare with?

[Deneb]

I got a baader one if you want to borrow it Luke..?

[lukebl]

I got a baader one if you want to borrow it Luke..?

That's very kind of you, Nadeem. I am going away for a week or so, but will get back to you on my return. I notice that the Baader corrector has a T-thread so it will fix directly to the filter wheel, whereas my Skywatcher one has a different thread which is why I need an adaptor to fit it. That might be causing the corrector to be not exactly square, and could be the source of the problem, so trying with the Baader may well be a solution. But surely someone here is using a Skywatcher corrector and a filter wheel successfuly?

[Deneb]

im sure i've got your address, just send the correct postcode to me via pm, i'll do the rest.. Seriously im in no rush, check it & see if it resolves your issues...

Nadeem.

[TheThing]

That's what SGL is all about. What a cool offer Nadeem! Salute!

I've been reading this thread with interest. Hope that sorts you out.

TheThing.

Sent from my iPhone using Tapatalk

[Starlight 1]

I have offed to come over and help you out and bring my sw 250pds + sw Coma Corrrector and any more parts if you like luke.

[Starlight 1]

This maybe a daft thought, is it possible to screw the end of a bellow onto your ccd, effectively this will add something like 30mm to the focus length but at the same time will let you move the focuser out of the main tube reducing spaces in between the

coma corrector and the ccd, but will take a bit of playing with to try and get right and the bad news is it will affect the focal length I think. I may be totally wrong on all of it, but anything is worth a try.

[lukebl]

Thanks for all the input and kind offers, gents. Starlight, if I understand your suggestion correctly, I think that would result in the sensor to corrector distance being too great. Anyway, having ruled out most other options, I'm inclined to think that it can only be the alignment of the coma corrector to the filter wheel, which can't be helped by all the adaptors and spacers I'm using. If I try the Baader one which will fit the filter wheel without the need of adaptors, plus a single wide spacer on the camera side, then I should know either way if that is the cause of the problem. Will keep y'all posted!

Incidentally, why don't Skywatcher use a 'standard' T-thread like everyone else!

[Starlight 1]

Try and look at it this way round,

If you put the ccd in just the focuser and trying to get focus but the focuser will not go in the tube any more to get focus, by adding the end from a 2 x bellow lens to the ccd will let you move ever thing out more by about 30mm right,

So I am thinking by moving ever thing out more with a bellow lens on the end of the ccd it may see the coma corrector at a different alignment.

[lukebl]

Try and look at it this way round,

If you put the ccd in just the focuser and trying to get focus but the focuser will not go in the tube any more to get focus, by adding the end from a 2 x bellow lens to the ccd will let you move ever thing out more by about 30mm right,

So I am thinking by moving ever thing out more with a bellow lens on the end of the ccd it may see the coma corrector at a different alignment.

Sorry, but I really can't visualise what you mean. Any chance of a sketch? I'm thinking that the issue might be caused by the scenario shown in an exaggerated form in this image, whereby the corrector is not precisely square due to a fault with the adaptors. I may be wrong, though. It looks right to me on a visual inspection, but I guess the positions are critical, so even a slight, almost imperceptible, misalignment would cause a problem.

[Starlight 1]

When you got it set up and look down the tube can you see the focuser on the inside of the tube (I may have work this one out)

[Starlight 1]

OK Lets go right back to when everything was right and lets try and create a couple of experiments from this point.

Re-set up with just the digital camera in the focuser and check how far the focuser is inside the tube at focus point.

Do the same again and check this point with the coma corrector in place, check this focuser adjustment inside the tube again

Let me know if everything is as it should be as in nice star shapes and everything across the field in view.

[lukebl]

OK Lets go right back to when everything was right and lets try and create a couple of experiments from this point.

Re-set up with just the digital camera in the focuser and check how far the focuser is inside the tube at focus point.

Do the same again and check this point with the coma corrector in place, check this focuser adjustment inside the tube again

Let me know if everything is as it should be as in nice star shapes and everything across the field in view.

You've lost me completely. Not sure what that would tell me. The Coma corrector changes the focus point anyway, so all that would do would tell me the difference in the focus position, which is irrelevant. It will have no effect on the requirement to have a 55mm spacing between the corrector and the sensor, which we know doesn't produce nice star shapes. I know there's nothing wrong with the coma corrector itself, as it works fine with my DSLR.

Anyway, I'm away for a few days and won't be able to continue with this till my return!

[Starlight 1]

I think I work it out it the chip on the ccd board is not flat.

So I am just looking to see that evey thing is right up to the ccd.