Strange effect on edges

[04Stefan07]

I have taken a few subs using different filters and I am getting a strange effect on the edges.

It can't be the filters as it's doing it on all of them (unless thats possible?). Maybe something to do with the 1600MM-Pro? I used to use a 178MM-Cool without an OAG and never had this issue.

Running an ES ED102, 36mm Baader LRGB, Field Flat 0.8x, OAG, 1600MM-Pro

[symmetal]

That's vignetting which is quite normal on optical systems. Using the smaller sensor size of the 178 it was outside the area covered by the camera so wasn't noticeable. The larger sensor on the 1600 shows it up.

To correct for it you need to take some flats and associated flat darks and use them in calibration as mentioned here.

Alan

Edited August 11, 2021 by symmetal

[sterec]

I would agree with the above.

[04Stefan07]

6 hours ago, symmetal said:

That's vignetting which is quite normal on optical systems. Using the smaller sensor size of the 178 it was outside the area covered by the camera so wasn't noticeable. The larger sensor on the 1600 shows it up.

To correct for it you need to take some flats and associated flat darks and use them in calibration as mentioned here.

Alan

Ah, so thats what the flat darks fix in the images! Thank you

[symmetal]

2 hours ago, 04Stefan07 said:

Ah, so thats what the flat darks fix in the images! Thank you

No problem. 🙂

Though it's the flats that correct the vignetting in the image. The flat darks reduce the extra noise introduced by using the flats during calibration. 😊

Alan

[michael8554]

The 1600MM sensor is over twice as large as the 178MM, so it's edges are much closer to the vignetted regions of the ED102.

Michael

[04Stefan07]

I am running a series of subs and I where I get the above effect the stars are elongated. Could it be the field flatner I am using with my setup that isn't compatible?

I am using the Altair Astro 0.8x Reducer/Flat.

[michael8554]

The corner stars in the images you posted look fine to me

Michael

[04Stefan07]

3 hours ago, michael8554 said:

The corner stars in the images you posted look fine to me

Michael

 

[michael8554]

Which of those two images was that from, I still don't see it in the originals ?

Square, but not elongated.

Michael

[symmetal]

Getting the field flattener back-focus spacing correct is more critical the larger the sensor. For the small sensor 178 getting the spacing within a few mm would be sufficient but for the 1600 it needs to be set more accurately. Incorrect spacing will produce increasing star distortions the further their distance away from the centre of the image, like you've shown.

As Michael says your original images don't seem to show such distortion, though their small size may be masking it.

Alan

[michael8554]

The latest image that actually shows the elongation:

Depending on which corner it's from, the camera could be too close, or too far away, from the FF.

Or it could be guide errors if the whole frame is the same.

Who knows.

Michael

[04Stefan07]

23 hours ago, michael8554 said:

The corner stars in the images you posted look fine to me

Michael

Yep been running various test the last two nights to try and correct the problem. Tried rotating the camera, refining the spacing but still the same issues. Was thinking tilt was an issue so I swapped a push fit adapter with a thread, the star shaped changed from one corner to another. It could be a combination of tilt and spacing but I am also thinking maybe the flat. Maybe 0.8x reduction is too much? I should just go for a flat or a 0.9x?

I am now running tests without the OAG just to see. I was getting elongated stars in the OAG and it was a pain for PHD to track.

I saw diagrams about it being too far and too close but adjusted it didn't change anything.

Possibly a 1600MM and the field flat not liking each other?

[04Stefan07]

OK so I managed to improve it quite a bit. It's still there in some corners but no way near as bad. I removed the OAG entirely and put back the guidescope.

So far so good.

[symmetal]

I don't think the flattener would be incompatible especially with the 1600 sensor size, which although larger than the 178 is not really a large sensor.

It looks like the OAG setup is introducing some tilt which isn't helping. For a refractor a separate guide scope is normally not an issue.

For flattener spacing adjustment you need to refocus each time you've altered the spacing and preferably at the centre of the image each time. Autofocus routines will often focus at an edge of the frame when there's tilt, which makes the opposite edges suddenly appear much worse so it may be worth using a bahtinov mask on a bright centre star each time for consistancy.

Start at flattener spacing like 5mm closer than the specified distance and all the corners should have elongated stars pointing towards the centre maybe some worse than others. Move the spacing out by 1mm, refocus and check star shapes again and whether one or more corners look better. Keep moving out by 1mm or so until you're say 5mm beyond the specified distance and all the corners are distorted again, probably in a different direction to when you started. You should find all four corners have given good stars at some point in your tests, but perhaps unfortunately not all corners at the same flattener spacing distance, due to tilt.

Hopefully you can find a distance where the corners aren't too bad overall and can live with that. If not you're in the realms of tilt adjustment which can be another can of worms in itself.

Alan

 

[04Stefan07]

On 17/08/2021 at 12:35, symmetal said:

I don't think the flattener would be incompatible especially with the 1600 sensor size, which although larger than the 178 is not really a large sensor.

It looks like the OAG setup is introducing some tilt which isn't helping. For a refractor a separate guide scope is normally not an issue.

For flattener spacing adjustment you need to refocus each time you've altered the spacing and preferably at the centre of the image each time. Autofocus routines will often focus at an edge of the frame when there's tilt, which makes the opposite edges suddenly appear much worse so it may be worth using a bahtinov mask on a bright centre star each time for consistancy.

Start at flattener spacing like 5mm closer than the specified distance and all the corners should have elongated stars pointing towards the centre maybe some worse than others. Move the spacing out by 1mm, refocus and check star shapes again and whether one or more corners look better. Keep moving out by 1mm or so until you're say 5mm beyond the specified distance and all the corners are distorted again, probably in a different direction to when you started. You should find all four corners have given good stars at some point in your tests, but perhaps unfortunately not all corners at the same flattener spacing distance, due to tilt.

Hopefully you can find a distance where the corners aren't too bad overall and can live with that. If not you're in the realms of tilt adjustment which can be another can of worms in itself.

Alan

 

Thanks for the advice! I have been doing more testing without the OAG. Managed to get this. Bottom Right, Top Left and Top Right (apologies, the clouds came in but you can see the stars in relation to their corners, bottom left I couldn't get but the stars aim inward).

So I have been doing some researching and forum searching and tried various configurations. This confuiguration is better than some others.

- Flat
- M48 - 11mm
- M48 - 16.5mm
- T2-T2 - 2mm
- EFW - 20mm
- 1600MM - 6.5MM

= 56mm

So I have met the 55mm and 56mm with different configs and still no good! According to some charts I found online, when the stars are this shape it means that camera is too close to the field flat, but how can that be?

I am just at a standstill I got no idea what to try next.
 

 

[symmetal]

I was thinking you had an adjustable flattener in my earlier reply suggesting 1mm increments, but if you haven't it's a bit more hard work adding/removing extenders.

Your stars do indicate the flattener spacing is still too close as they are pointing towards the centre. I've found than when the spacing is too far, while some flatteners do produce elongated stars appearing to rotate around the centre as is often suggested, others can produce more coma like aberrations instead.

One point to note is that having filters, or any other glass, in the path between flattener and sensor requires you to increase the specified spacing distance by around 1/3 the thickness of all the glass in the way, to get the actual distance required. The specified flattener spacing distance is quoted for just air between the flattener and sensor. With say 1.5mm filters and 2mm camera protect glass, the distance you want to aim for is the specified distance + (1.5 + 2) / 3 or just over 1mm beyond the specified distance.

It's good to know whan too far a distance looks like, so if you can, add say a 5mm spacer and check you get stars other than pointing towards the centre and how bad they look. This gives an indication of where you should be heading regarding optimum distance. It's quite normal to find that to get the optimum stars you need to have the actual distance a little different from what you've calculated by sometimes a mm or so, so treat the calculated distance as a starting point rather than an absolute value.

Alan 

[04Stefan07]

4 hours ago, symmetal said:

I was thinking you had an adjustable flattener in my earlier reply suggesting 1mm increments, but if you haven't it's a bit more hard work adding/removing extenders.

Your stars do indicate the flattener spacing is still too close as they are pointing towards the centre. I've found than when the spacing is too far, while some flatteners do produce elongated stars appearing to rotate around the centre as is often suggested, others can produce more coma like aberrations instead.

One point to note is that having filters, or any other glass, in the path between flattener and sensor requires you to increase the specified spacing distance by around 1/3 the thickness of all the glass in the way, to get the actual distance required. The specified flattener spacing distance is quoted for just air between the flattener and sensor. With say 1.5mm filters and 2mm camera protect glass, the distance you want to aim for is the specified distance + (1.5 + 2) / 3 or just over 1mm beyond the specified distance.

It's good to know whan too far a distance looks like, so if you can, add say a 5mm spacer and check you get stars other than pointing towards the centre and how bad they look. This gives an indication of where you should be heading regarding optimum distance. It's quite normal to find that to get the optimum stars you need to have the actual distance a little different from what you've calculated by sometimes a mm or so, so treat the calculated distance as a starting point rather than an absolute value.

Alan 

Yep I’ll give that a go. So how come the recommend 55mm (Flat) and 56mm (ZWO) is incorrect if I need to add more space?

I’m guessing there’s really no way of telling how much I need other than adding more spacers and seeing what happens. 

[symmetal]

1 hour ago, 04Stefan07 said:

Yep I’ll give that a go. So how come the recommend 55mm (Flat) and 56mm (ZWO) is incorrect if I need to add more space?

I’m guessing there’s really no way of telling how much I need other than adding more spacers and seeing what happens. 

I'm not sure what the 56mm ZWO spacing is referring to, as the only critical distance is the one specified by the flattener data sheet, presumably the 55mm you mentioned. As I mentioned above, any extra glass in the way requires the spacing to be increased from that specified, in which case I would have thought your 56mm setting would have been closer to what's required.

All you can do is add more spacing, initially in larger amounts, until you see a definite change in the star shapes and go from there to find the optimum distance. It's not a quick process, especially if you don't have an adjustable flattener. At least you don't appear to have any significant tilt which is a bonus. 🙂

Alan

[04Stefan07]

4 hours ago, symmetal said:

I'm not sure what the 56mm ZWO spacing is referring to, as the only critical distance is the one specified by the flattener data sheet, presumably the 55mm you mentioned. As I mentioned above, any extra glass in the way requires the spacing to be increased from that specified, in which case I would have thought your 56mm setting would have been closer to what's required.

All you can do is add more spacing, initially in larger amounts, until you see a definite change in the star shapes and go from there to find the optimum distance. It's not a quick process, especially if you don't have an adjustable flattener. At least you don't appear to have any significant tilt which is a bonus. 🙂

Alan

https://www.altairastro.com/lightwave-08x-reducer-290-p.asp

Thats the flat I use. Specifies it needs 55mm.

It's so bloody confusing. According to the chart its too close so I will just keep adding space.

Let's make a bet, I reckon it will end up in the neighbours garden haha.

[symmetal]

The spec for your flattener does say 'approx 55mm' which is a more realistic figure. Those specified with close tolerances are probably those made to match a particular model of scope. The flattener distance is affected by the focal length so a flattener advertised as suitable for a range of scopes can only give an approximate spacing distance.

I've generally found that even allowing for the extra glass I invariably have to add extra spacing around 1 or 2 mm to get the best stars. I don't remember having had to reduce the distance from the calculated value. Perhaps the 1/3 glass thickness is too low a figure in reality.

I think you'll manage it in the end without having to impinge on your neighbour's property. 😁

Alan

Edited August 20, 2021 by symmetal

[04Stefan07]

16 hours ago, symmetal said:

The spec for your flattener does say 'approx 55mm' which is a more realistic figure. Those specified with close tolerances are probably those made to match a particular model of scope. The flattener distance is affected by the focal length so a flattener advertised as suitable for a range of scopes can only give an approximate spacing distance.

I've generally found that even allowing for the extra glass I invariably have to add extra spacing around 1 or 2 mm to get the best stars. I don't remember having had to reduce the distance from the calculated value. Perhaps the 1/3 glass thickness is too low a figure in reality.

I think you'll manage it in the end without having to impinge on your neighbour's property. 😁

Alan

It might be clear tonight so I will give it a test and report back with my findings. I will keep adding spacing. Anything else I should try?

[symmetal]

I keep the camera in the same orientation each time for testing after making spacing adjustments to try and minimize any tilt errors from confusing the results. Pointing towards the zenith for your test images also reduces any possible sideways focuser slop from causing an issue.

 Your corners look similar so hopefully tilt isn't a problem for you. When you get good results pointing straight up you can take test images pointing at low Dec, both sides of the Meridian and fingers crossed there is no change.

Good luck.

Alan

[04Stefan07]

Ok so did some testing and I give up. I’ve spent 4 nights on this issue and it’s been an issue since I changed camera and filter wheel. I used to use a 178mm-cool now using the 1600mm-pro.

I tried adding more space, the stars direction indicated to add more, still no change then I couldn’t focus at all!

So I went the opposite and removed space and now the stars have the different effect that I need to go further in. I took all spacers out so it was the camera, EFW and flat and still no good,

i am now adding space between the camera and EFW and still no good, 

The scope needs 2 of the 2” spaces which I think is just stupid, no idea why the focus point is so far back.

So my conclusion is there is an issue with either the camera, field flat or scope itself.

[04Stefan07]

Stand by, I may have found something but will need to upload photos.