Stars don't look right. Help

[souls33k3r]

Hi All,

I do sincerely apologize if this is not the correct place to be asking this question. Feel free to move it to the correct location.

I finally completed my first image of NGC6888 ... Below are the image details

NEQ6 Pro mount, EdgeHD 800 scope, Polar aligned using Polemaster, Guided 21 x 200s all NB filters. The image was acquired on two different days.

Ignore the processing, this is only rough to get something out to show. 

The stars don't look right, i just wanted to find out what could be causing this? 

Also the focusing do not look right as well. I mean the nebulosity isn't sharp. I focused using Bahtinov mask (only once with L filter and then checked once again when i moved to Ha filter). 

I'm sure it's not backfocus as well because on a Celestron EdgeHD 800 after introducing reducer the back focus is 105mm. What i have is a T-adapter (78.35mm), ZWO 8 Pos FW (20mm) and ASI1600MM (6.5mm) which is 104.85mm altogether. What could be causing this as well. 

Sorry if i've asked too many questions here, i'm just very concerned and would like to avoid any issues if i'm being silly.

Thank you once again for looking & reading

[ronin]

What filters are you using? And more relevantly have you taken into account the change caused by the filter thickness. Generally filters seem to be 3mm and if I recall they alter the path by 1/3 of the thickness so that makes about 1mm.

Just wondering if you have taken all the distances assuming air, since that is how they are specified, and not accounted for what could be a 1mm shift caused by the filter itself.

Also I have recently seen people comment that even 0.1mm can make a difference in focus/sharpness.

[Earl]

focus will shift when you change filter, so getting focus with the filter you are going to use to image with is worth it, and that alone could solve the issue.

[souls33k3r]

13 minutes ago, ronin said:

What filters are you using? And more relevantly have you taken into account the change caused by the filter thickness. Generally filters seem to be 3mm and if I recall they alter the path by 1/3 of the thickness so that makes about 1mm.

Just wondering if you have taken all the distances assuming air, since that is how they are specified, and not accounted for what could be a 1mm shift caused by the filter itself.

Also I have recently seen people comment that even 0.1mm can make a difference in focus/sharpness.

Hi Ronin, cheers for replying to this.

I'm using Astronomik 31mm 6nm Ha, SII & OIII filters.

I am a very much a noob in AP, like i said this is my first attempt to do any sort of imaging. I've battled my way through learning of the software (image acquisition, guiding, platesolving) but never thought there was any other factor that would cause this so pardon my ignorance on the subject if i didn't quite understand what you meant by any of it and how would it impact.

[souls33k3r]

5 minutes ago, Earl said:

focus will shift when you change filter, so getting focus with the filter you are going to use to image with is worth it, and that alone could solve the issue.

Cheers Earl. I am definitely going to be investing in a auto focuser. Found someone having the same scope as mine and he just literally was able to sort his focuser issues out so that's a result

I was also told that the sharpness could lack due to not having enough data, could that be an issue?

[steppenwolf]

Even apparently parfocal filters will not necessarily all come to focus at the same plane so it is vital that you focus for each filter as you change to it (see caveat later). In addition, you need to re-focus in keeping with the change in temperature over an imaging session. I always re-focus every hour at least.

The caveat? If you have an autofocus system with reliable re-positioning, you can focus with your L filter using a Bahtinov mask, note the focus position from the software and then refocus manually with the mask and each filter in turn noting the new focus positions. The difference in the focus positions of the other filters in relation to the position of the L filter will give you an 'offset' which can be applied automatically by your software. This means that in future, you can autofocus with your L filter, swap filter under software control and the autofocuser will automatically apply the offset for that filter and off you go again! Because of temperature change, you should carry this calibration check in as short a time as you can, ideally after the ambient temperature has settled down

[souls33k3r]

14 minutes ago, steppenwolf said:

Even apparently parfocal filters will not necessarily all come to focus at the same plane so it is vital that you focus for each filter as you change to it (see caveat later). In addition, you need to re-focus in keeping with the change in temperature over an imaging session. I always re-focus every hour at least.

The caveat? If you have an autofocus system with reliable re-positioning, you can focus with your L filter using a Bahtinov mask, note the focus position from the software and then refocus manually with the mask and each filter in turn noting the new focus positions. The difference in the focus positions of the other filters in relation to the position of the L filter will give you an 'offset' which can be applied automatically by your software. This means that in future, you can autofocus with your L filter, swap filter under software control and the autofocuser will automatically apply the offset for that filter and off you go again! Because of temperature change, you should carry this calibration check in as short a time as you can, ideally after the ambient temperature has settled down

That is super nicely put Steve. I am obviously saving this information because this has already helped me understand how the auto focuser will want to work.

So you think my image is not in focus right?

Any idea about the eggy stars?

[steppenwolf]

12 minutes ago, souls33k3r said:

So you think my image is not in focus right?

Any idea about the eggy stars?

To be honest, I don't think your focus is that far out but focus is critical in capturing the fine detail in an object like the Crescent Nebula so lust a little soft will become apparent. Again, the eggy stars are not actually that bad and there isn't a really stand-out pattern to them which makes me think that the corrector is probably working fine with this spacing although if I do detect a pattern, it is a circular one. There is a chart somewhere (can't find it right now) that may tell you if this indicates too short or too long a spacing of your sensor from the rear lens element of the corrector. Hopefully another member will remember where the chart is downloaded from!

[swag72]

This one? 

Spacing - Too close

Spacing - Too much

[souls33k3r]

So with regards to spacing here's what i have my image train looks like at the moment

 

OTA > Celestron 0.7x Reducer > Celestron T-Adapter for EdgeHD 8 (part # 93644) to make up the spacing > ZWO 8-Position FW > ZWO1600MM

this in numbers goes to this

From the Celestron 0.7x reducer, i have only 105mm so

Celestron T-Adapter for EdgeHD 8 (part # 93644) is 78.35mm > ZWO 8-Position FW is 20mm > ZWO1600MM is 6.5mm ... total is 104.85

 

Still think it's the spacing?

[steppenwolf]

1 hour ago, swag72 said:

This one? 

Thanks, Sara, I knew someone would know its whereabouts!

44 minutes ago, souls33k3r said:

Still think it's the spacing?

I'm far from convinced that it is the spacing but to my eye there is possibly a very slight rotation of the stars - it certainly isn't obvious coma (inconsistent) or poor tracking (star shape angles alter across the field). Furthermore, although I suggested the charts above may provide an answer, I am not personally convinced that they are conclusive but I do know that some people believe they are!

[souls33k3r]

the plot thickens ... i'm using Celestron EdgeHD version scope because it's supposed to have flat field corner to corner so it can't be the scope now can it?

[Racey]

1 hour ago, souls33k3r said:

Celestron T-Adapter for EdgeHD 8 (part # 93644) is 78.35mm > ZWO 8-Position FW is 20mm > ZWO1600MM is 6.5mm ... total is 104.85

 

Still think it's the spacing?

Might be...

I'd point out that you've made no allowance for the inclusion of the actual filters and the extra 1mm they require. I think your calculations, if you are using filters is 1mm out.

I say this as when I was enquiring about pretty much the same situation, but with an ED80 as an option to consider pressing into service instead of our 8" EdgeHD I got some great advice from FLO.

I quote from part of the email exchange:

"The ED80 with the reducer fitted requires 55mm from the reducer to the camera sensor (56mm when using filters).

The Edge requires 105mm from the telescope to the camera sensor when using the reducer (106mm when using filters)..."

As you're at 104.85 I think you are 1.15mm out. A Delrin ring may sort that...

Whether that resolves everything; time will tell. But in a game of fine tolerances it's quite a bit to be out if you are using filters as I think I read...

Simon

[iapa]

2 hours ago, swag72 said:

This one? 

Spacing - Too close

Spacing - Too much

LOL

just what I was going to post after seeing it in the past couple of days

[iapa]

28 minutes ago, Racey said:

Might be...

I'd point out that you've made no allowance for the inclusion of the actual filters and the extra 1mm they require. I think your calculations, if you are using filters is 1mm out.

I say this as when I was enquiring about pretty much the same situation, but with an ED80 as an option to consider pressing into service instead of our 8" EdgeHD I got some great advice from FLO.

I quote from part of the email exchange:

"The ED80 with the reducer fitted requires 55mm from the reducer to the camera sensor (56mm when using filters).

The Edge requires 105mm from the telescope to the camera sensor when using the reducer (106mm when using filters)..."

As you're at 104.85 I think you are 1.15mm out. A Delrin ring may sort that...

Whether that resolves everything; time will tell. But in a game of fine tolerances it's quite a bit to be out if you are using filters as I think I read...

Simon

here we go

next requirement is a good quality reliable micrometer

[Racey]

31 minutes ago, iapa said:

here we go

next requirement is a good quality reliable micrometer

I've missed the point you're making here. OP asked about his spacing. I've tried to share data I got from FLO...

Care to explain the "here we go" comment ?

[iapa]

27 minutes ago, Racey said:

I've missed the point you're making here. OP asked about his spacing. I've tried to share data I got from FLO...

Care to explain the "here we go" comment ?

It was tongue in cheek.

This is a money pit exciting mentally stretching hobby where we progress through a variety of stages, with each demanding greater accuracy than the last.

e.g. get a mount and OTA, then add cameras, guiding, find the mount is lacking, and replace that, change OTAs, add field flatteners etc, filters and things which need greater accuracy of measurement than previously to the point where you need something better than the wooden ruler used at school

Once you get on to 0.5mm measurements we are looking at additional purchases - oh no not again!! LOL.

I'm in that place just now  fixed and variable extensions, deleon spacers, but not necessarily convinced that my 'rule' is accurate enough 

[Racey]

18 minutes ago, iapa said:

It was tongue in cheek.

This is a money pit exciting mentally stretching hobby where we progress through a variety of stages, with each demanding greater accuracy than the last.

e.g. get a mount and OTA, then add cameras, guiding, find the mount is lacking, and replace that, change OTAs, add field flatteners etc, filters and things which need greater accuracy of measurement than previously to the point where you need something better than the wooden ruler used at school

Once you get on to 0.5mm measurements we are looking at additional purchases - oh no not again!! LOL.

I'm in that place just now  fixed and variable extensions, deleon spacers, but not necessarily convinced that my 'rule' is accurate enough 

Well pardon me for trying to help. 

The question was "is my spacing correct" and I tried to help. 

Your approach might stop people trying to help...

"Tongue in cheek" of course... ?

[knobby]

Are you taking the 105 mm from the thread or face of the reducer ... ?

And how are you guiding, could be flexure ?

Doesn't seem to have any pattern, so spacing looks fine I guess.

 

[DaveS]

Astronomik filters are 1mm thick, therefore the spacing needs to be adjusted by 0.3mm. I don't know whether this is enough to make a noticeable difference.

[souls33k3r]

16 hours ago, Racey said:

Might be...

I'd point out that you've made no allowance for the inclusion of the actual filters and the extra 1mm they require. I think your calculations, if you are using filters is 1mm out.

I say this as when I was enquiring about pretty much the same situation, but with an ED80 as an option to consider pressing into service instead of our 8" EdgeHD I got some great advice from FLO.

I quote from part of the email exchange:

"The ED80 with the reducer fitted requires 55mm from the reducer to the camera sensor (56mm when using filters).

The Edge requires 105mm from the telescope to the camera sensor when using the reducer (106mm when using filters)..."

As you're at 104.85 I think you are 1.15mm out. A Delrin ring may sort that...

Whether that resolves everything; time will tell. But in a game of fine tolerances it's quite a bit to be out if you are using filters as I think I read...

Simon

Cheers for the Racey, i never realized i need to factor in the spacing when using filters. If such is the case then i'm 2mm out. I will speak to someone about the delrins

14 hours ago, knobby said:

Are you taking the 105 mm from the thread or face of the reducer ... ?

And how are you guiding, could be flexure ?

Doesn't seem to have any pattern, so spacing looks fine I guess.

 

Knobby, mate, i'd hoped your chime in  The 105mm is exactly from the end of the reducer to the camera chip just like the diagram.

I've been told that i might have tilt issues and now i'm off to some goose chase. Everything looks tight enough but then only thing that i have my super doubts on is this wonky Celestron T-adapter for EgdeHD scope. Have you ever had any tilt issues using this contraption? Other than that, i've also been told that this could be collimation so will check that. Camera orientation will need to be checked by rotating it 180 degrees. I have uploaded my acquired files to a friend who will run them through CCD Inspector for me. So lets see what he says.

I'm guiding using Qhy MiniGuidescope along with QHY5LII-Mono .... if guiding was an issue then all my stars would be weird but they're not

[souls33k3r]

2 minutes ago, DaveS said:

Astronomik filters are 1mm thick, therefore the spacing needs to be adjusted by 0.3mm. I don't know whether this is enough to make a noticeable difference.

Cheers Dave. Are they really 1mm thick? I thought they were 2mm ... They're the 31mm 6nm version filters ... and pardon my ignorance but why 0.3mm adjustment is required and not the full 1mm?

[Racey]

6 minutes ago, souls33k3r said:

Cheers for the Racey, i never realized i need to factor in the spacing when using filters. If such is the case then i'm 2mm out. I will speak to someone about the delrins

Do research further... As @DaveS says, different filters may be requiring different allowances. But it at least is a lead to follow up on...

FLO were very helpful and stock Delrins... 

[DaveS]

Here we are:

http://www.astronomik.com/en/fotografische-emissionslinienfilter/schmalbandfilter-h-alpha-ccd-6nm.html?___store=en

Look down to the Technical Data section, where it says 1mm. I think they are all 1mm.

The 0.3mm comes from the T/3 "rule" assuming a refractive index of 1.5 for the glass substrate.

[MarsG76]

I must have just been lucky. I slapped on the FR and no major distortion issues.