First image, blurry circle why??? setup correct?

[sokubus]

Hi, I am struggling with my first imaging, and I am not 100% sure I have put everything correctly together to achieve the right back focus.

My setup is as follows: Zwo Am5 mount, WO 81 GT, WO 6III Flattener, ZWO 36mm EFW, ZWO EAF, ZWO 2600MM and Antila LRGB and Narrowband Filters.

My fist images are looking like follows: (see upload) this is with a Luminance filter

 

Where is the blurry ring coming from? I have done calibration frames but you can still see it in photoshop.

 

To achieve 55mm backfocus I have used the  

 

I am unsure if this is correct because after the flattner the WO 81 GT is below f 5.4 ??? I have used 7.1mm as disctance on the 6III Flattener.... any ideas?

 

Thanks a lot

soku

[iapa]

I believe that with the flattened installed the back focus (measure from flattened to camera sensor) is 62.1mm when used with the GT81.

Your current back focus from M41-M48 adapter to sensor is 72mm (0+16.5+36+2+17.5).

Filters are nominally add 2-3mm to back focus, so, your total is required from flattener is around 65.1mm. 

You’ll likely not get the OAG and filter wheel together.

So, I think you need 

camera =17.5mm

Camera->filter wheel adapter = 2mm

filter wheel = 36mm

filters = 3mm

 

Total = 17.5+2+36+3 = 58.5

adjust WO 6 III to 3.6 mm to give 62.1

My opinion - worth 1% of what you paid for it.

Edited October 27, 2022 by iapa
Corrected calculation

[Elp]

I would use 0.3mm to 1mm to add onto the total backspacing required due to filters.

[pipnina]

This looks like a more dramatic issue to me than backspacing... Are there any bright light sources near the telescope when it is taking images?

[iapa]

11 minutes ago, pipnina said:

This looks like a more dramatic issue to me than backspacing... Are there any bright light sources near the telescope when it is taking images?

I don’t disagree;  my take was to sort the simpler issue (it is just a case of measure it things and doing the some).

Anyway, I’ve put in tu’pennurth and will the more experienced take it.

Edited October 27, 2022 by iapa

[sokubus]

thanks, not really. street lights are far away and i used a red headlamp during the session. 

Will sort the backspacing issue now, thanks a lot for that help. Really appreciated!!!

[Elp]

What do your flats look like?

[scotty38]

The Flat 6AIII needs to be adjusted just as WO says it does which is 7.1mm or 7.2mm, I forget which but it's something like that. The backfocus distance of 55mm or whatever it needs to be after slight adjustment for filters etc is measured from the back of the flattener irrespective of what it has been set to (the 7.1/7.2).

Well..... That's what mine is set to and regardless of anything else my stars aren't too bad across the entire frame of my 294 🙂

 

As said above there seems to be more of an issue with your flats causing the  problem.

Edited October 27, 2022 by scotty38

[iapa]

50 minutes ago, Elp said:

I would use 0.3mm to 1mm to add onto the total backspacing required due to filters.

I proposed 3mm from data in FLO website

• 2mm+/-0.05mm thickness for 1.25", 2" mounted, 31mm and 36mm unmounted
• 3mm+/-0.05mm thickness for 50mm and 50x50mm unmounted

[scotty38]

3 minutes ago, iapa said:

I proposed 3mm from data in FLO website

• 2mm+/-0.05mm thickness for 1.25", 2" mounted, 31mm and 36mm unmounted
• 3mm+/-0.05mm thickness for 50mm and 50x50mm unmounted

That's the actual thickness, not the effect it has on the optical path though.

[sokubus]

thanks to all, so just to summarize if I understand this correctly?

I will set the Flattener to 7.1mm (this will always be 7.1 regardless of the rest of the imaging train as I will measure from after the flattener???)

from there measured I need a 55mm back focus incl Filters or does it need to be 62.1mm according to this diagram??

 

I have the camera with 17.5mm

Camera to Filter wheel adapter 2mm

Filter wheel 36mm

Filter 3mm

so that;s in total 58.5mm

so thats it... ???? no spacer etc. and i am already 3.5mm over due to filter?

I read that I could screw the camera onto the filterwheel and to get rid of the 2mm spacer. Shall I do this as well?

Thanks for all your help

[newbie alert]

Did you add flats as calibration frames to the image as it looks like over correcting flats.. 

I'd check the backspacing spec, not sure why you want a oag with a 80 mm scope

Add a third of your filter thickness to the spec'd spacing distance 

[alacant]

6 hours ago, sokubus said:

Where is the blurry ring coming from?

Maybe it would be easier for us to troubleshoot if we had -links to- a light frame and one each of whatever calibration frames you used.

Cheers 

[sokubus]

thanks, here a link to the files: https://drive.google.com/drive/folders/1nFkVcIZzbFh58U6johaOC2_ydKlmDtIV?usp=sharing

 

would be great if someone can clarify how much back focus, I need now to add from the 6II Flattener and if this is always set to 7.1mm and I exclude this when measuring the back focus. Got a bit confused now how best to approach this.

 

[bosun21]

The way I’m reading this is that you need 55mm from your flattener. If you add this to your 7.1mm of your flattener you have the 62.1mm as per the WO recommendation 

[Stefan73]

9 hours ago, sokubus said:

thanks, here a link to the files: https://drive.google.com/drive/folders/1nFkVcIZzbFh58U6johaOC2_ydKlmDtIV?usp=sharing

That effect on the lights looks very like what I had on a very dewy night when the various mirrors of my 130PDS ended up pretty sodden.  Could it be dew?

[scotty38]

12 hours ago, sokubus said:

thanks to all, so just to summarize if I understand this correctly?

I will set the Flattener to 7.1mm (this will always be 7.1 regardless of the rest of the imaging train as I will measure from after the flattener???)

from there measured I need a 55mm back focus incl Filters or does it need to be 62.1mm according to this diagram??

 

I have the camera with 17.5mm

Camera to Filter wheel adapter 2mm

Filter wheel 36mm

Filter 3mm

so that;s in total 58.5mm

so thats it... ???? no spacer etc. and i am already 3.5mm over due to filter?

I read that I could screw the camera onto the filterwheel and to get rid of the 2mm spacer. Shall I do this as well?

Thanks for all your help

The filters add a third of their thickness to the backspace consumed so only about 0.7mm in reality so that takes you to about 56mm so I'd just try that.

[alacant]

+1 for all the above.

Here are the frames. 

The flat frame is of no use. It is saturated and will not correct the light frames. Aim for around 20000. 

Redo the flat frames correctly and try again. It should get you closer. This will make it much easier to diagnose.

Next, remove the filter and take another flat frame. Otherwise, it's probably local to the session.

Cheers and HTH

--- --- ---

flat:

 

dark:

 

bias:

 

light:

Edited October 28, 2022 by alacant

[barbulo]

Agree with @Stefan73. Could it be dew?

[sokubus]

thanks all, I will change the backfocus and redo the flats and than replicate all in a new session. Will keep you up to date about the progess.. Thanks for all the help!