Narrow Band Reflections

[Tandum]

Is it possible to get reflections in the imaging train when using NB filters? I got a couple of new NB filters and seem to be seeing reflections in the image from the reducer.

This S2 image is a stack of 20minute subs shot at x2 binning. If it is a reflection it's also in this single Ha shot. I didn't know they where there until I did a ddp stretch on the images.

Concidering how difficult it is to get any data from an S2 filter, it seems odd to have reflections like this. When it stops raining I'll try again and rotate the camera to see if it moves.

[Moon]

Robin,
Try a bright star and see what happens. Also compare to some flats with those filters in place. It will be interesting to find out the cause.
James

[Tandum]

Flats are out, I took a 10minute x2 binned Flat with the light box running full bottle and the result looks like a bias file. I guess there's no S2 coming out of the LEDs in the light box :( Unfortunately I didn't keep it either, in hind sight a good stretch of that file might have shown something worth seeing.

[JohnH]

Robin,

What is the effective F no with the FR in line? The NB filters are reflection filters - they do not work with steep light cones, that could be the issue, it does not look like reflections to me.

[Tandum]

Those are from the TAK @ F6 John. Rain is forecast here for the next week so I can't do anything till it clears.

[jase]

F/6 is fine. Cone isn't steep enough and even then you'd only miss the emission line bandpass ever so slightly. You sure its not present in any of your broadband data sets? SII or for that matter most narrowband filters require flats to be taken at considerably longer exposure times to reach the target ADU. I suspect you'll find that the issue will be corrected by flats. At least if its optical train configuration related anyway. Check there's no shiny edges of accessories in the optical path.

[JohnG]

Is your Metal Back Distance correct, it should be 83.7mm from the Reducer/Corrector to the CCD chip.

Cheers

[Tandum]

It's as close as it's going to get to 83mm with this qhy wheel and I can't find any f6 broadband shots using this camera, all the f8 shots look fine though. I'll need to rotate the camera and compare when the weather clears. Saturday might be ok.

I'll need to look at sky flats for this filter I guess, the light box doesn't work.

[marki]

Robin I had this very problem when I tried to use T threads in front of the FW. Chenged everything to 48mm on the front side and it went away. I was also using a refractor at F6.

Mark

[Tandum]

No t-threads to the Tak Mark. Only need them on the newt with the mpcc.

I find it odd that there are reflections when bugger all light gets through the filter in the first place.

[marki]

Robin, I dont believe mine were reflections. The center of the image was clearly more illuminated then the edges. I do get some internal reflection on the broadband filters when there is a really bright star around but never from the narrowband filters. If you have opened up the aperture in front of the FW I have no idea what could be causing your troubles. Good luck sorting it though :thumbsup:.

Mark

[Tandum]

They are forecasting the rain to stop for the week-end so I'll have another look then. At least I've learnt not to blindly launch off into an extended imaging session without checking what's coming out as it's coming out :lol:

[Tandum]

Got some sky flats tonight and it looks like they will clear these images up nicely. It appears the camera is no longer square which is giving these offset results. I've had it apart several times to clean it after frosting issues, so one of the new qhy tilt adjust nose pieces will fix that issue. It should look like this image. NB newbie alert :D

All clear in SE QLD for a change ... Just for tonight I'm sure :(