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markarian

Ha showing secondary image

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Need some help with this please. Just started narrowband imaging with Baader filters and I'm seeing some very strange effects that I can't explain. Anyone seen this before and how do I get rid of it? Basically it looks like the primary, secondary and spider are showing up on the images.

Here is a 200s sub in Ha (2x2) and an associated flat. I get seomthing similar for OIII too - but not SII.
Telescope is 250mm f4.8 newt with an MPCC, Atik 460EX and Baader filters. 

Thanks
Mark

2.jpg

1.jpg

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Is that an unstacked sub? If not, the problem seems to be with the flat. Maybe you took it against a bright light and you've picked up reflections? Just a thought.

Louise

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It is an unstacked sub but you can see the same pattern in both the sub and the flat. All subs in Ha and OIII have this. It was reasonably dark last night (or as good as it gets here on the edge of Edinburgh) so don't think it was against light.

Both images seriously stretched to show the issue.

Thanks

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13 minutes ago, markarian said:

It is an unstacked sub but you can see the same pattern in both the sub and the flat. All subs in Ha and OIII have this. It was reasonably dark last night (or as good as it gets here on the edge of Edinburgh) so don't think it was against light.

Both images seriously stretched to show the issue.

Thanks

Hmm... Sometimes, where I am, I'll get momentary headlights which are very annoying. However, that wouldn't explain your flat. Were all your subs the same? Has it happened before? If not has something changed?

Louise

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Flats and subs all show the same in Ha and OIII. SII doesn't show these that I can see.
The only thing that's changed is that this is the first real full narrowband image I've done - so a big change really :)

Wondering if it's out of focus reflections from the MPCC to the Baader filters?

Mark

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The weird thing is the flats - they show an almost perfect image of the primary, secondary and spider.

Final images show it too.

 

IC343 NBx2.jpg

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One other possibility is the acrylic over the  EL panel I use for flats being reflective?

Suppose I need to try and change everything to see what works.

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2 minutes ago, markarian said:

One other possibility is the acrylic over the  EL panel I use for flats being reflective?

Suppose I need to try and change everything to see what works.

That wouldn't explain the uncalibrated subs showing the problem. Did you buy the filters new? If you don't get the problem without the filters, it must be something to do with them, I think. I'm sure someone on here will know what's causing the problem.

Louise

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This is so weird!

It looks like image that can be seen when you look at eyepiece with naked eye.

Strange thing is that it is "focused" at that particular distance.

Reflection of flat panel can actually be the issue here coupled with CC. It cannot be problem on its own - many people have reflective light boxes, but in your case something is creating focus at that exact spot.

I have two suggestions that you can try:

1. Put a piece of thin paper over your flat panel to dim it, but point is to make it non reflective rather than dim the light

2. Move your flat panel a bit further (as you probably can't get it closer) to aperture opening. Just to see what sort of effect this will give you.

Try combination of the two above as well.

Interestingly SII does not produce that? I find this very strange, because if it were due to flat panel - it would show up in SII being so close to Ha, don't think that there would be such dramatic difference in reflectivity or anything.

Another thing that you can try is to figure out what reflective surface is causing this, you can use this distance calculator for that:

http://www.wilmslowastro.com/software/formulae.htm#REFLECT

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Thanks vlaiv - will give those a go.

16 minutes ago, Thalestris24 said:

Did you buy the filters new? 

Filters were new Louise.

Mark

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Totally wild guess.. are the filters facing the correct way? (Fw facing the right way?)

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4 minutes ago, markarian said:

They are screw in filters so no other way to go in that I can see.

Depending on the FW model, the FW itself can be fitted a different way around. Dunno if that applies here?

Louise

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From the calculator above I suspect I may have 2 possible sources: Coma Corrector and EL lightbox.

Can try and eliminate both to prove the point.

M

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No need to eliminate flat box - it is certainly not due to that

You Ha sub of horse head is showing this artifact and flat box was not nowhere near when you took that. It's reflection and one fairly distant because the size of it.

Now that I'm thinking about it - nothing special in the artifact except the size - and that means reflective surface is very far away. What sort of CC are you using? I suspect that it's one with large back focus and not standard 55mm one, although it depends on speed of scope - faster scopes throw larger defocus image for same reflection distance.

All reflection artifacts look like that - but they are smaller and secondary is usually not seen due to star in center, but spikes are always present - defocus image represents aperture - so spikes and secondary will both be seen in it. You can get the same image by defocusing enough bright star.

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6 minutes ago, vlaiv said:

No need to eliminate flat box - it is certainly not due to that

You Ha sub of horse head is showing this artifact and flat box was not nowhere near when you took that. It's reflection and one fairly distant because the size of it.

Now that I'm thinking about it - nothing special in the artifact except the size - and that means reflective surface is very far away. What sort of CC are you using? I suspect that it's one with large back focus and not standard 55mm one, although it depends on speed of scope - faster scopes throw larger defocus image for same reflection distance.

All reflection artifacts look like that - but they are smaller and secondary is usually not seen due to star in center, but spikes are always present - defocus image represents aperture - so spikes and secondary will both be seen in it. You can get the same image by defocusing enough bright star.

Baader mpcc is standard but I recall the gso cc is not.

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16 minutes ago, Thalestris24 said:

Baader mpcc is standard but I recall the gso cc is not.

Ah, yes, now I see OP mention MPCC as coma corrector of choice. Maybe F/4.8 can account for such a large defocus reflection image then, and the fact that 460 has relatively small sensor size at 16mm?

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Are the spacers that you use to get the correct back-focus distance before or after the FW?

I had a really odd internal reflection when I got my new Chroma NB filters and the only way I could deal with it was by putting the spacers between the FW and camera, rather than the recommended FW closest to the sensor.

Might be worth a try...

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Yes Baader MPCC. Spacers may not be exactly correct - I will need to check that out.

Thanks everyone for your help. I have a few things to try out now.

Mark

Edited by markarian

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First check is that MPCC distance is wrong. I've adjusted it  (not totally correct yet but I'll sort that) and at first glance the pattern has gone! May have been chance that the position I chose just happened to produce a very nicely focused image of the spider and secondary!

I'll do more tests to be sure.

Mark

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Well I've finally decided that it is caused by the MPCC and the Baader filters together - even at the correct spacing there's still some effect.

The really weird thing is that my images look far better without the MPCC. I only used it because it was necessary with my 8 inch Newtonian, but this 10 inch seems not to need it at all - and it certainly gets rid of the the horrible reflections / artifacts.

Thanks again for everyone's help and suggestions.

Mark

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Glad you have got to the bottom of it, and thanks for coming back to us.  It was certainly a mystery, but what are you going to do now?  Not use the MPCC?

Carole 

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37 minutes ago, markarian said:

Well I've finally decided that it is caused by the MPCC and the Baader filters together - even at the correct spacing there's still some effect.

The really weird thing is that my images look far better without the MPCC. I only used it because it was necessary with my 8 inch Newtonian, but this 10 inch seems not to need it at all - and it certainly gets rid of the the horrible reflections / artifacts.

Thanks again for everyone's help and suggestions.

Mark

Ah yes, I see your sensor is quite small so it looks like you can get away without a flattener. APS-c size, - not so much!
Glad you got it going ok.

Louise

 

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