Jump to content

Banner.jpg.b89429c566825f6ab32bcafbada449c9.jpg

Ha showing secondary image

markarian

By markarian
in Imaging - Discussion

Recommended Posts

markarian

markarian

Posted
Posted

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

Link to comment
Share on other sites
  • Replies 28
  • Created
  • Last Reply
markarian

markarian

Posted
  • Author
Posted

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

Link to comment
Share on other sites
Thalestris24

Thalestris24

Posted
Posted
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

Link to comment
Share on other sites
markarian

markarian

Posted
  • Author
Posted

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

Link to comment
Share on other sites
Thalestris24

Thalestris24

Posted
Posted
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

Link to comment
Share on other sites
vlaiv

vlaiv

Posted
Posted

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

Link to comment
Share on other sites
vlaiv

vlaiv

Posted
Posted

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.

Link to comment
Share on other sites
Thalestris24

Thalestris24

Posted
Posted
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.

Link to comment
Share on other sites
vlaiv

vlaiv

Posted
Posted
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?

Link to comment
Share on other sites
Pompey Monkey

Pompey Monkey

Posted
Posted

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...

Link to comment
Share on other sites
markarian

markarian

Posted
  • Author
Posted

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

Link to comment
Share on other sites
  • 2 weeks later...
markarian

markarian

Posted
  • Author
Posted

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

Link to comment
Share on other sites
Thalestris24

Thalestris24

Posted
Posted
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

 

Link to comment
Share on other sites

Archived

This topic is now archived and is closed to further replies.

Go to topic listing

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.