Jump to content

Stargazers Lounge Uses Cookies

Like most websites, SGL uses cookies in order to deliver a secure, personalised service, to provide social media functions and to analyse our traffic. Continued use of SGL indicates your acceptance of our cookie policy.

sgl_imaging_challenge_celestial_motion.thumb.jpg.a9e9349c45f96ed7928eb32f1baf76ed.jpg

Recommended Posts

What's making my diffraction pikes look strange? Not the spikes themselves... but the fan shapes in between.

fin.jpg

Share this post


Link to post
Share on other sites

My bet? Internal reflections.

Are the edges of your vanes black? Tube? Any shiny surfaces in there at all?

Edited by MikeWilson

Share this post


Link to post
Share on other sites

Ahhh.... With prime focus, my draw tube sticks into the OTA quite a way... and its bare aluminum.

Black paint?

Share this post


Link to post
Share on other sites
Ahhh.... With prime focus, my draw tube sticks into the OTA quite a way... and its bare aluminum.

Black paint?

My focuser's draw tube is bare aluminium too and I don't get fan shapes like that. Still, it almost certainly contributes to the lack of contrast and internal reflections.

You could try blackboard paint or self-adhesive flocking material. I'm waiting for someone in the UK to get hold of some Protostar before I take off my secondary and flock the whole area neatly.

Do you have any stray light sources shining into your OTA?

Share this post


Link to post
Share on other sites

I have a neighbour who leaves her bathroom light on all night, and that was around 40 degrees off the OTA's axis. Could that be it?

The way they fan out from the star however, makes me think it's the star's light that is the source of the problem... it must be hitting something.. so I think I'll paint the draw tube and see what that does.

Edited by pook

Share this post


Link to post
Share on other sites

The bright/dark fan shapes are caused by the primary mirror clips sometimes and are usually seen around bright stars. They are 120deg apart should match the mirror clips, the clips cause the dark space.

John.

Share this post


Link to post
Share on other sites

If that's the case, is there anything I can do?

Share this post


Link to post
Share on other sites

I am not sure but processing every bright star with PS layers would be one way but painstaking. Professional scopes show the same thing so your in good company, I have this too - see attachment below.

You should make sure all your surfaces in the light path are blackened (bare aluminium draw tube is a worry), stop stray light getting up back of scope, having a shade on scope end with streetlights - basically cut out all possible stray light.

Halos caused by filters make it worse and could be the main cause of the glare, my image are with Astronomik filters, Baader filters are better. What filters did you use?

John.

post-15219-133877542096_thumb.jpg

Edited by johnh

Share this post


Link to post
Share on other sites

I didn't use any filters at all.

I'll paint the draw tube first, as that seems a likely candidate.

All mirrors have retaining clips though, so how come others, even those with Skywatcher scopes the same, or similar to mine don't get it?

That seems weird. I think the bare ally draw tube is the first port of call.

Share this post


Link to post
Share on other sites

I have a 200p and I have noticed that if I have knocked one of the struts holding the secondary mirror so that it is no longer "horizontal" and at a slight angle it can either add diffraction spikes or cause them normal diffraction spikes to fan out.

might be worth checking to make sure that they are not at an angle and if so correcting them

Share this post


Link to post
Share on other sites
I have a 150P. Can you take a photo of your tube?

Will do that when I get home tonight.

Share this post


Link to post
Share on other sites
Will do that when I get home tonight.

Thanks. And Lunalight's suggestion is good too - your vanes might not be edge-on.

Share this post


Link to post
Share on other sites

An interesting read, but are you worrying about nothing? Personally I think they add to a image and they are only going to be noticeable on the brightest of stars in your shot.

Share this post


Link to post
Share on other sites
An interesting read, but are you worrying about nothing? Personally I think they add to a image and they are only going to be noticeable on the brightest of stars in your shot.

With diffraction spikes in general, I agree. If I ever upgrade to a Mak or something, I'll probably even tape threads across the aperture to put them back in!

Those fan shapes however... they just shout out "optical error" to me.

Anyway... here's the spider.

_DSC2672.jpg

Looks OK to me. They all seem to be edge on.

Hard to shoot the mirror with such a small scope, but....

_DSC2673.jpg

All looks pretty normal to me. I've certainly not modified anything.

Share this post


Link to post
Share on other sites

Agree with John -- you're seeing the diffraction from the mirror clips.

If you really want to get rid of this, you could try making a circular mask for the primary mirror. Size it so it just covers the mirror clips and leaves you with a truly circular aperture. You'll lose a bit of aperture (only a few mm by the looks of it), but I bet the image will clean up a lot...

Share this post


Link to post
Share on other sites

Eek! with such a small scope I don't lie the thought of losing any aperture! I'll see if I can fabricate something and give it a go.

Share this post


Link to post
Share on other sites

Yeah, always painful to cover up good mirror surface :(

If you make a cardboard mask though, you can take it on/off and see if it makes a difference. I don't think you need much -- roughly the pink area in the attached picture... You could just tape it to the top of the mirror clips maybe?

post-18754-133877544541_thumb.jpg

Share this post


Link to post
Share on other sites

The question is though.. why aren't people with the same scope getting these weird spikes?

I hope MikeWilson doesn't mind, but in an earlier thread he posted up a stacked raw TIFF file, so I've processed it with a fairly big stretch to bring out more of the diffraction patterns, and it's below.

M45-11-11-20107x6minstacked.jpg

That was with a SW 150P

There's got to be more to this than clips... as I'm fairly confident that Mike's scope is identical to mine. If you look carefully at Mike's there's clearly the draw tube interfering a little, but I could live with that. My damned "iron cross" pattern just looks stupid though. How can two identical scopes perform so differently?

fin.jpg

????

Arrgh! So annoying.

Edited by pook

Share this post


Link to post
Share on other sites

Not sure if your image is actually in focus as diff spikes should be sharper?

John.

Share this post


Link to post
Share on other sites

Maybe not perfect focus, but that is also an enlarged section from a larger image.

Share this post


Link to post
Share on other sites

Hmmm, why does the one going upper left to lower right seem to be splitting?

Olly

Share this post


Link to post
Share on other sites

I have no idea. I have no idea why any of this is happening tbh. Why Does Mike's SW150P show perfect diffraction spikes and my identical scope doesn't?

Share this post


Link to post
Share on other sites
Hmmm, why does the one going upper left to lower right seem to be splitting?

Olly

Spider vanes not exactly parallel with each other?

If you look carefully in the other image, I think you can see a similar effect. It's not quite as obvious as in your images -- but the same shape is there I think, i.e. in the left-most bright star. You also have to be a bit careful comparing with Pleiades stars, as they have a 'natural halo' around them which will mask diffraction effects.

Share this post


Link to post
Share on other sites

Hi Pook,

I don't mind you comparing or editing my images at all.

I do get internal reflections. You'll notice some quite prominent ones especially up and right of Atlas and Pleione. Those are caused I think by reflections between the Astronomik CLS-CCD and the MPCC. I've also managed to improve contrast since this image was taken by taking a black permanent marker to the objective-facing side of the secondary mirror, the edges of the vanes and the reflective silver metal screws. I haven't touched the focus drawtube yet, nor the mirror clips.

With these upgrades, I took a 30 second (it was just a quick test) image of Capella.

5482840317_c9cd31a635.jpg

Capella. How many stars? by MikeWPhotos, on Flickr

You'll notice if you zoom in and look carefully that I have (as TeaDwarf points out) a similar but not as harsh reflection effect. You can see the effect of the mirror clips, it's minor but it is there all the same.

As a side note, it's also possible that your collimation is a little out and the focus is out.

Having said all of this, my money is on stray light entering your tube and reflecting off something (probably the mirror clips).

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.


  • Recently Browsing   0 members

    No registered users viewing this page.

×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.