Problem - "diffraction spike" on refractor

[dmahon]

I bought whole load of kit over the last few months, specifically for imaging. The scope is a Skywatcher Equinox 80 bought new from FLO, the camera is a Canon 450D. There is a 2" Baader Neodymium filter and a Televue TRF-2008 reducer/flattener in the optical path.

I note a "diffraction spike" type effect in all my images - any idea what could be causing it? Here is a typical example (single exposure, unprocessed - click for link to 11MB RAW file):

[jgs001]

It looks to be on all the bright stars... Do you have a small hair or something like that on any part of the optical train ? Might be worth giving all the components a blast with a rocket blower. I've had something similar, and cleaning everything helped.

[Merlin66]

Just double check that one of the spacer foils on the objective isn't protruding into the aperture. This has been a common cause in the past.

[dmahon]

Can't see anything obvious, but not exactly sure what I should be looking for. Do I look down the scope from the front, or up the scope from the eyepiece end? Happy to take a picture for others to look at if you tell me exactly what to take.

Looks like a process of elimination is in order, when the clouds go away. I will take the filter out, and will then take the reducer out (have just ordered a standard T-mount from FLO).

Then I'm stuck, as I only have one scope and one camera.

[Merlin66]

Look into the front of the objective lens...

You should see three small spacers situated at 120 degree around the edge. These hold the lens elements apart.

Sometimes they are misplaced or physically too large and encrouch into the objective. One "longer" tab will cause a double diffraction spike...

Hope this helps

[nightfisher]

great pic by the way

[dmahon]

Can't see anything (here is a picture taken up from the focusser)

[step_hen]

A pic of the objective end would be more useful

[Nillchill]

check there is not anything in the camera that could cause the diffraction.

just a thought!

[dmahon]

I tried to take the same picture as the chap, who was havving multiple diffraction spikes in this thread. His problem looks obvious, yet my spikes are far bigger and I can't see anything wrong anywhere!

[dmahon]

Here is a photo I took with the camera, 500W security light for 1/50 second.

If it was the camera, that should have a spike too, right (the bit above and left of the light is the roof)?

[step_hen]

Clips on the lens look ok. Must be something somewhere else in the imaging train- have you checked all the optical surfaces and edges? If you check through your subs, does the spike occur on all of them?

[MikeWilson]

A hair (as already mentioned) could replicate that, as could a small piece of string.

If you haven't had these issues before, use a blower to clean off any debris from the objective surface and try again.

[dmahon]

Visible on all images I've ever taken. No hairs visible anywhere.

I've ordered some Baader cleaning fluid from FLO. I've also ordered a T-mount, so I can try without the reducer that I use. I'll give it a go without the filter too.

[dmahon]

Quick thought:

If I keep everything that is attached to the camera firmly secured, and simply rotate everything by 90 degrees where it is inserted into the focuser, if the diffraction spike moves 90 degrees (in relation to the rectangular frame), it must be on the scope. Conversely, if it remains the same, the scope must be OK and I need to look elsewhere.

Is that right?

[astromerlin]

Quick thought:

If I keep everything that is attached to the camera firmly secured, and simply rotate everything by 90 degrees where it is inserted into the focuser, if the diffraction spike moves 90 degrees (in relation to the rectangular frame), it must be on the scope. Conversely, if it remains the same, the scope must be OK and I need to look elsewhere.

Is that right?

Yes, good point. I've been watching your post as I have a similar problem with my reflector. I'll give that a go too next time I'm out.

[Peter Drew]

I would be surprised if it was a "front end" problem. You usually need an edge to centre obstruction to produce the effect your image shows. I've seen this effect from an internal baffle in a refractor that had come loose and turned through 180 degrees.

[dmahon]

OK, here are a few images - very cloudy, so all I could manage was Jupiter and I couldn't polar align well. I don't have my T-mount yet (the current one is integral to the flattener) nor do I have my blower and cleaning fluid. So minimal options, but here we go. 30 second exposures.

1) Full setup, 0 degrees

2) Full setup, rotated T-mount 90 degrees

3) Removed Baader filter, rotated T-mount 90 degrees

4) Removed Baader filter, 0 degrees

5) Removed Baader filter, rotated focuser 90 degrees

Not ideal images, but it looks like the filter may be the culprit! Pity, as it makes a hell of a difference to the glow, but at least it's the cheapest item there (other than the T ring).

I'll take long, guided exposures, when I have time (will be busy most evenings until Christmas) and the clouds go away. I will post the results up here.

Looks like it is the filter.

[dmahon]

Or to make it more obvious:

[Kef9]

Where did you get the filter from. If I was you I would try to take it back.

Alternatively the baader coatings are quite hard so you could try to give it a clean and see if that works?

[MikeWilson]

I've found the answer to your problem....

[Merlin66]

Hmmmm

When you inspect the Baader filter....what do you see??

Is there smearing (finger print smudge..) or coating damage.

Most unusual

[dmahon]

You picked the worst Bond!

[Kef9]

No way.... Dalton is in fact the best bond!

[dmahon]

Where did you get the filter from. If I was you I would try to take it back.

Alternatively the baader coatings are quite hard so you could try to give it a clean and see if that works?

It came from FLO, so hopefully no problems there - but I'll give it a clean first, as there are a few specs of dust on it.