Star halos

[Demonperformer]

My first shot through my new ccd camera.

10x15-second exposures of M50.

All the bright stars have fuzzy halos.

Is this a focussing issue (I have terrible trouble focussing) or is there some other reason why I would be getting this effect?

Any help/advice welcome.

Thanks.

[Merlin66]

I'd say 90% focusing then there may be dewing....

Which lens/ telescope/ camera were you using?

[shaunster]

imaged through which scope and any filters?

[kookoo_gr]

i would also check the colimation

[Demonperformer]

Sorry, should have included those details:

102SLT

SXV-H9

Was trying to use a number thingy that is supposed to reach maximum as you get into focus, but the number changed every time it took a new 'focus exposure'. Possibly exacerbated by using Sirius to focus on (low down - twinkling) - may have better luck with a slightly fainter star but higher in the sky.

No filters yet ... when I get this basic kit sorted, will start adding the filter wheel into the mix. On which note (to save me starting another thread) as I have to use a 40mm extension tube to achieve (anything like!) focus, which is better ... ?

• ota - filter wheel - extension tube - camera

or

• ota - extension tube - filter wheel - camera

Thanks

[Merlin66]

If the extension tubes are tight and no slop it shouldn't really matter.

Putting smaller filters nearer the camera will reduce the likelyhood of vignetting and you may also find your filter wheel is "threaded' to go straight onto the camera body. Probably the better solution!

[FraserClarke]

Was trying to use a number thingy that is supposed to reach maximum as you get into focus, but the number changed every time it took a new 'focus exposure'. Possibly exacerbated by using Sirius to focus on (low down - twinkling) - may have better luck with a slightly fainter star but higher in the sky.

Might also help to focus on a fainter star such that you can use 1--2 second exposures when focusing. If you're too quick, you'll 'chase the seeing' with the focus. Better to use a slightly longer exposure so that the atmospheric effects average out, and you're really seeing the focus of the instrument.

[Demonperformer]

Thanks for your advice, guys.

Will try to put it into practice next time out.

One more small piece added to the puzzle ...

[mark7331]

Might also help to focus on a fainter star such that you can use 1--2 second exposures when focusing. If you're too quick, you'll 'chase the seeing' with the focus. Better to use a slightly longer exposure so that the atmospheric effects average out, and you're really seeing the focus of the instrument.

Very useful advice! Cheers.

[Ben Ritchie]

Is the 102SLT an achromat? It's described as a "short tube refractor" on the Celestron website

The very soft, fuzzy stars are what i'd expect from an achro with no filters as the CCD will be much more sensitive to unfocussed IR than the eye will be.

[shaunster]

allmost certainly caused by the optics and nothing else here

I think its CA, I would expect when you do colour you will find the blue stars are very bloated and will cause blue halo's

[Demonperformer]

Ben, Yes, it is an achro, but it is the same scope that I have used for (most of) my imaging with the Canon, and this has not been a problem before

[Ben Ritchie]

You'll have much greater IR sensitivity with the SXV-H9, even if the DSLR was modified (and especially so if it wasn't, as the unmodified camera has an IR-cut filter built in). The ICX-285 has a QE over 50% at the H-alpha line (part of the reason it's such a great CCD) and significant out to near 1000nm. In an achromat the colour correction will be diverging strongly at those wavelengths, hence the fuzzy, out of focus stars.

[Demonperformer]

So am I basically wasting my time trying to get decent images with this system without adding the filters into the mix?

I had been hoping to come to grips with my new gear one step at a time, but if my understanding is correct, then I need to have the filters/wheel in there from the start?

[Ben Ritchie]

With an achromat, yes, you really need filters - at a minimum you want a UV/IR cut filter in there to remove the regions where the correction is most divergent.

Even with an apochromat you'd typically use a UV/IR cut filter, as very few apochromats are well corrected out to 1000nm - the FSQ is, I think, but cheaper apochromats tend to diverge in the far red, better ones will diverge somewhere in the I-band but, as the response curve shows, the ICX-285 is still very sensitive in the near-IR.

[Demonperformer]

All right, next stupid question ...

I have had a rummage through my "things I've bought but never really used" box, and found an "IR-pass filter 685nm". Is this the same animal you are talking about by a different name, or do I need to splash out on another one if I want to do monochrome (I presume my colour filters - Opticstar 1.25" LRGbB Imaging Filters. - only pass the colours they are supposed to and so the problem will not arise if I use them).

You can probably tell that filters are a little like quantum mechanics to me: I am just about convinced of their existence ... and that's about it

Thanks for all your advice.

[Ben Ritchie]

An IR Pass filter is almost the opposite of what you want - it passes the far-red/near-IR light above 685nm (slightly redwards of H-alpha) and blocks the visible spectrum and UV. Typically they're used for planetary imaging to minimise the effects of seeing, which is less pronounced in the near IR. Unfortunately here it would mean imaging only in a wavelength region where your colour correction is divergent.

I presume my colour filters - Opticstar 1.25" LRGbB Imaging Filters. - only pass the colours they are supposed to and so the problem will not arise if I use them

Sadly not. Your Opticstar filters aren't IR-blocking - for visual use there's no need for them to be, as the eye can't see the IR leak, but your CCD can. So they would need to be used in conjunction with a UV/IR blocking filter too - which gives an extra surface for internal reflections ... and, you guessed it, haloes. Imaging colour filters are IR blocking too, which is part of why they're more expensive.

Best place to start is one of these

UV & IR Filters - Baader UV/IR Cut Filter

which will help control the fuzzy stars you're getting, but won't completely eliminate them. Note that in a full imaging set

http://firstlightoptics.com/proddetail.php?prod=Baader_LRGBC_filter_set

there's a difference between the Luminance filter, which is UV/IR blocking, and the clear filter, which isn't. If you have a very well corrected apochromat (e.g. the FSQ) then you might use CRGB rather than LRGB to gain additional luminance signal in the IR

[shaunster]

Even when using filters with the achromat you will still get fat bloated stars, especially in the blue

[Merlin66]

Shaunster,

Sorry mate, I disagree with that.

With the right choice of filter + a UV-IR there should no or very minimal bloating.

[shaunster]

i guess we will have to agree to disagree on that one then mate

[Demonperformer]

Well, I have ordered a UV & IR Filters - Baader UV/IR Cut Filter from FLO

When I get it I will post a pic and you can fight it out then

[Demonperformer]

Well, on Friday evening I managed to have a go with my new UV-IR cut filter on the H9. OK, the stars leave a lot to be desired in terms of being circular (I have a horrible feeling I am going to be spending lots of cash on an EQ mount before too long:eek:), but the halos seem to have disappeared ... unless, of course, you think different ...

Thanks for all the advice.