Identifying the cause of reflections

[Tim]

Just found this very interesting formula for working out where in your imaging train any reflections/halos are being caused.

All credit to Neil Fleming, this is his site;

Fleming Astrophotography

[StarlightKnight]

Alternatively for dust donuts

If your maths isn't that good (or your just lazy !!!) you could try this link

Dust Donut Calculator

Don

[MikeWilson]

Thanks guys, those links look useful for finding dust and debris, but the artifacts I was thinking about are these.. they're consistently to the upper right of any bright object anywhere in the frame.

Did you see a different artefact in the image, TJ?

[MikeWilson]

Ok, so if I take a look at one of those artifacts and measure the diameter of the brightest/worst part of it, it's between 150px and 300px. The sensor size is 5.8 microns. The focal ratio is f/5. Therefore the distance from the sensor to the artefact is 2.1mm to 4.3mm. I don't think that's right because it doesn't account for the mirror or the distance between the sensor and anything else that I've introduced (the CLS-CCD and the MPCC).

I would say that result would be the Astronomik CLS-CCD clip in filter. I can experiment to be sure, but say it is then what should I do about it? What does one do about this other than to clean it?

[FraserClarke]

There's probably very little you can do. These are reflections, and any bit of optics is going to generate reflections at some level (even super expensive anti-reflection coated optics aren't perfect). It is a very common issue in telescopes and imagers unfortunately (right up to professional level).

[Tim]

Is it definitely sat nice and square Mike?

Try without the filter in place first, then refit and try again. I have one of those filters, you shouldn't get that really. If it IS your filter, I'll send you mine, try it, and if that's ok, then go back to the dealer and get a replacement, assuming it was bought new.

Just pm me if you want to do that Mike

[MikeWilson]

Hi TJ,

That's awfully kind of you. My filter is second hand but there doesn't appear to be anything wrong with it. And it's sat what appears to be perfectly comfortably in place. It is conceivable that (given how reflective it is) that some light could reflect off it back up towards the coma corrector and (given how reflective that is) then back down and through the CLS-CCD clip filter and onto the sensor.

At least, that's what I think is happening. Did you use a coma corrector also, or were you perhaps using an SCT, refractor or coma-free Mak-Newt for your imaging?

Best,

Mike

[Ibbo!]

I had a nice shiny black draw tube which caused awful reflections with the Ha filter untill I flocked it

Steve

[MikeWilson]

I had a nice shiny black draw tube which caused awful reflections with the Ha filter untill I flocked it

Steve

Flocking is an interesting idea. I've asked johninderby about procuring some Protostar flocking material but I don't think he's got enough interest to put a decent sized order through.

I might just wait until I can get the 200P DS (if this would fit my mount) and flock that instead. I'm pretty sure I'm going to want a bigger reflector within the next six months.

[Tim]

I've used it all ways round Mike, with and without MPCC, and in various newts from 6" F4 to 8" F4 and the mn190.

Just to be sure, the filter is in the right way round, ie, gold face outwards?

And how's your collimation Mike? If you centralise a bright star, then defocus it in and then out, does it move much?

I dont know the 150P, but is there anything bright/chrome that could be causing those blips?

[MikeWilson]

Hi TJ,

Thanks for your advice. I've taken a look and can't see what might be causing it. There are reflective surfaces within the tube, the secondary mirror and it's supports are rather reflective and on inspection it looks like they're a little dirty too and could use a clean at some point soon.

The coma corrector looks like it is mounted correctly and the CLS reflector is the right way around:

Collimation is spot on according to the focus/defocus test as well as the laser collimator. I tend to collimate obsessively, checking before each session.

In the meantime, processing removes the blips and they only occur on very bright images on long exposures, however one night I'll try a few test shots on Vega and see if I can eliminate the CLS-CCD and MPCC as the cause.

Best,

Mike

Best,

Mike