Collimation Error?

[BlueAstra]

At the start of my last session I focussed on Vega with a Bahtinov mask. I took a 30s exposure at iso1600 with and without the mask. I noticed in the overexposed image without the mask what I take to be a couple of reflections from internal optics. The setup was C8, F/6 focal reducer, 1.25" SW LPF, Canon 1000d & 1.25" T adapter.

Is this evidence of a collimation error? Other possible causes could be smudges on the optics, mis-aligned focal reducer, the presence of a focal reducer, 1.25" aperture on 2" reducer....

The vignetting is because I used 1.25" optics in the system.

Any comments welcome.

[Doc]

Could be tube thermals. How long did you leave C8 to cool down for?

[Merlin66]

I'd guess it was internal reflections from the filter and reducer. Vega's a very bright object and your image at f6 would be well over exposed at 30s

[Mikea]

I'm no lensing expert (my field is 10Gb/s pulses in 125 micron glass fibres), but I am intrigued by your second image. The surrounding stars seem pin-sharp, so that would suggest to me that collimation is ok, yet Vega has a peculiar flare round it - maybe an artefact of the camera CCD? Then there is that peculiar secondary out-of-focus image at the 4 o'clock position: I agree it looks like a secondary reflection of some sort. In what order was the LPF, reducer?

[BlueAstra]

Could be tube thermals. How long did you leave C8 to cool down for?

Probably not long enough. It was at the start of the session, and the OTA had probably been outside for about 30 min. Should have repeated the photo at the end of the seesion!

[BlueAstra]

I'm no lensing expert (my field is 10Gb/s pulses in 125 micron glass fibres), but I am intrigued by your second image. The surrounding stars seem pin-sharp, so that would suggest to me that collimation is ok, yet Vega has a peculiar flare round it - maybe an artefact of the camera CCD? Then there is that peculiar secondary out-of-focus image at the 4 o'clock position: I agree it looks like a secondary reflection of some sort. In what order was the LPF, reducer?

It was OTA O/P (2") - reducer (2") - 1.25" visual back - LPF (1.25") - T mount tube (1.25") - Camera (1000d).

As you say, the background stars are sharp, and my subsequent DSO photos showed no evidence of flare. I thinking its some very low level retro-reflection from some optical element, only visible with a super bright objest.

[Doc]

Probably not long enough. It was at the start of the session, and the OTA had probably been outside for about 30 min. Should have repeated the photo at the end of the seesion!

It looks like this.

But it could also be reflections.

[roundycat]

If you want to have nightmares photograph any area with stars in it and make sure you over expose. The reflections you see will be present around every star all the time, it's just that you normally don't see them. Looking at the dust shadow your collimation appears to be near-perfect.

Reflections from reflecting surfaces in the optical train will always be present but tend to be more apparent with the cheaper filters and possibly reducers. They are often more obvious in blue than in other colours.

Regarding your exposure here, if I took a picture of Vega with my ST10 at f6 it would probably set it on fire!!

Dennis

[Mikea]

Dennis, may I ask what you mean by a dust shadow?

[roundycat]

anywhere there is dust in the optical system, and that means just about everywhere, it will cast a shadow on the film or chip. The closer to the chip the sharper will be the shadow. The doughnut shaped mark at 4 O'clock is the sort of thing you see but this one is actually the 'wrong' polarity. It should have a dark outside ring but as the ring and its centre are concentric it shows your collimation to be pretty good.

Dennis