Wierd Bahtinov DSLR problem

[Tommohawk]

Bit of an odd question!

I've used my Bahtinov mask quite happily with the SW PD200 and Canon 550D for a couple of years, and also found the same mask works passably well with my Tamron 300mm camera lens.... or at least it did until recently.

For some reason I now get quite swirly lines rather than the nice cross effect that I used to get. The camera settings are the same, and I've tried positioning the mask differently, but still the same result. This just doesn't seem possible, but the last 2 outings have had the same result. I suspect folk will think I've completely lost it, but I'm sure this isn't just my imagination!

Any ideas anyone please?

[Thalestris24]

Hi

An example might be useful  Generally speaking the Bahtinov mask should be scaled to the size of the aperture.

Louise

[Tommohawk]

Hi Louise - yes I wish I'd done a screen shot last time out, but my patience was getting thin and the frost was getting thick!

Most particularly I wish I had done a screen shot when everything was OK so I could compare. I'll try and do something next time out but goodness knows when that'll be. I'm off to La Palma in a few weeks so would really like to get sorted before going. I realise the mask design should take account of the lens/telescope, but my point is I'd used that mask with that lens previously and got a fair result.

One other odd thing. When using the Liveview previously, a very bright star would show CA around it (witohut the mask) and a nice cross with the mask.

When I first tried it with the UHC clip-in filter, I noticed the CA was no longer apparent, although there was an odd fringe around the image if defocused. Also that the lines weren't so bright. I put this down to the attenuating effect of the filter.

Its almost as if this same effect is persisting, even though I've now removed the filter. Sounds bonkers, but thats what it seem like. Without UHC, there is now no CA around any bright star, wheras there was previously.

BTW I've done some test images on various terrestrial objects and the results seem normal. The lens look OK, the iris work OK etc.

Open to, in fact desperate for, suggestions!

[Thalestris24]

1 hour ago, Tommohawk said:

Hi Louise - yes I wish I'd done a screen shot last time out, but my patience was getting thin and the frost was getting thick!

The frost might account for it!

[Tommohawk]

On 02/02/2017 at 00:04, Thalestris24 said:

The frost might account for it!

Nice idea but no - the funny focus issue was there at the outset. I wonder if its something to with the camera settings but I haven't changes anything - not intentionally anyhow. Maybe its just the liveview which is odd, but again no settings have been changed.

[kens]

It really does sound like the lens had fogged up

[Tommohawk]

2 minutes ago, kens said:

It really does sound like the lens had fogged up

Thanks for input, and agree it does sound like that - but 100% it wasnt fogged up. I leave this lens outside sometimes for hours without dew issues, and the problem occurred immediately on set-up.

[iapa]

the moisture may be on the camera sensor rather than the lens, or, the rear glass of the lens?

I have had that before, front glass completely clean, turned out the rear glass has mis build up - lens had been out of hours, camera only an hour to cool.

I figured the camera boy temp was enough different to cause that

<shrug> may be wrong, usually am

 

[ollypenrice]

The mask hasn't lost its non reflective finish?

Olly

[Tommohawk]

2 hours ago, iapa said:

the moisture may be on the camera sensor rather than the lens, or, the rear glass of the lens?

I have had that before, front glass completely clean, turned out the rear glass has mis build up - lens had been out of hours, camera only an hour to cool.

I figured the camera boy temp was enough different to cause that

<shrug> may be wrong, usually am

 

Again, thanks for the input. However, the camera is the last thing I take out and I'm pretty sure its not dew on the sensor cos there's enough heat in the body to stop that for quite some time, and once running the sensor is warm enough to stop dew even when well sub zero - sensor temp in winter is usually 12-15C.

But thanks fr the idea and it did get me thinking on a slightly different tack.... I changes my laptop recently and the screen runs much cooler. I have had frost on the screen a couple of times. Maybe what I'm seeing is simply a screen artifact?? TBH I think that's unlikely too though cos again the problem occurred immediately on set up. All adds to the process of elimination and further tests!

41 minutes ago, ollypenrice said:

The mask hasn't lost its non reflective finish?

Olly

Olly - another great idea.. but the mask is not so old and still much as new. But again it has me thinking about stray light. I have some significant outdoor lights (Boo... Hiss...) which I sometimes use just to get me set up. Maybe I still had these on and some stray light from that could be mucking things up?

I'll bear all this in mind the very next time we have a clear sky .....

[Tommohawk]

Some time later.....

A partial explanation! Last night I got out briefly just to test my portable rig and new guidescope. In the process I made a discovery - I am an idiot.

Reason is, I'd completely forgotten that recently I installed "f.lux". It's a neat and free program which runs in the background and cuts blue light out from the screen in the later hours - blue light from screens is reckoned to cause sleep problems.

So when focused on a the bright star,  the CA I was accustomed to seeing as a blue-ish ring became a grey-ish ring - and thats the thing that seemed oddest. Needless to say the clouds rolled in before I got a chance to check the guidescope... no change there then.

TBH the Bahtinov image still isn't great but I think thats as good as it gets with this lens - see below. BTW if anyone has any thought on getting a better Bahtinov image I'm all ears!

 

 

 

[Thalestris24]

Hi

Which particular Tamron lens is it? Is it a zoom? Fwiw I was never able to get decent images from my Canon 70-300mm kit lens - no good for astro. I think it was a collimation problem between the moving part and the fixed part. You could maybe try stopping down. TS do a microfocusser for lenses but I'd say only worth it for a decent astro lens.

Louise

[Tommohawk]

Hi Lousie.

Its a fixed focus 300mm SP which is the better glass and I've had some pretty good results with it.... well good for me anyhow!

Fine focusing is a bit of a pig, but not too bad. I looked at the microfocuser but trying to avoid any more bits if possible.

I've found I need to stop down with the UHC filter, but F2.8 is OK without filter.

Latest image done with this lens below - limited number of subs though. (always some excuse!)