Astronomik UHC EOS clip filter queries esp. focus

[Tommohawk]

Hi all. I recently acquired a UHC filter and have a couple of issues using this with 300mm tamron lens.

Firstly fitting it into the camera seems a bit iffy - I guess I must have this correct as it seems to have stayed in place. any tricks for this? For removal do you just somehow pul it forward to release it?

More importanly I have some focus probs. I was worried I wouldnt see stars easily but as it turns out this was OK - but the live image seems strangley blurred almost like a a sort of halo. The Bahtinov was also very peculiar kind og wavy looking and not possible to focus.  Similarly the final RAW images, although seeming to show more detail, had a halo type blur around the brighter stars. 

Couple of thoughts - there is some CA with the Tamron lens, and its almost as though what would normally appear as a purplish fringe without the filter is rendered as a kind of white halo with the filter in place.

Also I'm working at f2.8 and I know there can be some issues using filters with fast lenses - any ideas?

I'm pretty gutted that the results seem so poor so any thoughts gratefully received!

[D4N]

What mod do you have on the camera, is it full spectrum or replacement filter?  If it is full spectrum you need the CCD filter as the standard clip filters don't have UV/IR cut.

If this is the case it could easily be out of focus IR or UV light as the Tamron lens won't be designed to bring these to the same focal point as visible light.

 

You could be right about the speed as well, the angle of incidence at f2.8 could cause a problem on the filter if it isn't designed for it.  Filters designed for imaging often state what speed they are designed for.  The way around it is of course to put the filter over the objective but this would mean buying a bigger filter.  I've not heard of this problem with UHC just NB but that doesn't mean it doesn't exist.

[Tommohawk]

OK Dan thanks for that - the mod was done by Juan Fierros at Cheapastrophotography. TBH I can't recall now exactly what was done!

Its a real shame if this filter cant be made to work cos there is definitely more detail. When I get a mo I'll post images, but still catching up with work and trying to decide what to do tonight if skies clear. I might try stopping down as an experiment but bit of a shame having F2.8 and not using it!

[StargeezerTim]

I did notice some halo/coma stuff when i used mine at 2.8 on my nifty 50. when i stop down to 4 or 4.5 its much better. I do find the filter makes it harder to find stars in live view, less contrast, but the end result is better. I too use it on a cheapastro modded camera without probs. They are a bit fiddly. I find it fits more securely in my 100d than the 1200d. Could be dew on the filter or lens causing problems? Tim.

[Tommohawk]

15 hours ago, StargeezerTim said:

I did notice some halo/coma stuff when i used mine at 2.8 on my nifty 50. when i stop down to 4 or 4.5 its much better. I do find the filter makes it harder to find stars in live view, less contrast, but the end result is better. I too use it on a cheapastro modded camera without probs. They are a bit fiddly. I find it fits more securely in my 100d than the 1200d. Could be dew on the filter or lens causing problems? Tim.

Hi Tim - ta for that. I think I must have the clip in OK it seems to have seated OK - just a bit fiddly to remove.

I tried again last night with F5 ish which seemed to improve things but even 8 min exposures were dark. I havent had time to process yet. I can see why you've opted for ISO1600 instead of 800 with the heart neb you posted earlier.

The halo effect occurred from the outset so defo no condensation etc.