Annoying imaging problem

[Matthew.Blake]

So, I got the Cannon 50mm f/1.8 lens for my camera and thought I'd try some almost widefield photography. Normally I can't go above 30 second exposures with my scope because the alignment and PEC aren't good enough and I'm not patient enough to sort them out. However, at 50mm I reckoned I could get away with 1 minute subs as each pixel represents that much more angle in the sky. To my delight I found I was right!

To my horror, however, 1 minute at ISO 400 F/2 is enough to saturate the sensor with the sky glow from light pollution where I live :-( Coventry sucks, the coucil won't even turn the lights off at night because they seem to think, contrary to empyrical evidence, it will increase crime and more old ladies will fall over.

[brianb]

Try a CLS filter ...

[Takahashi]

Indeed, I was amazed at the difference when I experimented by pointing my camera upwards, and resting a cheap 2" LPR filter on the skylight filter that is permanently attached to the lens. On a 3 minute exposure, the area that the LPR filter covered was still a nice, contrasty dark-grey/black, and the outlying area was, yep, orange-mush. I'll post the pi©s later when I get home from work.

[Matthew.Blake]

I've the Skywatcher UHC filter and a LPR filter somewhere, I wish they fit the filter thread on the camera though :-( anyone know where I can pick up a 52mm LPR filter?

[Takahashi]

If you find one, could you ask the maker if they have any 72mm versions?

[brianb]

anyone know where I can pick up a 52mm LPR filter?

The B+W #491 "Redhancer" (didymium glass) is nearly as good, it blocks sodium glow just as effectively as the CLS filter, but it doesn't clobber mercury vapour lamps too.

[jgs001]

You can get a reducing ring to fit the 2" SW LPR filter nicely. The lens is quite narrow, much narrower than the 2" filter so you shouldn't get vignetting. A CLS Clip filter works great though. You do get some quite rough stars around the edges of the FOV at under f/3.2 (it wasn't great even then). I've not had a chance to try since then, so perhaps f/4 would be best.

[Matthew.Blake]

The B+W #491 "Redhancer" (didymium glass) is nearly as good, it blocks sodium glow just as effectively as the CLS filter, but it doesn't clobber mercury vapour lamps too.

It looks like it cuts out lots of green and blue though, or am I just misinterpretting?

[Matthew.Blake]

You can get a reducing ring to fit the 2" SW LPR filter nicely. The lens is quite narrow, much narrower than the 2" filter so you shouldn't get vignetting. A CLS Clip filter works great though. You do get some quite rough stars around the edges of the FOV at under f/3.2 (it wasn't great even then). I've not had a chance to try since then, so perhaps f/4 would be best.

Any idea where to get one?

[jgs001]

I got a 52mm to 48mm ring from ebay I think... I know psychobilly has posted somewhere to get reducing/stepdown rings from... I can't find a 52to48... I'll have to dig mine out and check the details on it later.

[jgs001]

I just checked, it's defo a 52mm to 48mm step down ring... on my searches I could find 52-49 and 52-46 but not 52-48... it's probably worth you having a search though...

either as a step ring, stepping ring, stepdown ring, or reducing ring... something like that.

[brianb]

It looks like it cuts out lots of green and blue though, or am I just misinterpretting?

The Redhancer was designed to work with film cameras ... colour transparency films contain layers sensitive to red, green and blue, and the red & green tended to overlap a bit (in the yellow). The Redhancer removes this overlap, making the differentiation between green foliage and "autumn colours" much better defined. Transmission in the green & blue is over 80% ... Trust me, it works, it's a narrowband notch filter centred on the sodium D lines.

[Matthew.Blake]

Awesomeness. Only £20 too! Comes in 72mm as well as 52mm according to froogle!

[simon84]

I had this problem too but with so many different lenses with different sized filter threads it could have got really expensive buying a filter for each lens.

In the end I went for the Astronomik CLS CCD clip in filter which goes into the body of the camera, you cant use EF-S lenses but the EF lenses work a treat, and now I can use one filter with all my lenses.

[Takahashi]

That EOS clip filter looks like the one for me. I sold my 18-55mm EF-S kit lens to part-fund my one multi-purpose lens, a Tamron 18-270mm zoom, which I'm pretty sure has enough rear clearance to use the clip-on. The only thing that protrudes beyond the bayonet flange is the seven-copper-contact mount, about 2 - 3 mm in height. Can anyone who uses the clip-on say whether I might have clearance issues or not? Also, my 500D is conspicuous by it's absence from the "Will fit these cameras" list...

[Takahashi]

This is the wee experiment I carried out by planting a standard SW LPR filter on the camera lens and opening the shutter for three minutes on the EQ6. Quite a staggering difference between the filtered centre-circle and the unfiltered periphery.

[Matthew.Blake]

This is the wee experiment I carried out by planting a standard SW LPR filter on the camera lens and opening the shutter for three minutes on the EQ6. Quite a staggering difference between the filtered centre-circle and the unfiltered periphery.

Awesome, just pure awesome! I'll be giving that a go!

[simon84]

You'll be fine with that lens and the clip in filter, the EF-S lens has an extra rubber/plastic ring that protrudes a further 3mm beyond the contacts panel.

I can take a piccy of both lenses if you would like to see.

Here ya go, EF-S on the left, EF on the right.

[Takahashi]

Thanks Si, most valuable info.

Feel free to send piccies... any reference is a help.

The optimist in me is telling me the only reason the 500D isn't on the compatibility list of the retailers I've checked, is down to them not updating said lists, yet.

EDIT : Ah, yes... quite a difference.

[simon84]

No problem at all.

I think I remember reading somewhere once that the EF-S lenses can be modded but its not something I have ever considered.

[Takahashi]

Yeah, I've seen a few modification attempts; some tidy and some downright messy, so I'm now glad I don't even have to try.

[simon84]

It does seem to be a bit pointless that extra ring. My one has a rubber ring around it which can be removed so I'm guessing its only for guiding purposes, or maybe to remove any vibration in the lens when the mirror swings up. Not too sure.

The only thing I'm missing out on is being able to use my CLS filter at 18mm, my next widest setting I have is 28mm on my EF lens.

[simon84]

Just been looking around for a solution to the EF-S lens problem and found this, 7dayshop.com - Online Store

Not a cheap solution but will give that much needed to space to allow all EF-S lenses to work with clip in filters........I guess at the end of the day it all depends how much you really need to use an EF-S lens for your widefield imaging.

For me it gives me access to 18mm so probably well worth the £59, but then saying that if Canon make one I'm sure someone else makes something like this aswell......I'll keep looking.

[brianb]

Just been looking around for a solution to the EF-S lens problem and found this

Sorry but this will prevent you getting anywhere near infinity focus.

[Matthew.Blake]

I've just ordered a reducer ring that will allow me to attach the 48mm thread astro filters onto a 52mm thread. It was only £2.50 and there were all sorts of other sizes available so, once I've tested it, I'll probably get one for every lens I buy.