Covering up your bottom.....

[Uranium235]

Of your newt that is

Just a quick query as to whether this is a good idea or not because Ive noticed that if your imaging location isnt as dark as it can be Ive seen that stray light can actually leak in through the bottom of the OTA and cause gradient issues.

Ive already tried blocking off the bottom with a cut-down bin liner, which kinda worked but that wasnt ideal because I noticed it seems to attract dew on the inside of the bag, which cant be good for your primary.

Has anyone thought of any novel solutions to this light leakage problem?

[martin_h]

How about a black cloth? Or for the bin liner, chuck in a couple of silica bags?

[barkis]

Use Camping Mat material, and make Dew shield as you would for the front, only put a lid on it, then slide it onto the rear of the OTA.

Ron.

[Uranium235]

Cheers Ron, thats a good idea. I still have some bits of lightweight foam underlay left over from when I made my last dewshield, so I'll put one together and give it a go, should be a 5 min job... well, as long as it takes the superglue to dry

[ollypenrice]

Are you in danger of inducing tube currents this way? Isn't the open back is needed for cooling?

Olly

[johninderby]

Some flocking on the bottom part of the tube should take care of it. Stops the light from reflecting off the tube wall and across the surface of the mirror and thus reducing contrast. It's more important to do this than flocking at the focuser end.

John

[Telrad]

I have a dew shield that seems to do the trick rather well, although it's always good to read other members opinions and I thank you for the enlightening post U235. I've learned a great deal from this, v.educational !

[Uranium235]

Its been one of those things where you wonder if anyone else has tried cutting out stray light in this way. Olly may have a point in regard to cooling, but once the scope temperature is equalised with the ambient air temp I shouldnt think covering up would produce any serious currents.

Flocking is a good idea, but my newt is well collimated at the moment so I would have to think twice before taking it apart again

[Tim]

Use Camping Mat material, and make Dew shield as you would for the front, only put a lid on it, then slide it onto the rear of the OTA.

Ron.

Yep agreed, you def need something. I found even the light from light pollution was getting in, bouncing back off my patio

I guess ventilated would be better, and possibly a cap with a chimney type top would work. I'll be needing one soon for my new toy so i'll have a play.

[Tim]

FWIW, the tube I had issues with is an F4 GSO 8", there was a ring artifact visible on the subs that really stood out with stretching, like a coffee mug ring. The tube is flocked throughout, but it seemed to me at the time that the problem was caused by light coming up through the primary itself, which was a worry

[UTMonkey]

Really, I had no idea of this - this isn't a wind up?

[blinky]

Nope, I see the same effect on my GSO newt

[Tim]

Yep agreed, you def need something. I found even the light from light pollution was getting in, bouncing back off my patio

Should perhaps point out that these were long subs, 20 min+

[Adamski]

Has every one removed the back plate from behind the primary or am i missing something completely different. I have read that you can take out the plate to aid cool down time but didn't think it really important cos cooldown time is not that long, 1-2 hrs.

[Uranium235]

Should perhaps point out that these were long subs, 20 min+

Yeah, thats what I was getting at also. Its not such a big deal with short exposures (though it will show up once theyre all stacked), but on longer subs it starts to get noticeable. Ive found that binning at 5 or 6x then taking a 20 or 30 sec exposure will start to show a gradient if there is light leakage.

Its not just LP that causes this, it could be the glow from your laptop/netbook (mines covered up with a carpet tile now). I also taped over the led on the qhy5 becuase that can be a bit bright.

[northernnutter]

Thanks for this thread! I have also been wondering about this issue....some good ideas to try!!

Has every one removed the back plate from behind the primary or am i missing something completely different.

Some scopes don't come provided with a backplate. I queried this on the arrival of my SW 200p Dob...thought it was missing..but was informed that this model isn't provided with one.

[Mik3]

It just made this out of the packing foam that the scope came in, I will spray the back and sides if needed. It pops in and out easy so you can remove it to speed up cooling.

[Mik3]

I stuck some foil on the back as it was still letting a lot of light in.

Before the plug.

0.6 S Exposures.

After:

Much better

[Uranium235]

Ingenious! (and the best price.... free!)

[dph1nm]

Took me ages to spot this one. I kept getting an arc of enhanced sky across my images, but only when pointing in a very precise certain direction. I kept trying to shield out street lights and all sorts. Turned out that was the direction where the bottom of the tube was pointing exactly at the red light on the power supply (which was on the ground) - light was coming straight round the primary and off the secondary into the camera!

NigelM