light coming in from the back of a newtonian?

[dobbie]

hi all.. has anybody else notice this? i went to collimate my 200pds today and looked down the tube to see a lot of light coming in from the back of the scope , the scope i have already flocked so was a bit shocked to see the amount there was, since i observe from my garden where there is a lot of ambient light, would this make a difference in  contrast. 

  

this is the back of my 200pds with the light coming in..

and now with the dust cap placed on the other end and the result 

as you can see there is a remarkable difference the light you now see is from the mirror reflecting light from the open end ,

i have not had chance to see any marked affect  this will have ,, has any body else done this and seen better results .

[long_arms]

I have noticed the light leak before. 

I never thought about trying the dust cap down that end, I just slap a regular black woolly hat over the end which solves the problem 

Dan

[sbooder]

When I had the 8" years ago I made a cardboard cap for the back, covered in gaffa tape on the outside and flocked on the inside to stop light leaks.

[steppenwolf]

Yes, I've noticed this too - it also creeps in at the tube to rear  plate junction so I have run black tape around the join -  a CCD camera and a torch are great tools for finding these leaks, I found that the tilt adjuster on my SXVF-M25C also leaks light!

[crashtestdummy]

I used a horse feed bucket cover on my 12" but it was only necessary when there's a lot of stray lights nearby

[dobbie]

i never notice it before , until i tried to collimate it daylight , i will try it with the cap off then retry it with it on to see if there is a difference  and will post results . 

[Peter Drew]

If necessary, cover the end once the mirror has cooled, before that you need to keep the air circulating. 

[stargazerslivingroom]

I have read somewhere of folk using those shower caps you find in hotel bathrooms.

HTH

Steve

[SnakeyJ]

This does cause issues, particularly with longer exposures.    For high speed planetary imaging the effects are negligible, at least with my kit/experience!   I probably have more of an issue in this area as I have no warm room, but two displays in my obs (fixed and laptop) - I do drape/curtain these, but need to unmask to see what's going on a fair bit.

+1 for Dan's woolly hat, though fleece is better than open knit for light leakage     i cut a circular plate from 2mm black opaque plastic, with cut out notches to allow the collimation/locking screws.   This simply velcros in place with three strips.

The flip side is that the open end helps the mirror cool quickly and some ventillation helps prevent tube currents, maintain the mirror at ambient temp and help keep the dew off - I intend to fit a 12v fan to mine to assist ventillation.

[catman161]

I noticed this also on my 200P which used to be on a dob base but now I use on an NEQ 6. I combat it by using one if Dion (from astronomy sheds) cooling fans. The mounting plate of the fan is laser cut very precisely in a shape not dissimilar to a "Radioactive" warning symbol with the fan mounted in the centre. This shape covers the central circular opening right to the edge and the cut outs allow you to still access the collimation knobs/bolts. I will upload a pic when I get a chance later as I am not great at explaining things! Hope that helps anyway

[catman161]

Below are a couple of pics (one with,one without) of my primary so you can have a visual of what I was trying to explain above!

[sbooder]

I did a similar thing on my 10" but used flock covered foam bed roll.

[Moonshane]

I prefer to keep air circulating and would tend to put a baffle ring with a hole in the centre for cooling. in all honesty though I never bother.

[dobbie]

mmmm like the idea of the fan & mounting plate. 

[SnakeyJ]

Below are a couple of pics (one with,one without) of my primary so you can have a visual of what I was trying to explain above!

Nice idea with the rechargeable battery pack to keep things light and mobile, especially with the neat speed control - though as I'm obs mounted I shall run the power to a transformer fed PWM controller.

[catman161]

It's powered via a tip positive plug rather than batteries. I used to use a two way splitter with one plug going into the power socket of the mount and the other clipped down the dovetail and to the plug in the fan

[SnakeyJ]

It's powered via a tip positive plug rather than batteries. I used to use a two way splitter with one plug going into the power socket of the mount and the other clipped down the dovetail and to the plug in the fan

Ah my mistake, I thought it looked like a small LIPO and speed controller in one - It is a very neat unit and looks well made, I should have a look at some of the other Astronomyshed bits as I've made great use of their video tutorials over the last year and a half!

[catman161]

Astronomy shed is brilliant and Dion is such a friendly knowledgable guy