Newtonian rear cell cover ?

[RyanJones]

Hi all,

I have a question regarding covering the rear cell in my newtonian. I have an 8” f/4 newt and the rear mirror cell is open to air ( and light ). It has a fan to cool the primary but the rest of the cell is open. I live in quite a light polluted area so I figured there may be some benefit, even if small to making a cover for the rear of the tube. Is there any reason not to ?

Thank you in advance for any advice

Ryan

[Pepper (Steve)]

Here is my rear cover for my 8inch.
The cutout is for a 90mm fan

[Startrek (Martin)]

Ryan
I use 6” and 8” newts in Bortle 8 suburban Sydney and don’t feel the need to cover the back end of the scope
I think it’s the front opening that’s a real issue for newts, let’s in everything including my neighbours flood lights on either side
I don’t suspect the rear cell to be an issue otherwise someone would have designed a rear shroud or deflector by now for newts
Only my thoughts
Cheers

[RyanJones]

Thanks Steve and Martin,

Steve - that’s pretty much what I was going to make. Only difference maybe was I was going to use my fan screws to retain the cover. Did you find it made any difference ? What was your reasoning behind doing it ?

Martin - From the research I’ve done, brands like Takahashi and Farpoint imaging scopes don’t have open rear cells. I’d imagine there is a reason for that ?

[Pepper (Steve)]

Haven’t had a chance to use yet. Scope is a work in progress.
I did it to see if it had any effect at all with dew and in summer when it’s hot here.

[Ken]

Just about to do the same thing for my 18" Newt. which is a truss style and very open. As it is only used for astro imaging it picks up stray light from all sorts of places. I have had to put tape over all the leds including the usb hub and make a box for the laptop.
Clear skies Ken.

[RyanJones]

Thanks Steve,

Dew reduction was also a consideration as the fan will be forced to move air past the mirror which I guess will help too.

Ken,

That was another thing that I saw when I was researching the idea. Some of the truss style newts had obviously open front ends but still didn’t have open rear cells. Obviously an open truss would be useless in a highly light polluted area without at least a cloth cover but I’d imagine that there should theoretically be some benifit in reducing the potential light hitting the glass of the primary. Probably not a huge difference but I’d imagine it would be much like people covering the view finder in their DSLRs.


Cheers

Ryan

[luka]

Just make sure your fan is blowing out of the scope (not blowing onto the mirror). A very nice reasoning why can be found here.

[RyanJones]

A very interesting read Luka. Thanks for posting the link

[Ken]

My Truss Newt. has a very open mirror cell, good for cooling but if you look between the mirror edge and the tube side wall you can see the secondary mirror. Off axis light from behind might end up in the camera. I have a black cloth over the truss but will remove it today as it works as a good sail in the breeze.
Clear skis Ken.

[raymo]

I removed my 8" f/5 Newt's rear cover[it was steel, and heavy]; used it for over 8 yrs for visual and AP, never had a problem, in fact it cooled quicker
without the cover. A 9" steel disc is a bit of a heat sink. Oh, and the secondary was not even close to visible.
raymo

[RyanJones]

So there’s clearly positives and negatives. After reading the link that talks about the fan drawing air out of the back which helps remove the boundary layer off the front of the mirror I have a greater understanding of the need for air flow rather than just a cooling of the mirror glass. As far as stray light goes though we have to compare apples with apples. A truss telescope wouldn’t be great in light polluted skies one would imagine, and to the same effect open rear cells in dark or at least darker skies aren’t going to be affected in the same way they “ might “ in High LP skies. All good food for thought

[ChrisV (Chris)]

I have to cover the rear of my newt as its on a verandah just backing off our overly-lit living room. I forgot to cover the back this month. When I started to process there was a terrible bath ring of light around the Helix Neb.

25 hours of lights down the drain ..

[RyanJones]

Originally Posted by ChrisV

I have to cover the rear of my newt as its on a verandah just backing off our overly-lit living room. I forgot to cover the back this month. When I started to process there was a terrible bath ring of light around the Helix Neb.

25 hours of lights down the drain ..

Very interesting Chris. Although I wouldn’t be dealing with as much light as your living room light, it is the kind of issue I’m thinking about. You’re also using a very similar newt to mine all be it an f/5 rather than f/4. 25hrs is a huge amount to lose ! You’d be bleeding I’m truly sorry for your loss