#1  
Old 22-08-2020, 09:19 AM
RyanJones
Registered User

RyanJones is offline
 
Join Date: Jun 2018
Location: Melbourne,Australia
Posts: 1,439
Newtonian rear cell cover ?

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
Reply With Quote
  #2  
Old 22-08-2020, 02:18 PM
Pepper (Steve)
Registered User

Pepper is offline
 
Join Date: Dec 2018
Location: Daruka
Posts: 394
Here is my rear cover for my 8inch.
The cutout is for a 90mm fan
Attached Thumbnails
Click for full-size image (DE5D1FAE-1CF6-47EA-96F0-106E95E1B8A9.jpg)
194.8 KB65 views
Reply With Quote
  #3  
Old 22-08-2020, 02:23 PM
Startrek (Martin)
Registered User

Startrek is offline
 
Join Date: Dec 2017
Location: Sydney and South Coast NSW
Posts: 5,990
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
Reply With Quote
  #4  
Old 22-08-2020, 03:12 PM
RyanJones
Registered User

RyanJones is offline
 
Join Date: Jun 2018
Location: Melbourne,Australia
Posts: 1,439
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 ?
Reply With Quote
  #5  
Old 22-08-2020, 03:39 PM
Pepper (Steve)
Registered User

Pepper is offline
 
Join Date: Dec 2018
Location: Daruka
Posts: 394
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.
Reply With Quote
  #6  
Old 22-08-2020, 03:54 PM
Ken
Registered User

Ken is offline
 
Join Date: Dec 2005
Location: Wattle Ponds via Singleton
Posts: 365
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.
Reply With Quote
  #7  
Old 22-08-2020, 04:02 PM
RyanJones
Registered User

RyanJones is offline
 
Join Date: Jun 2018
Location: Melbourne,Australia
Posts: 1,439
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
Reply With Quote
  #8  
Old 22-08-2020, 07:04 PM
luka's Avatar
luka
Unregistered User

luka is offline
 
Join Date: Apr 2007
Location: Perth, Australia
Posts: 1,164
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.
Reply With Quote
  #9  
Old 22-08-2020, 08:39 PM
RyanJones
Registered User

RyanJones is offline
 
Join Date: Jun 2018
Location: Melbourne,Australia
Posts: 1,439
A very interesting read Luka. Thanks for posting the link
Reply With Quote
  #10  
Old 28-08-2020, 06:36 AM
Ken
Registered User

Ken is offline
 
Join Date: Dec 2005
Location: Wattle Ponds via Singleton
Posts: 365
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.
Reply With Quote
  #11  
Old 02-09-2020, 02:39 PM
raymo
Registered User

raymo is offline
 
Join Date: Sep 2011
Location: margaret river, western australia
Posts: 6,070
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

Last edited by raymo; 02-09-2020 at 02:41 PM. Reason: more text
Reply With Quote
  #12  
Old 02-09-2020, 07:23 PM
RyanJones
Registered User

RyanJones is offline
 
Join Date: Jun 2018
Location: Melbourne,Australia
Posts: 1,439
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
Reply With Quote
  #13  
Old 03-09-2020, 04:23 PM
ChrisV's Avatar
ChrisV (Chris)
Registered User

ChrisV is offline
 
Join Date: Aug 2015
Location: Sydney
Posts: 1,737
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 ..
Reply With Quote
  #14  
Old 03-09-2020, 07:31 PM
RyanJones
Registered User

RyanJones is offline
 
Join Date: Jun 2018
Location: Melbourne,Australia
Posts: 1,439
Quote:
Originally Posted by ChrisV View Post
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
Reply With Quote
Reply

Bookmarks

Thread Tools
Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT +10. The time is now 06:54 AM.

Powered by vBulletin Version 3.8.7 | Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Advertisement
Testar
Advertisement
Bintel
Advertisement