Go Back   IceInSpace > Equipment > Equipment Discussions
Register FAQ Calendar Today's Posts Search

Reply
 
Thread Tools Rating: Thread Rating: 3 votes, 5.00 average.
  #1  
Old 07-08-2009, 06:21 PM
Astrobserver99's Avatar
Astrobserver99 (Rob)
Starlit Night

Astrobserver99 is offline
 
Join Date: Apr 2009
Location: Bellarine Peninsula, Victoria
Posts: 505
SCT Dew Shields

Will a long dew shield have any vignetting effect on incoming rays in a SCT? With a newtonian, vignetting of rays will obviously occur if the tube diameter is too long and narrow.

Last edited by Astrobserver99; 13-08-2009 at 07:30 PM. Reason: Corrected Spelling
Reply With Quote
  #2  
Old 07-08-2009, 06:39 PM
multiweb's Avatar
multiweb (Marc)
ze frogginator

multiweb is offline
 
Join Date: Oct 2007
Location: Sydney
Posts: 22,079
I don't think so. I have a dew shield made out of flute panel, painted and flocked on my C11. It is as long as the OTA. I get better contrast and no increase in vignetting. It also keeps the corrector plate from dewing and I don't have to push the heaters as hard.
Reply With Quote
  #3  
Old 07-08-2009, 09:13 PM
Astrobserver99's Avatar
Astrobserver99 (Rob)
Starlit Night

Astrobserver99 is offline
 
Join Date: Apr 2009
Location: Bellarine Peninsula, Victoria
Posts: 505
That sounds right, there wouldn't be any visual indication of vignetting, but I thought there might be some loss of brightness on some objects due to less light falling on the corrector?

On the the plus side, unwanted stray light falling on the corrector is reduced or removed completely.

I have no choice but to use a heated dew shield in winter as we get heavy dews, but I am also wondering if using a heater strap has any advantages?
Reply With Quote
  #4  
Old 07-08-2009, 09:44 PM
multiweb's Avatar
multiweb (Marc)
ze frogginator

multiweb is offline
 
Join Date: Oct 2007
Location: Sydney
Posts: 22,079
The light falling on the corrector will be the same wether you have a dew shield or not as you look right ahead. The contrast will be greatly increased because stray light won't reach the corrector plate as it will be deeply nested half way into the whole tube. You'll still need to heat it because when you're pointing up it will get very cold very quickly. I have an electric wire around he steel rim that I feed 12V to. Keeps me out of trouble. Doesn't have to be very warm. Just above the dew point is all you need.
Reply With Quote
  #5  
Old 11-08-2009, 12:37 PM
Don Pensack's Avatar
Don Pensack
Registered User

Don Pensack is offline
 
Join Date: Aug 2006
Location: Los Angeles
Posts: 536
A dewshield as long as the tube will still not intrude on the narrow fields of view in SCTs.
But it will protect well against dew.
I recommend at least 1.5X the aperture in dewshield length, as measured from the corrector plate out.
Reply With Quote
  #6  
Old 11-08-2009, 05:06 PM
mithrandir's Avatar
mithrandir (Andrew)
Registered User

mithrandir is offline
 
Join Date: Jan 2009
Location: Glenhaven
Posts: 4,161
I've put an Orion FlexiShield
https://www.bintelshop.com.au/Product.aspx?ID=5513
on my C8. Since then, I've had no sign of dew on the corrector, even without a heater and the tube was dripping wet.

However it is heavy enough to need the mount rebalanced.
Reply With Quote
  #7  
Old 12-08-2009, 03:02 AM
citivolus's Avatar
citivolus (Ric)
Refracted

citivolus is offline
 
Join Date: Sep 2007
Location: Carindale
Posts: 1,178
The downside of length of the dew shield can be that it acts as a sail in the wind. Gusts can have more of an impact than normal with the shield in place, although in gusty conditions I believe you are also less likely to have significant dew.
Reply With Quote
  #8  
Old 12-08-2009, 10:37 AM
mldee's Avatar
mldee (Mike)
Photon sorter

mldee is offline
 
Join Date: Aug 2008
Location: Near Warwick, Qld, Australia
Posts: 657
Quote:
Originally Posted by mithrandir View Post
I've put an Orion FlexiShield
https://www.bintelshop.com.au/Product.aspx?ID=5513
on my C8. Since then, I've had no sign of dew on the corrector, even without a heater and the tube was dripping wet.

However it is heavy enough to need the mount rebalanced.
Being a cheapskate, I wanted to try a DIY dewshield before comitting to any heaters. After a few abortive attempts, I came up with a cheap and simple shield that seems to work and meets the length = 1.5x diameter recommendation for my C8. Finding suitable black dew-proof materials at low cost became a slight hassle. I found Officeworks had some.

It's just 4 matt black plastic covers from two A4 plastic ring binders bought for $1.54 each at Officeworks, stuck together with duct tape, then fixed to the C8 OTA with $5 worth of velcro. I also used the Velcro to fasten the "ends" of the shield, so it can be "unzipped" for removal. I then used double sided tape to stick three .65c matt felt sheets inside, also bought from the same section of Officeworks. Some Photos attached.

Officeworks Plastic folder Item No. 2007002, Felt piece item No. 50541

It still lets the OTA cap be put on with the shield in place.

Not overly elegant, and I'll do the cosmetic touches if it becomes permanent , it's adequately rigid for use in my obs, and a good way to trial if a dew shield is sufficient for your needs. I've had no further corrector plate dew problems since installing it.
Attached Thumbnails
Click for full-size image (Imgp0519.jpg)
100.8 KB74 views
Click for full-size image (Imgp0520.jpg)
105.6 KB54 views
Click for full-size image (Imgp0521.jpg)
80.1 KB63 views
Click for full-size image (Imgp0522.jpg)
121.3 KB58 views
Click for full-size image (Imgp0523.jpg)
97.3 KB62 views
Reply With Quote
  #9  
Old 12-08-2009, 04:44 PM
g__day's Avatar
g__day (Matthew)
Tech Guru

g__day is offline
 
Join Date: Dec 2005
Location: Sydney
Posts: 2,901
Isn't the old saying if you were in a very, very deep well - looking straight up - you'd see stars? I've heard that before - not sure if its true or not - but essentially long dew shields help block stray light - I super size mine!
Reply With Quote
  #10  
Old 15-08-2009, 11:29 AM
Karls48 (Karl)
Registered User

Karls48 is offline
 
Join Date: Jan 2006
Location: Sydney
Posts: 753
Quote:
Originally Posted by g__day View Post
Isn't the old saying if you were in a very, very deep well - looking straight up - you'd see stars? I've heard that before - not sure if its true or not - but essentially long dew shields help block stray light - I super size mine!
It doesn’t work that way. I was on bottom of 380 feet mineshaft of copper mine that we worked long time ago. Looking straight up I could see blue sky above. But when I look down the shaft from the top of mine I could not any reflection of the sky in the water on the bottom. Just a black hole. I suppose if there was bright star overhead I may see its reflection in water below.
Reply With Quote
  #11  
Old 16-08-2009, 10:25 AM
Robert9's Avatar
Robert9 (Robert)
Registered User

Robert9 is offline
 
Join Date: Dec 2008
Location: Mt. Waverley, VIC, Australia
Posts: 741
Mike, that's great with the velcro stuck to the circumference of the OTA corrector. One addition/alternative which just occurred to me is to have a resistance strip inside the velcro band so that you have effectively a dew-heater as well. Then, rather than sticking a long length of velcro to the OTA, probably 3 to 6 spots of velcro (depends I guess on diameter) would suffice to adhere the dew-heater band cum dew-shield attachment to the OTA.
Reply With Quote
  #12  
Old 16-08-2009, 10:38 AM
mldee's Avatar
mldee (Mike)
Photon sorter

mldee is offline
 
Join Date: Aug 2008
Location: Near Warwick, Qld, Australia
Posts: 657
Quote:
Originally Posted by Robert9 View Post
Mike, that's great with the velcro stuck to the circumference of the OTA corrector. One addition/alternative which just occurred to me is to have a resistance strip inside the velcro band so that you have effectively a dew-heater as well. Then, rather than sticking a long length of velcro to the OTA, probably 3 to 6 spots of velcro (depends I guess on diameter) would suffice to adhere the dew-heater band cum dew-shield attachment to the OTA.
Yep, I thought of that approach and tried in in an earlier version. My problem was aligning the dots when putting the shield on (joking)

Actually it was just laziness, it was easier to put a strip on than a few dots. As you may notice from my photos, the C8 is an older, pre-loved version with many battle scars, mostly due to my various experiments, so I wasn't overly concerned with protecting the cosmetics of a multi-thousand dollar Tak, RC or such

I will probably make a less cosmetically-challenged version of the actual shield next week.

I was considering a dew heater as well, using the old-jug-element wire approach, but decided it was all too hard, as I would also have to either acquire an RS-type PWM controller or build/buy one. It turned out that the shield is quite adequate for my needs at the moment. I'll do the dew heater eventually when my inertia subsides a little or dew increases!
Reply With Quote
  #13  
Old 16-08-2009, 11:16 AM
Robert9's Avatar
Robert9 (Robert)
Registered User

Robert9 is offline
 
Join Date: Dec 2008
Location: Mt. Waverley, VIC, Australia
Posts: 741
Mike, do you think you really need a controller for the dew-heater. I have just a number of resistors in parallel wrapped in a bit of cloth and held in place by the omni-present velcro. This draws only a fraction of an amp from my 12V source and provides sufficient heat to avoid dew formation.
The design I used is from IIS Projects.
Reply With Quote
  #14  
Old 16-08-2009, 02:01 PM
mldee's Avatar
mldee (Mike)
Photon sorter

mldee is offline
 
Join Date: Aug 2008
Location: Near Warwick, Qld, Australia
Posts: 657
Quote:
Originally Posted by Robert9 View Post
Mike, do you think you really need a controller for the dew-heater. I have just a number of resistors in parallel wrapped in a bit of cloth and held in place by the omni-present velcro. This draws only a fraction of an amp from my 12V source and provides sufficient heat to avoid dew formation.
The design I used is from IIS Projects.
I daresay you're right, Robert. I do have a tendency to want to over-engineer things!

I might just read up on the project and incorporate the resistors in next week's pimp of the dewshield. Since I will then have 12V up to the scope OTA, I might as well then install the little red LED I have been threatening to put in the plastic eyepiece of my SW 9x50 finderscope, so I can see the x-hairs more easily. Bling is good!

I'm also busy today making a small 12V motor-drive focuser, using Jaycar parts, for my GSO 2:1 Crayford on the back of the C8, using the rubber-band belt drive approach. I'll post a thread on that if it's successful. Unfortunately it involves use of the complete array of my precision hand tool collection; battery hand drill, hacksaw and screwdrivers

Plenty of uses for the 12V supply once it's at the OTA
Reply With Quote
  #15  
Old 16-08-2009, 06:43 PM
Robert9's Avatar
Robert9 (Robert)
Registered User

Robert9 is offline
 
Join Date: Dec 2008
Location: Mt. Waverley, VIC, Australia
Posts: 741
Mike, it sounds as if you're a great constructor - such a fantastic array of tools. I shouldn't laugh, its what one can achieve with them that counts. I'd be very interested to see your mod to your finderscope e.p. to insert the LED. having a SCT, fine focussing is mirror movement although I'm sure a better arrangement could be dreamt up for it at the e.p. end. Probably worth a bit of thought.
What have you got for 12V. I amusing an el cheapo jump start from "Super Cheap". Cost me under $65. Had no problems. Never run it down below about 40-50% and recharge immediately after. Now if I had an astro shed......... I can only wish. My suburban block is just not suitable.
Reply With Quote
  #16  
Old 16-08-2009, 07:08 PM
mldee's Avatar
mldee (Mike)
Photon sorter

mldee is offline
 
Join Date: Aug 2008
Location: Near Warwick, Qld, Australia
Posts: 657
Quote:
Originally Posted by Robert9 View Post
Mike, it sounds as if you're a great constructor - such a fantastic array of tools. I shouldn't laugh, its what one can achieve with them that counts. I'd be very interested to see your mod to your finderscope e.p. to insert the LED. having a SCT, fine focussing is mirror movement although I'm sure a better arrangement could be dreamt up for it at the e.p. end. Probably worth a bit of thought.
What have you got for 12V. I amusing an el cheapo jump start from "Super Cheap". Cost me under $65. Had no problems. Never run it down below about 40-50% and recharge immediately after. Now if I had an astro shed......... I can only wish. My suburban block is just not suitable.
I found mirror flop on the C8 annoyed me, that's why I went to the Crayford before the EP. Gives me three times as many knobs to adjust

I am fortunate to visit Mt Palomar obs quite often, it's been a never ending source of admiration to me since I was a kid in the 50's. I guess that's the sort of scope stability I'm trying to achieve in my little obs.

For power, I started off with a 12V 7AH Jaycar gel cell and small electronic charger, then hung a Jaycar small solar charger on it as well for daytime trickle, (gadgets, you know) then decided a decent 12V supply was better, so bought the GH1379 Jaycar 12V 5A switchmode, only $30, and works well. Very satisfied. Still have the battery for backup, but never use it, it justs sits there on trickle charge.
Reply With Quote
Reply

Bookmarks


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 11:37 PM.

Powered by vBulletin Version 3.8.7 | Copyright ©2000 - 2025, Jelsoft Enterprises Ltd.
Advertisement
Bintel
Advertisement