#1  
Old 02-05-2008, 09:20 AM
Kyle
Registered User

Kyle is offline
 
Join Date: Apr 2008
Location: Newcastle
Posts: 6
Question Focal Reducers

Hi there
my recent aquisition of a Mak pro 150 (1800x150) on a HEQ5 goto is quite impressive. There is one thing i dont understand. When a camera (dslr) is mounted, why is there a massive vignette on the images? it looks sweet when i look through it..
is this something to do with chip size v adapter size?
suggestions have brought forward a focal reducer. what is it and why would i want to reduce my nice 1800mm to say 1200?

if that is what i need, what would be a sufficient reduction?
Also, what eyepeice would be needed? or is this an image dependant question?

TIA.
Kyle
Reply With Quote
  #2  
Old 02-05-2008, 10:37 PM
Jeff's Avatar
Jeff
Starry Eyed

Jeff is offline
 
Join Date: Dec 2007
Location: Wonga Park
Posts: 692
Hi Kyle,

Not sure I understand your setup ... are you saying that you're using the Focal Reducer attached to an Eyepiece, and then have the camera?

I'm not familiar with this technique, attaching the focal reducer to my DSI CCD camera then inserting it "prime focus" the focusser (without an eyepiece). Is that what you should be doing (with the aid of a T-Ring and Prime Focus Adapter ), or are there more cunning methods afoot that I'm still noobie oblivious to?

Cheers,
Jeff

Last edited by Jeff; 02-05-2008 at 11:17 PM. Reason: typos
Reply With Quote
  #3  
Old 03-05-2008, 04:27 PM
Kyle
Registered User

Kyle is offline
 
Join Date: Apr 2008
Location: Newcastle
Posts: 6
well im still a noob myself, but atm i have a camera adapter with a 10mm eyepeice in it, then a t-ring onto my dslr.
on the images, there is vignetting on the edges and i have been told to use a focal reducer, which goes infront of the camera adapter, ie, right off the scope, to fix this.

I guess my main question is, what type would i need?

thanks for the reply.
Reply With Quote
  #4  
Old 04-05-2008, 01:25 AM
Jeff's Avatar
Jeff
Starry Eyed

Jeff is offline
 
Join Date: Dec 2007
Location: Wonga Park
Posts: 692
Okay, I think I'm with you now.

Sounds to me like the exit pupil of the image is too small when it hits the CCD (as you suspected). Since you're using a 10mm eyepiece with a f/12 scope, exit pupil will be only 0.83mm, and therefore only illuminating part of the CCD.

ie. Image Exit Pupil = 10mm / 12 = 0.83mm

Suggest you first try using a longer focal length eyepiece to increase the field of view and Exit Pupil size.

I image without an eyepiece (prime focus), and just use a focal reducer when I want to get a wider field of view such as the moon or a large nebula. There are quite a few availabe ... I'm using a $49 GSO budget version while I learn the ropes.

Bintel Focal Reducer 0.5x & 25mm extension (1.25")

https://www.bintelshop.com.au/Images/Stock/7458.jpg
$49.00
Stock code: 7458
New Bintel 0.5x focal reducer is threaded to screw onto any 1.25¨ eyepiece barrel and provides a nominal reduction in magnification of 0.5x.
Double the amount of sky on your DSI chip!
This reducer will be of particular interest to anyone with DSI or Starshoot type imagers where it is difficult to fit much sky on the chip of the imager.
Halving the effective focal ratio of the telescope means shorter exposures will show the same or even more detail than at the original f/number
The actual focal length reduction will vary a little over or under 0.5x depending on where the focal reducer is placed in the optical path.
For years people have purchased Barlow lenses to double the magnification of their eyepieces. Now you can halve the power of any eyepiece you own.
The reducer features 2-element fully multi-coated optical design and blackened lens edges in a black anodize aluminum filter cell. It also has additional filter threads, so you can screw/stack additional filters on to this reducer. Included is a 25mm extension tube to allow variation in the effective reduction.
Without 25mm extension tube reduction is around 0.7x
With 25mm extension tube reduction should be close to 0.5x original image scale.
Some Newtonian and refractor telescopes may have difficulty reaching focus with the reducer attached to an eyepiece. For example:
Focal reducer without extension tube requires ~10mm in travel to reach focus.
Focal reducer with 25mm extension tube requires ~35mm in travel to reach focus.

Cheers,
Jeff
Reply With Quote
  #5  
Old 04-05-2008, 10:25 AM
Kyle
Registered User

Kyle is offline
 
Join Date: Apr 2008
Location: Newcastle
Posts: 6
well that answers my question. i guess i need to to some more shopping

Thanls very much for your help.

Kyle

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 07:39 PM.

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