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Optical window or just use the MFR-3?

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#1 smokin oakum

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Posted 10 January 2008 - 07:11 AM

My question is two fold. Why would one need an optical window to seal out dust and dew if you use the MFR-3 most of the time. The only time I figure I won't use it is when I need more mag, and even then, couldn't I use the zoom feature.

My second question is, do you guys know what f ratio for my f/4.9 becomes with the MFR-3, then with the MFR-3 and the 5mm extension ring, then with the MFR-3 and the 5mm, and the 10mm extension rings? I've never been able to figure this stuff out, and this is actually the first time I've used a focal reducer on my DOB. Thanks for your help.

Still no sucker holes for my first light.
Stay tuned
Neil

#2 Jack Huerkamp

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Posted 10 January 2008 - 07:19 AM

Neil,

On my 8' f/4 Newtonian, I don't need or use an MFR-3. I do use the Optical Window to help isolate the cooled CCD sensor from the moisture in the air.

If you are using the MFR-3, you don't need the Optical Window.

Jack

#3 smokin oakum

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Posted 10 January 2008 - 07:26 AM

Thanks Jack. Can you belive my CSC! I've had blue in there for weeks, up until I recieved my camera. If you live near me, I'm sorry! The curse holds true I guess.

Any Idea, of how to find out what my f ratio would be with the rings? Is there a chart or something?

Neil

#4 ccs_hello

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Posted 10 January 2008 - 07:30 AM

Neil,

If you can measure the MFR-3 lens' focal length (or find it in the actual spec.), then it's easy to calculate the net result. Focal Reducer calculator http://timosastro.1g...calreducer.html

Clear Skies!

ccs_hello

#5 Jack Huerkamp

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Posted 10 January 2008 - 07:32 AM

Neil,

Look at the graph I posted in the MallinCam Group Files section:

http://tinyurl.com/2ryhwk

It shows the effects of adding the MFR-3 and spacers to the camera. Based upon Greg Stones measurements on images he took, I developed the graph.

Your f/4.9 scope changes to f/3.8 with just the MFR-3.
It becomes f/3.5 with the addition of the 5mm ring.
It becomes f/3.2 with a 10mm ring.
With 15mm of extension it becomes f/3.

Again this is all based on measurement data Greg obtained by taking images with different camera-focal reducer-ring combinations and then measuring the separation of certain stars on prints.

Jack

#6 smokin oakum

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Posted 10 January 2008 - 07:36 AM

I found the Focal Reducer Calculator but I don't have the specs for the MFR-3. Do you have them Jack? I looked on the Mallincam web site without any luck.

Neil

#7 smokin oakum

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Posted 10 January 2008 - 07:37 AM

Sorry Jack! You're quicker than I can type.

#8 rolandskythree

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Posted 10 January 2008 - 01:14 PM

Jack, where do you get the optical window. I want one. Roland


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