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Thread: Help Please on Cassegrain Baffle Source

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    Default Help Please on Cassegrain Baffle Source



    Hi All! I'm a newbie to the forum but not to amateur astronomy as I have been involved in the hobby since I was 13 and built a 12.5" classical Newtonian and 8" Classical Cassegrain in the mid 80's.

    Recently, I have restored both scopes making improvements as I went along. I was never able to get a perfect image on my Cassegrain even with excellent collimation and just now realized that my light baffle (2" OD") was actually too short. I bought it from Novak and at the time it was the longest baffle offered at 2" OD. My math on the length was also definitely off but it was probably influenced by the insufficient length of the available baffle.

    I could not find a source for Cassegrain Baffles on the Internet and just using 2" OD aluminum tubing is not really and option due to the taper hole of the primary mirror.

    My request is for any Cassegrain Baffle source or resources so that I may complete my project.

    Thank you in advance for your assistance.

    Bill

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    Default Re: Help Please on Cassegrain Baffle Source

    Hello and welcome, now there is a request I have not dealt with before...
    Currently the brain trust is located elsewhere.

    While I am checking to see if I can find anything to help, please take a look at this post from the site moderators
    Announcements - Forum Welcomes Introductions
    Refractors: Antares 105 f/15, Celestron 150 f/8, Stellarvue NHNG 80 f/6.9, TAL 100RS f/10, TS 102 f/11, UR 70 f/10, Vixen SD115s f/7.7
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  3. #3
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    Default Re: Help Please on Cassegrain Baffle Source

    I check in here very infrequently now and urge you to join TSS

    Fortunately I considered making my own Cassegrain in the past. There are two resources I can recommend for the baffling problem:

    Chapter 19 of https://www.willbell.com/tm/tm6.htm contains maths considerations from which you can derive your baffling requirements and design.

    In addition Chapter 9 of https://www.willbell.com/tm/tm3.htm has a description of a particular project but not the reasoning from first principles that you need.

    I wish you luck on your restoration and hope to see you report back on the other forum!
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    Default Re: Help Please on Cassegrain Baffle Source


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    Default Re: Help Please on Cassegrain Baffle Source

    Thank you for the assistance!

    Unfortunately, I have found all but one of these resources on the Internet prior to my original posting. I also have the Second Edition of Novak's Cassegrain Notes from 1979 which was instrumental in my original build in 1986.

    There are 2 specific areas for which I need help:
    1) All baffle length formulas require a value for "Final Image Size" for which there is no information provided on how this is determined. I lost the original specs to my Enterprise Optics 8" f16 optics set but do know everything else (separation distance, etc.) required for the various formulas from my build notes. Does anyone know how to calculate Final Image Size?
    2) Regardless of the baffle length, I still need to find a source for a baffle tube. Does anyone know of a vendor or source?

    Thank you all. I'm only a baffle tube away from finishing this scope and any information is greatly appreciated.

    Bill

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    Default Re: Help Please on Cassegrain Baffle Source

    The final image size is not "determined", you specify it, so it's easy. What size image do you want? Are you using 1.25" or 2" eyepieces for instance? What is the size of the sensor you are using if for astrophotography? The inner diameter of your focuser is a good starting point. But if you have chosen a large focuser the baffling will be less effective unless you accept vignetting.

    The question is really "what is the size of the 100% illuminated image you desire". You might need to consider accepting some vignetting if you've chosen a focuser that is too large for the optics.

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    Default Re: Help Please on Cassegrain Baffle Source

    This may work and probably much cheaper than getting a small piece through a astro retailer.
    https://www.grainger.com/category/pl...luminum-tubing
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    Refractors kick arse precisely because they don't hide behind excuses. That is, they have no obstructions to hide behind. - Jon Isaacs (CN)

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    Default Re: Help Please on Cassegrain Baffle Source

    Quote Originally Posted by Gabby76 View Post
    This may work and probably much cheaper than getting a small piece through a astro retailer.
    https://www.grainger.com/category/pl...luminum-tubing
    Of course it needs to be flat blackened in and out.....

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    Default Re: Help Please on Cassegrain Baffle Source

    Hi again All!

    Thank you for your quick and informative replies. I can report that there seems to be no resource for Cassegrain Light Baffles. I have had an offer from a fellow ATM and supplier to assist with fabrication of an extension for my baffle and will report on it when ready. This tweaking was the last task required to complete my 8" Classic Cassegrain. I'm currently installing a Byers Drive on my 12.5" Newtonian (need to take it slow and fabricate a bracket, etc.) but I've found my next project to keep me busy while I am in the hurry-up-and-wait mode of my current projects.

    Doing searches for items that I was looking for regarding the baffle, I found a Criterion Dynascope RV8 for $100 and could not pass it up. It seems to be all there except for the Clock Drive. I will probably look for a Dynascope forum for some restoration ideas, parts and substitutes. Unfortunately the Barlow is chipped and all 4 eyepieces are more or less toast (excessively dirty and the black paint on the tops are chipping). Whoever originally purchased it went for 2 high power ARs of very short focal lengths (4, 6, mm) and a 9 and 18. The 18 is the only one with a reasonable aperture by today's standards. The scope itself seems to be in better condition than the flaking paint, tarnish and dirt would first indicate. I'm really looking forward to work on this project and invite comments, advice and introductions to resources.

    Regards,
    Bill

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    Default Re: Help Please on Cassegrain Baffle Source

    Hi Not_Fritz,

    Thank you for the information regarding image size considerations. I have a 2" focuser on my Cassegrain and it is a f16. Rather than trying to "do the math", I'm taking a trial-and-error approach regarding final Baffle length. Since the Secondary is a 4X multiplier, minor changes of the mirror separation distance produces larger changes to the Back Focus and where the image appears behind the Primary (Baffle End, Glare Stop, Focuser Barrel, 2" first surface Diagonal, Eyepiece). The long focal ratio alone will limit my shorter fl eyepiece selections and I've collected quite a few over the years. I would be interested in which eyepieces you feel would perform best. I realize that when I read most negative eyepiece reviews that many times the viewer has inadvertently chosen an eyepiece type and fl that is not optimum for their instrument. I have an old 25mm OR (EDSCORP/Circle T) that still holds its own.

    BTW, YES I have used Krylon's Industrial Ultra-Flat Black spray paint for interior OTA surfaces and parts now for 35 years. It dries quickly and is the visually flattest black I've ever seen when dried. It seems to micro-flock giving more actual paint surface/s to absorb light. Unfortunately, I can no longer find it locally and must order via online.

    Again, thank you for taking the time to address my ATM challenges.

    Best,
    Bill

 

 
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