My post is a little blurry between equipment and solar system observing but I chose to post here because I didnt want to start a "my scope is better" debacle on the other forum. I am building a dedicated planetary scope not bigger than feet on ground. It will be either a 10" F6 or a 12.5" F5. I know many of you have years of experience in planetary observation with many scopes. Would you trade 2.5 inches of aperature for a little longer focal length mirror maybe getting a better figure? I am leaning toward the 12.5 F5 to get the extra resolution. The seeing at my planetary site is consistently fair to good P5 to P6. I know I couldnt go wrong with either scope. Any comments about your FIRST HAND visual experiences comparing planetary observations in similar scopes is appreciated. thanks, Vince
Experienced visual planetary observers
Started by vbert, 05/04/2004 05:27PM
Posted 05/04/2004 05:27PM
Opening Post
Posted 05/04/2004 05:59PM
#1
I have a 10" f/6 with full thickness mirror with an outstanding figure. I also had a 12.5" f/5.2 modified truss tube, full thickness mirror and excellent figure. I live in central FL with good to excellent seeing most of the time.
I could never see any major improvement in the 12 vs the 10, so sold the 12.
I recently purchased an older orange tube C8 that turns out to be one of those one in a thousand. My 10" has been gathering dust ever since. It only goes to star parties under really dark skies where the extra photons will be put to use.
I would say go for the 10" with an excellent figure, or maybe an 8". Mark Harry and others are noted for some really fine mid size mirrors that make killer planetary systems.
Walt
I could never see any major improvement in the 12 vs the 10, so sold the 12.
I recently purchased an older orange tube C8 that turns out to be one of those one in a thousand. My 10" has been gathering dust ever since. It only goes to star parties under really dark skies where the extra photons will be put to use.
I would say go for the 10" with an excellent figure, or maybe an 8". Mark Harry and others are noted for some really fine mid size mirrors that make killer planetary systems.
Walt
Posted 05/05/2004 04:46AM
#2
Vincent,
You have received good advice on your instrument choices but I will add my 2 cents worth. Both the 10" f/6 and the 12.5" F/5 Newtonian reflectors can make excellent planetary instruments. The difference in focal lengths is not significant (1,524mm (10" f/6) vs. 1,588mm (12.5" F/5)). For planetary observation and imaging I would prefer a longer f/ratio as the cone of light is less steep (f/5 vs. f/6). The size of the secondary in the 10" f/6 Newtonian may be theoretically smaller than that used on the larger 12.5" f/5 instrument (preferably less than 20% obstruction). The figure on the 10" instrument may be easier to produce than that of the 12.5" instrument (possibly even to 1/6 to 1/8th wave). The optical axis (collimation) maybe harder to maintain in the larger instrument as well. The 10" f/6 instrument may be easier to move around unless both are made using a truss-tube design. If your seeing conditions are typically P5 to P6 then the 10" instrument may be the better performer in the majority of observing sessions (obviously the 12.5" instrument should theoretically resolve more detail if the seeing imroves to >6/10; please refer to my observations of Mars made using an excellent 12.5" f/6.2 Newtonian in 1986 http://www.astromart.com/messages.asp?message_id=207770&page= ). These are just some factors to consider in your choice of instrument.
I also prefer to be as comfortable as possible when observing the planets. Standing upon a ladder for an extended period of time (over 30 minutes and even up to 60 minutes or more!) waiting for that brief moment of steady seeing really takes it's toll on the observer. Fatigue will result in the observer missing fine detail as the body (and therefore the eye) is strained. For this reason I also prefer the Cassegrain design (Classical or Dall-Kirkham) as Jeff Beish (who is a world class ATM) suggests. A well constructed (including baffling) Cassegrain with a small secondary (<20%) can make an exceptional planetary instrument. Placed upon a solid mount this instrument can reveal all that there is to see over a planetary atmosphere or surface.
I believe that either the 10" f/6 or the 12.5" f/5 reflectors will make excellent instruments for your planetary observations but the above are some factors to consider in your final choice.
Carlos
You have received good advice on your instrument choices but I will add my 2 cents worth. Both the 10" f/6 and the 12.5" F/5 Newtonian reflectors can make excellent planetary instruments. The difference in focal lengths is not significant (1,524mm (10" f/6) vs. 1,588mm (12.5" F/5)). For planetary observation and imaging I would prefer a longer f/ratio as the cone of light is less steep (f/5 vs. f/6). The size of the secondary in the 10" f/6 Newtonian may be theoretically smaller than that used on the larger 12.5" f/5 instrument (preferably less than 20% obstruction). The figure on the 10" instrument may be easier to produce than that of the 12.5" instrument (possibly even to 1/6 to 1/8th wave). The optical axis (collimation) maybe harder to maintain in the larger instrument as well. The 10" f/6 instrument may be easier to move around unless both are made using a truss-tube design. If your seeing conditions are typically P5 to P6 then the 10" instrument may be the better performer in the majority of observing sessions (obviously the 12.5" instrument should theoretically resolve more detail if the seeing imroves to >6/10; please refer to my observations of Mars made using an excellent 12.5" f/6.2 Newtonian in 1986 http://www.astromart.com/messages.asp?message_id=207770&page= ). These are just some factors to consider in your choice of instrument.
I also prefer to be as comfortable as possible when observing the planets. Standing upon a ladder for an extended period of time (over 30 minutes and even up to 60 minutes or more!) waiting for that brief moment of steady seeing really takes it's toll on the observer. Fatigue will result in the observer missing fine detail as the body (and therefore the eye) is strained. For this reason I also prefer the Cassegrain design (Classical or Dall-Kirkham) as Jeff Beish (who is a world class ATM) suggests. A well constructed (including baffling) Cassegrain with a small secondary (<20%) can make an exceptional planetary instrument. Placed upon a solid mount this instrument can reveal all that there is to see over a planetary atmosphere or surface.
I believe that either the 10" f/6 or the 12.5" f/5 reflectors will make excellent instruments for your planetary observations but the above are some factors to consider in your final choice.
Carlos
Posted 05/05/2004 02:34PM
#3
Vincent,
I have a homemade 10" f6.3 that has an excellent figure. Uses a 2" secondary (20% co). Provides beautiful planetary images. I recently completed a 12" f5 travel scope with a 2.625" secondary (22% co). The primary is a Guan Chen (Tiawan) optic with a decent figure based on a ronchigram I performed.
I was shocked about a month ago that the 12" gave me the "best" view of jupiter I have ever seen! I only regret that I didn't have the 10" out the same night for a side by side comparison. From memory though, even on nights of great seeing, the 10" has never given an image that good. Based on that I would say that a 12" can provide better images, but I agree with everything Carlos has said about seeing, figuring, central obstruction, etc...
By the way, I am currently building what I think will be my ultimate planetary/lunar scope... a 10" f8 w/ a 1.375" seconday and curved vein spider. Of course it will be confined to my back yard. This atm stuff is really additive. Good luck and clear skies.
Bill
I have a homemade 10" f6.3 that has an excellent figure. Uses a 2" secondary (20% co). Provides beautiful planetary images. I recently completed a 12" f5 travel scope with a 2.625" secondary (22% co). The primary is a Guan Chen (Tiawan) optic with a decent figure based on a ronchigram I performed.
I was shocked about a month ago that the 12" gave me the "best" view of jupiter I have ever seen! I only regret that I didn't have the 10" out the same night for a side by side comparison. From memory though, even on nights of great seeing, the 10" has never given an image that good. Based on that I would say that a 12" can provide better images, but I agree with everything Carlos has said about seeing, figuring, central obstruction, etc...
By the way, I am currently building what I think will be my ultimate planetary/lunar scope... a 10" f8 w/ a 1.375" seconday and curved vein spider. Of course it will be confined to my back yard. This atm stuff is really additive. Good luck and clear skies.
Bill
Funding Member
