Back to the GSO secondary size, I have found with my 200mm f4 that the position of focal plane is actually further away from the secondary than it really needs to be, and at that distance my secondary (according to the OWL Diagonal calculator) doesn't actually give 100% illumination even in the centre of the field. The secondary is simply too small. The solution is either a different slightly longer optical tube to bring the focal plane closer to the secondary, or a larger secondary (or just accept that the scope is performing less efficiently than it needs to). If that was the case with James' 300mm f4 too, then I guess that would necessitate the secondary mirror upgrade.
That's just typical of commercial scopes. They're a compromise between visual and imaging. I'm not sure why they have such long focusing units. Possibly because old style photography needed the racking space for adapters.
Here is my 10 inch f10 newt with a 1 inch diagonal , thats her in 2 pieces .
the Dobbo base , next to Goldie , and the top 1/2 , now you want to see Jupiter in this baby !! .
Dont knock the quality of a mirror Wulfgar , I made and figured both of these , along with their mountings ,
Go buy an 10 inch f/5 , it will give great views up to about 200x , thats where my 5 inch f/13 is just comming on song ..
Dont go here with my M210 ,,,
Brian .
Quote:
Originally Posted by wulfgar
It's long known standard. An F12 Newt with average optics can take on the best. That long focal length isn't as sensitive to optical faults as short focal lengths are. That's something fast Newtonian fans need to be aware of......................your F4 optics have to be good to work well. Any figure errors get multiplied by the fast optics. For that matter a similar rule applies to fast refractors.
Originally I used a 3/4 inch Royal secondary and a recessed eyepiece collar. But I got tired of Hex keying the eyepieces into the collar to change them, so I shifted the main mirror up an inch and put in the one inch secondary.
The downside of course is the step ladder required for Newtonian focal lengths over 50".
Yep there dam big its called the starfinder 16" image below. I had a starfinder eq mount myself and they are built that tough its crazy, they make the eq6 look like a kids toy in terms of build. The only issue is the PE and that leaves them only good for visual. They also made a dob called the starfiinder but just add eq to your search.
I have an almost identical mount made by cave optical in 1970. One question though, what is "the PE" an abreviation of? My mount only has a single axis drive but I have been using it for astrophotograpy for 25 years. Hand guiding with a drive corrector and illuminated recticle eyepiece is not too hard if the scope is drift aligned. I have marks on the concrete slab where the legs go and only have to slightly tweek the alignment. This has been great for imaging comets, eclipses, transits and rock solid for piggybacking telephoto lenses. Never did much deep space prime focus stuff though, mainly due to the hour long exposures needed for 35mm film. I am learning digital......finally.
And the 2 week wait for 1 out of 24 exposurers to be delivered ( for us without dark rooms ) .
I hear you , love this digital age .
Brian
Quote:
Originally Posted by doppler
I have an almost identical mount made by cave optical in 1970. One question though, what is "the PE" an abreviation of? My mount only has a single axis drive but I have been using it for astrophotograpy for 25 years. Hand guiding with a drive corrector and illuminated recticle eyepiece is not too hard if the scope is drift aligned. I have marks on the concrete slab where the legs go and only have to slightly tweek the alignment. This has been great for imaging comets, eclipses, transits and rock solid for piggybacking telephoto lenses. Never did much deep space stuff though mainly due to the hour long exposures needed for 35mm film.
And the 2 week wait for 1 out of 24 exposurers to be delivered ( for us without dark rooms ) .
I hear you , love this digital age .
Brian
And you had to keep a diary to note down frame number, exposure time, focal ratio etc. But it was always a thrill when you got the photos/slides back from the lab and had managed to capture some decent images.
I caught this fireball while imaging comet Hyakutake.
An F12 Newt with average optics can take on the best. That long focal length isn't as sensitive to optical faults as short focal lengths are. That's something fast Newtonian fans need to be aware of......................your F4 optics have to be good to work well.
Wulfgar - Eyepieces, focus and collimation are certainly more forgiving at F12 . A 1/8 wave error on the surface of an F12 mirror causes exactly the same ( 1/4 ) wavefront error as at the focus of an F4 , which has exactly the same effect on the Airy Pattern. What is more time consuming is sculpting a deeper parabola, but the final surface error tolerances are exactly the same.
"Any figure errors get multiplied by the fast optics. For that matter a similar rule applies to fast refractors."
Your post implies that say a 1/2 wavefront F12 mirror will give a better image than a 1/2 wavefront F4 mirror. The Airy Pattern in both cases will be identical .
Rick. Have read that Bintel use GSO bits. If it helps, my 12 inch f5 GSO star tests to show about lambda/7 (wavefront) but it has a little TDE and some minor roughness. Nett result is very good - certainly not perfect, but still nice to look through up to about 500x with my eyes if the seeing allows.
attached image from the scope is a bit seeing affected, but may help.
As others have pointed out, the mechanical construction of these scopes leaves a bit to be desired, but it is good enough as a starting point.
My main dilema is about compromise. I would like larger scope for visual use , but I also like to dabble with astrophotograpy. The 12" f4 has the same tube length as my 8" f6 and fits on the back seat of any small car, but from what I have been reading is not as good visually as an f5. If I could find a 12" f5 ota that would be great, but these seem to be available in dob form. I guess because of the xtra heavy duty mount needed to carry it. Would it be possible to convert a dob tube, using tube rings to fit an equtorial pier mount?
I have a ex-dob 12 f5 on an EQ6 for solar system imaging, but any 12 inch would need a much heavier mount for DSO imaging. what mount do you have?
have you considered a 10inch f5 OTA, which would probably fit in your car, be a big step up visually and be OK for DSO imaging (just) on an EQ6 or (better) a G11?
I am a bit wary about recommending any lower end f4, based on my experience with a 200/f4 - it was stressful, not because of the optics, but because the standard GSO mechanical components are pushed a little past their comfort zone by the alignment tolerances required at this FR.
I have an old motor drive equtorial peir mount... it was designed to carry up to 16" f4.5 / f6 ota's. There is pic of a similar mount on an earlier post in this thread. I would like a big jump in appeture. I am thinking that a bigger scope might fit in the passenger side with the seat down.
I should add that the motor drive runs of 240v mains (I have a variable speed drive corrector that lets it run of a 12v car battery) It also has a fully adjustable clutch plate on the drive to adjust the load.
sorry Rick - I didn't pay enough attention to earlier posts.
Your mount presumably does not have an external guide input, so will probably not be accurate enough for long period DSO imaging. An f5 would be best for visual and solar system imaging if you can make it fit in the car. If you get a Dob you could leave the side plates on the OTA and use it either EQ or Dob mounted. Will need to get some rings made up though - I couldn't find any that were readily available.
Also would recommend a motor focuser if you go in for solar system imaging. Thermal expansion is enough to compromise the fine focus needed and it is a pain (almost impossible) trying to refocus manually.
Wulfgar - Eyepieces, focus and collimation are certainly more forgiving at F12 . A 1/8 wave error on the surface of an F12 mirror causes exactly the same ( 1/4 ) wavefront error as at the focus of an F4 , which has exactly the same effect on the Airy Pattern. What is more time consuming is sculpting a deeper parabola, but the final surface error tolerances are exactly the same.
"Any figure errors get multiplied by the fast optics. For that matter a similar rule applies to fast refractors."
Your post implies that say a 1/2 wavefront F12 mirror will give a better image than a 1/2 wavefront F4 mirror. The Airy Pattern in both cases will be identical .
You mean fast optics will handle stuff like spherical aberration and zonal errors exactly the same?
Andrews has GSO 303mm internal diameter tube rings for $49 a pair, but I think a 12" scopes tube rings would have to be at least 400mm?
I think those 303 rings are for a 10". I put a ruler on this 12" GSO f4 and it's over 360mm.
I don't know of anyone in Australia who sells rings for a 12" tube.
I think some people have bought them from Orion UK but they where pretty expensive as I remember.
Rick its mean Periodic error http://www.cloudynights.com/item.php?item_id=2750 . The worm on mine was aluminum gear with a brass worm, they gears were badly turned and nothing like today's quality. You can get lucky with some parts of the worm gear and others are just bad, then the worm it self was pretty shotty limiting to around 20 secs. The mount has a number of things from the dc gear drive to the plastic bushes used for bearings which cause error. Some people have machined them out and added tapered roller bearings and even a new worm drive which then makes them very nice. For visual and planetery work though these things are the beast, lol gave up on the eq platform when i got mine because the thing could do a way better job
Here is my 10 inch f10 newt with a 1 inch diagonal , thats her in 2 pieces .
OOPs , heres my 10 inch , in its 2 pieces , 2500mm f/l 
And the 2 week wait for 1 out of 24 exposurers to be delivered ( for us without dark rooms ) 
.
I moaned when it hit 50c a gallon . ( MK 1 Zephyr ) and the fuel was pink. 
