Blind-Cyclops
sage
Reged: 11/28/05
Posts: 488
Loc: Kitchener, ON, Canada
|
|
Hello folks,
Many people (including me) may be interested in hearing
from folks with experience with reflectors having focal
ratios of f7 - f10.
I understand that shorter focal rations (f4-f5) create
many problems for folks trying to select eyepieces.
If anyone has had experience with reflectors (small or
large) with focal ratios of f7 - f10 or longer, please
share your experiences and knowledge (pros and cons)
with us.
If you looked through such a scope, would you like
to own one? ...and why?
Thank you,
Duncan
--------------------
Clear skies...
Duncan 
"Watch the skies, everywhere! Keep looking. Keep watching the skies!"
-- Closing line in movie spoken by newspaper report Ned "Scotty" Scott (Douglas Spencer) in the Sci-Fi movie
"The Thing From Another World", RKO Radio Pictures, 1951.
Antares (refractor) 127mm f/6.45. w/2-spd Crayford
Orion (Maksutov) 150mm f/12 w/2" EP adapter
Giro 3 (twin), 18" pier, EQ5 tripod.
Garrett 20x80mm, 410 head, 055 tripod.
|
grendel
sage
Reged: 04/12/09
Posts: 241
Loc: Canterbury, Kent, UK
|
|
I have a 6" f10 mirror to make up into a scope sometime., I just have a few other projects to finish first.
grendel
|
perfessor
sage
Reged: 12/30/07
Posts: 499
Loc: Northern Illinois
|
|
There was a recent thread on this topic . You might want to browse through that, a lot of good scopes and discussion.
--------------------
Tom
"Don't always know what I'm talkin about"
8" f/7
|
Biff
Carpal Tunnel
Reged: 09/04/05
Posts: 2372
Loc: Courtice, Ontario
|
|
I have a scope on the go using a 4.25" F/11.6 (IIRC) spherical mirror. TBH I was pretty shocked at the crisp views it gave of Saturn a while ago when I had it together to check final dimensions.
Such an instrument would pretty much be dedicated to higher power viewing but does a very good job of it.
--------------------
Ryan
Antares 200mm f/6 Dob & 130mm f/5 Travel Dob.
Projects on the go...
- a couple 80mm SS refractors on the back burner.
- a few small mirrors awaiting polishing
- 260mm f/7.15 mirror... still polishing
Member of DRAA
 My house.
 DRAACO
|
grendel
sage
Reged: 04/12/09
Posts: 241
Loc: Canterbury, Kent, UK
|
|
I have also just purchased a small (65mm) reflector at f7.7 so when I have had a chance to look through it I will let you know how it goes.
grendel
|
Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3121
Loc: Northeast
|
|
An F/7 Newt is diffraction limited at around 3/8ths inch at the focal plane. An F/10 is DL at about 3/4". This means it's essentially as good as it gets; and all without a corrector or other additional elements. These are fine scopes for planetary, and detailed medium/high power viewing. The simpler eyepieces like Orthos are fantastic with such scopes.
I had a 6" F/10 for quite a few years. Collimation was forgiving and it most always provided a very pleasing view on all but the cruddiest of nights. I observed the big comets, and the SL impact with that scope. Theory says that coma was at the edge of the field, but you had to really pay attention to that fact to notice it at all.
I had a nice 6" F/8 once, and it was the first scope I reached 100x/inch on the moon. Generally, it showed this DL field at about half the low power field, a good compromise. A little shorter and handier especially for a driven mount.
Generally, you can easily make a planet killer out of these longer scopes. A good combo was a 6" F/10 mirror with 1" secondary. They'd beat the pants off a 5" apo easily on everything except really low powers.
One prevailing myth about making long mirrors seems to hang on persistently: ease of mfr.
Theory being, that a long sphere is easy to make. Yes, they do eliminate the necessity of calculating the error through parabolization, but that's it. With long mirrors, the tendency to have a rolled edge increases (more than linearly, sorry to say) and things happen very fast with smoothing efforts. They're what I call squirrelly. Takes some practice, and a deft hand when smoothing one of these mirrors.
-BUT- when done right, there's nothing like 'em for delivering the goods!
Mark
--------------------
So many projects, so little time!
|
HiggsBoson
Pooh-Bah
Reged: 02/21/07
Posts: 1118
Loc: Kal-li-fornia
|
|
I remain very pleased with my 6” F/9. As stated elsewhere collimation is forgiving. I did it with an old film can the first time*. I am pleased with the views and to date have not seen a refractor for which I would trade.
I get 1.3 degrees with a Pan 27mm and 275x with a Radian 5 mm. It is my only scope.
* Now I use CATSEYE tools.
--------------------
Michael
ATM: 6" F/9 Newtonian Travel Scope
ATM: 12.5" F/4.5 Real Soon Now...
|
Luigi
Post Laureate
Reged: 07/03/07
Posts: 4933
Loc: MA
|
|
>>>I remain very pleased with my 6” F/9<<<
Sounds like it's a great scope. For my money though, a 9" f/6 (same focal length as the 6" f/9) can have a similar percentage CO for the same illuminated field and 50% greater resolving power. It will produce a larger exit pupil and because of that brighten the background. I can put on sunglasses if I find that troubling.  Of course,it'll also have a bit more coma and might be somewhat more demanding of the EP, though in my experience these are not significant factors at f/6.
|
oldtimer
sage
Reged: 11/13/08
Posts: 299
Loc: NW Illinois
|
|
I have been using something close (a 10" F6.65) newt for many years. It has given me the best planetary views I have ever had with the exception of a 16" F8 observatory newt. It has a low profile very early prototype Feathertouch focuser and a 1.83" secondary.
I also have owned several 4.25 F10.5 Edmund newts with spherical mirrors. They show amazing planetary detail for their size.
If you don't mind the size and weight the long focus 6 to 8 inch newt (with quality optics) is still going to beat the pants off any 5" APO and for a lot less $.
|
StarStuff1
scholastic sledgehammer
Reged: 04/01/07
Posts: 973
Loc: South of the Mason-Dixon Line
|
|
In the last few years I have built (with purchased mirrors)a 10-in f/9 and a 6-in f/10. Both gave wonderful,flat field views even with inexpensive Kellner eyepieces. The 10-in was such a beast, though.
To me, a 9 or 9.25-in f/7 would be a great compromise scope. I would give up my very nice 8-in f/6 in a heartbeat. Too bad I don't desire to push glass.
--------------------
Tools that make objects very far away appear much closer than they actually are.
|
Blind-Cyclops
sage
Reged: 11/28/05
Posts: 488
Loc: Kitchener, ON, Canada
|
|
Thank you for all the wonderful replies.
I hope some folks obtained some good information.
The link to the other similar topic provided by "perfessor" was also very good -- thanks Tom.
Here is why I thought this topic might be of interest to many folks...
Everyone is aware that the larger aperture scopes -- even the f4's - f5's need a ladder,
and this leads to safety issues, both for the scope owner and to star party guests.
In my latest issue of SkyNews (Canadian) I spotted a small advert about a new scope.
I visited their web site... http://www.planewaveinstruments.com/index.php
and saw a 20 inch f6.8 scope with an OTA length of only 50 inches.
Suddenly a thought hit me....
Why not have a cassegrain reflector (just 2 mirrors) on a dob mount with the EP on the end of the tube?
That would shorten the tube length by 50% and leave the observer standing on the ground behind the scope.
Of course this will require a slightly higher dob mount for observing comfort (standing or slightly bent over)
depending on the altitude being observed.
This would allow large apertures of 24-30-36-40 inches with an f7-f8 focal ratio
(a known "sweet-spot for apo refractors) and still have a tube length approximately
equal to many smaller aperture f5 Newtonian dobs.
Your comments are welcome.
--------------------
Clear skies...
Duncan
"Watch the skies, everywhere! Keep looking. Keep watching the skies!"
-- Closing line in movie spoken by newspaper report Ned "Scotty" Scott (Douglas Spencer) in the Sci-Fi movie
"The Thing From Another World", RKO Radio Pictures, 1951.
Antares (refractor) 127mm f/6.45. w/2-spd Crayford
Orion (Maksutov) 150mm f/12 w/2" EP adapter
Giro 3 (twin), 18" pier, EQ5 tripod.
Garrett 20x80mm, 410 head, 055 tripod.
|
Biff
Carpal Tunnel
Reged: 09/04/05
Posts: 2372
Loc: Courtice, Ontario
|
|
I think the issue with that is the relatively large secondary mirrors that cass's need, if I was looking at the same scope you were looking at the central obstruction is almost 40%. A general rule of thumb is anything above 20% kills contrast for planetary viewing.
As with everything there's a tradeoff somewhere and it depends on where your priorities lie. An added 'bonus' of the long focus newts is that you can get away with a smaller secondary mirror which further helps their planet killer reputation.
If you're looking for a large scope with a reasonable EP height maybe look into the Cheifspiegler. It's a sort of off axis/folded newt that is the brain child of a member here, Ed Jones. I'm dangerously close to ordering some glass to make a 8" f/8 or f/10 just to see what all the commotion is about. Down side is it's a more complicated design that may require custom optics and be a little more fiddly to get setup and tweaked properly but I hear that the views are worth it. Again with the trade offs.
--------------------
Ryan
Antares 200mm f/6 Dob & 130mm f/5 Travel Dob.
Projects on the go...
- a couple 80mm SS refractors on the back burner.
- a few small mirrors awaiting polishing
- 260mm f/7.15 mirror... still polishing
Member of DRAA
My house.
DRAACO
|
Blind-Cyclops
sage
Reged: 11/28/05
Posts: 488
Loc: Kitchener, ON, Canada
|
|
Hello Ryan,
Thanks for the info.
I figured an f7-f8 would be really good,
if it was a large aperture -- 20-24 inches.
No need for two scopes -- one for lunar/planets
and one for DSO's.
Cheers,
--------------------
Clear skies...
Duncan
"Watch the skies, everywhere! Keep looking. Keep watching the skies!"
-- Closing line in movie spoken by newspaper report Ned "Scotty" Scott (Douglas Spencer) in the Sci-Fi movie
"The Thing From Another World", RKO Radio Pictures, 1951.
Antares (refractor) 127mm f/6.45. w/2-spd Crayford
Orion (Maksutov) 150mm f/12 w/2" EP adapter
Giro 3 (twin), 18" pier, EQ5 tripod.
Garrett 20x80mm, 410 head, 055 tripod.
|
Luigi
Post Laureate
Reged: 07/03/07
Posts: 4933
Loc: MA
|
|
>>>I figured an f7-f8 would be really good,
if it was a large aperture -- 20-24 inches.
No need for two scopes -- one for lunar/planets<<<
A 17.5 f/5 has a 3.1" secondary for a 18% CO. A 20 or 24" f/5 with equal field illumination could have an even lesser %CO. Going to a longer focal length and/or smaller secondary would make no detectable improvement on planets. Falling off the taller ladder would certainly make you see stars though.
--------------------
17.5" f/5 Dob. IM-715 MCT. 120ED. Lunt 60mm Ha.
Zeiss, Leica, Fujinon, Nikon, Pentax, Bushnell bins
|
skywolf856
journeyman
Reged: 01/25/08
Posts: 9
Loc: New Baltimore, Michigan (North...
|
|
Here is my 8" F7 Newt I purchased from University Optics back in the late 70'S as an OTA with a 2" focuser setup for AP.
It now rides on my Celestron CG-5 sharing it with my C-6S.
I love it!
|
ed_turco
member
Reged: 08/29/09
Posts: 27
|
|
You haven't lived until you've looked through a well made and well figured 6" f/10 with an optical window. You simply haven't.
|
Previous
Index
Next
Threaded
Edit
Reply
Quote
