I keep wondering why some premium dobsonian makers use a 2.14" secondary mirror for their 12.5" scopes. Will the 4% CO difference make any improvement in planetary viewing?
Pls vote for the best secondary mirror size I should order for my ATM project. The objective is to get the best performance from the scope for BOTH planetary and DSOs.
Secondary size
Secondary size
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- Canopus Lim
- Posts: 1144
- Joined: Fri Jun 03, 2005 12:46 pm
- Location: Macpherson
It is always better to have a smaller secondary.... I have seen the effects of a large obstruction. To me it is noticeable. Just compare a SCT to a refractor and you will know what it means high contrast on the moon/planets and what is called low contrast; also that is why reflectors tend to perform better than SCT on planets. By the way, central obstruction affects planetary contrast as it modifies the MTF (Modulation Transfer Function) reducing its low contrast ability..darkness of the sky background depends a lot on the baffling/flocking. A refractor is easier to baffle and hence it can have very dark sky background. Inch for inch nothing beats the refractor.
Also, most of the time in Singapore we are using high magnifications (smaller field) and hence vignetting is NON ISSUE. Also, most of the time we are looking at planets, smaller secondary means better performance.
Lastly, if one is NOT using a paracorr, even if there is vignetting (which I do not observe that easily unless you specifically spent time looking at the edge with the lowest power eyepiece and looking at a star at the limit of visibility.. I don't do that since the telescope performs best on-axis), the coma is already a dominant factor that will detract the view unless you are one of those who do not see coma ..vignetting again is NON ISSUE.
Anyway, for those who are getting Z mirrors, it is really meant for planetary (which is the most demanding thing for the optics)...so it would be wiser to get a smaller secondary to enhance its planetary peformance. The difference in performance is mainly on planetary at very high magnification. We are talking small percentage of improvement..and anything that helps will add up.
So from my experience with my scope.... smaller secondary rules especially in Singapore.
Unless you are a DSO only person and goes to Mersing 12x a year and love to observe at such low magnification...
Also, most of the time in Singapore we are using high magnifications (smaller field) and hence vignetting is NON ISSUE. Also, most of the time we are looking at planets, smaller secondary means better performance.
Lastly, if one is NOT using a paracorr, even if there is vignetting (which I do not observe that easily unless you specifically spent time looking at the edge with the lowest power eyepiece and looking at a star at the limit of visibility.. I don't do that since the telescope performs best on-axis), the coma is already a dominant factor that will detract the view unless you are one of those who do not see coma ..vignetting again is NON ISSUE.
Anyway, for those who are getting Z mirrors, it is really meant for planetary (which is the most demanding thing for the optics)...so it would be wiser to get a smaller secondary to enhance its planetary peformance. The difference in performance is mainly on planetary at very high magnification. We are talking small percentage of improvement..and anything that helps will add up.
So from my experience with my scope.... smaller secondary rules especially in Singapore.
Unless you are a DSO only person and goes to Mersing 12x a year and love to observe at such low magnification...
AstroDuck
- Canopus Lim
- Posts: 1144
- Joined: Fri Jun 03, 2005 12:46 pm
- Location: Macpherson
- Canopus Lim
- Posts: 1144
- Joined: Fri Jun 03, 2005 12:46 pm
- Location: Macpherson
I see, so the portaball also uses 2.14". So it looks the same for the premium scopes. From the protostar website it is either 2.14 or 2.6".
I do not think there is a 'best' solution. There is always a trade off in design. If you want fully illuminated fields for imaging, then your planetary has to go. Vice versa. Unless you get a 12.5" Apo refractor.
Arief,
From what I read you like globs (they tend to be small in size... no greater than 1 degree TFOV) and planets.. so the smaller secondary will be a better choice.
I do not think there is a 'best' solution. There is always a trade off in design. If you want fully illuminated fields for imaging, then your planetary has to go. Vice versa. Unless you get a 12.5" Apo refractor.
Arief,
From what I read you like globs (they tend to be small in size... no greater than 1 degree TFOV) and planets.. so the smaller secondary will be a better choice.
AstroDuck
- weixing
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Hi,
I think you may want to take a look at the distance from diagonal to focal plane... 8.3 inch seem a bit too short to me.
Anyway, if you want the best image for planetary, use 2.14". But if you are more towards DSO, use the 2.6".
Have a nice day.
I think you may want to take a look at the distance from diagonal to focal plane... 8.3 inch seem a bit too short to me.
Anyway, if you want the best image for planetary, use 2.14". But if you are more towards DSO, use the 2.6".
Have a nice day.
Yang Weixing
"The universe is composed mainly of hydrogen and ignorance."
"The universe is composed mainly of hydrogen and ignorance."