4-inch to 8-inch: Not Enough?

Anyone out there who has had the experience of owning a 4-inch premium refractor, such as AP, Takahashi, or Televue, and then adding an 8-inch Dob?

I'm particularly interested in performance on galaxies and globulars. Can you describe comparatively what you see on M13, M3, M81, M51, M33, M101?

The question I' m trying to answer is whether to buy the 8-inch now, or save my bucks until I can get something in the 14-inch range. If the 8-inch provides an appreciable improvement, then of course I can buy it, use it, and sell it later, as I work my way up (is it up?) to a serious case of aperture fever.

My concern is that when differences in contrast are figured in, I may see brighter images, but not much more detail in the 8-inch, and it would just be a waste of time.

Replies to previous posts have yet to really answer the question for me, so I am trying again with a different question.

Steve

hi,

i have both and the 8" wins on DSOs hands down. It has 4x the light gathering capacity and a smaller airy disk then the 4 and it shows.

I also have a 10 and a 12 1/2" and the next really noticable step past the 8 is the 12 1/2.

A 8" F6 well made dob with a premium mirror will run circles around a 4" anything on DSOs.

My advice ... bring you 4" to a star party and set it up next to someone with a 8" and look see.


clear skies,

Larry Citro

"Hi Steve,

IMO, you won't notice much difference from the 4" APO and a 8" dob. I have the 4" Takahashi and a 6" dob and actually see more in the APO, DSO's included. On another forum there was some discussion about comparing the various designs (Warning: lots of controversy around this), and the suggestion was a 4" APO would be roughly equivalent to a 7" reflector. The problem is you are really comparing apples and oranges with light grasp vrs contrast, etc, etc, etc.

Anyway, I stick to my comments, save your cash and get the 12/14 inch when you're ready.

Mike"

Mike not to be rude or anything but this is typical APO owner hype of the performance of apo's. To say that you will not see much difference between a 4" scope and a 8" scope is funny really. Lets do the math real quick here: Back to geometry class, Formula for the area of a circle: A=3.14r^2... Now lets plug in: 3.14(2)^2=12.54in for the 4" APO. Now the 8": 3.14(4)^2=50.24. In other less mathematical words you are gathering 4 times!!!!!!! More light with the 8" scope. And you are going to go and tell folks that there is not much difference?? Basically it comes down to this: A 4" APO performs like a 4" scope when it comes to DSO. On planets it might come close to a 6" reflector when the seeing is not all that good. Nothing special about APO's they are just like any scope: they gather light and can’t magically defy the laws of physics.

Hi Steve.

I have almost the same doubt, but in my case to upgrade from a SCT 5" to a SCT 8".
Specifically this comparison (premium refractor 4"x dob 8") I never have experienced but last weekend for the first time I had the opportunity to make a comparison side by side between my Nexstar 5 and a Nexstar 8 during almost 6 hours.
I noticed a slightly better contrast for nebulas and globulars but just a tenuous, hardly noticeable difference for galaxies.
I confess I was expecting a "WOW, what a difference !!" but this not happened.
I have moderate light pollution in my city (mag limit = 5.3) and some people say this may limit the performance of larger apertures.
To check this with my own eyes, I decided to try this comparison again in a darker site.
Next 26 we go to a very dark site, an observatory of a friend in another place, located in a top of a hill (altitude 1200m), 15 miles away from the nearest town and with very low light pollution.
I'll report you my experience next sunday,27 if we have good weather on 26.

Fernando

I have owned 4" and 5" APOs and my 10" Portaball blows either away on deep sky and-given excellent seeing-planetary performance. The newtonian is more sensitive to seeing but when the sky co-operates the smaller APOs get a well deserved rest because they can not begin to compete. Just one example- M13 looks ok in the 4" and 5" APOs but is a whole other experience in the 10". I have an MN76 that out performs any 4" or 5" APO I have had a chance to look through but the Portaball leaves it well behind too.

The problem is that somewhere in the 10" to 12.5" range the balance between size and "useability", i.e., actually taking the scope out and using it, starts to drop off- at least for me. If I were faced with the choice you mention, I'd go for the 10" rather than the 8".

Bill,

Something about "...the average two scope household..." gives me a chuckle.

Steve