8" f/6 or 8" f/7
Posted 07 February 2004 - 06:07 PM
I'm still considering buying an 8" DHQ either f/6 or f/7. Which would you reccomend? This will be my first scope, and I realize that f/7 is better for planets than the f/6 and that the f/6 has a wider field of view, but I'm not sure which to choose. Also, I can switch the focuser for a Crayford; Is the Crayford focuser worth the extra bucks($60)? Also, should I get a Telrad instead of the 6x30 finder. The Telrad would cost an extra $10. Or should I get a 7x50 for an extra $45?
Posted 07 February 2004 - 06:15 PM
Posted 08 February 2004 - 01:22 AM
Posted 08 February 2004 - 11:08 AM
Take this for what it's worth. (Arguably not much.)
I've got a 6" Discovery f/5 and a Hardin DSH 8" f/6, and I think that the build quality on the Guan Sheng Hardin 8" is MUCH better than on the Discovery DHQ. I also think it's a much better value. The reasons I say that are that the Tiawanese GS "Hardin" (sold as several other brands) has:
-metal tube instead of sonotube
-tension springs to make the tube captive to the mount, (unlike the Discovery which relies solely on gravity to rest in the cradle
-is therefore easier to carry as a single unit)
-came with a 50mm finder
-seems to actually have a better mirror (maybe, possibly, hard to say when comparing different focal lengths
That gets me to your orig. question: f/6 vs f/7? If all else was equal, I'd go with the f/7. But, all else is rarely equal. Would I go for an f/7 Discovery or an f/6 Hardin? Well, in reality, when it came time to actually make the call, I went with the Hardin. But, that's just me. I wish more of these mid-sized scopes came with f/7 as an option, since it's a nice compromise between f/6 & f/8. But, even at f/6, it's slow enough to get good service out of relatively cheap eyepieces, (unlike f/4 & f/5).
The Discovery would probably have a slightly higher resale value sometime down the road, if that's at all important to you.
Just one guy's biased opinions. Take with salt.
Posted 08 February 2004 - 12:21 PM
Negative: narrower FOV, but it won't be THAT much narrower (for example, w/ 32mm EP, 1.2 deg in f7 vs. 1.4 deg in the f6).
Positive: better EP performance, and less-sensitive to precise collimation
If I was using an equatorial mount, I might lean toward f6 because it's smaller and thus a little easier to deal with. But for a Dob, I agree with everyone else...longer is better...(I'm still talking about the telescope, of course).
Posted 08 February 2004 - 01:13 PM
Posted 08 February 2004 - 04:23 PM
The difference in those two sets of optics is fairly substantial. The Premium mirror offers 94% ref. versus 89% for the standard. The premium also uses pyrex glass versus the sodium lime. There are plenty of other differences too. But I think most of those questions should be addressed in the optical forum and addressed to "AlM". He can give you a very specific answer.
I was actually considering one of there 10" mirrors for my project scope.