Hi All,
Well, I thought the big & fast refractor topic merited its very own thread.
A few weeks ago I had the chance to compare a 160mm f7 and a 130mm f6 APO under very dark skies and found a few surprises. It looks like people might be interested in what I found, or at least we could compare notes.
The short story is that the 130 is the instrument of choice for wide field views. Here's why:
1) It has a wider field. I was using a 31 Nagler for just about as big a field as is possible in these scopes. The 130 provided about 3.3 degrees, the 160 just over 2 degrees. It doesn't seem like much difference, but it is and it surprised me. The wider field really opens up. Wide objects like the veil and the North American were well framed, star hopping was easier, and the Scutum star cloud was just astounding.
2) The 130 is easier to handle. The 160 is a good 50% bigger, heavier, and scarier. I breathed a sigh of relief every time I got the 160 into its case. It is as much as I'm comfortable lifting onto a mount. The 160 also put you in some awkward positions when viewing close to the zenith.
3) The 130 puts less demands on the mount. I used a losmandy G-11 for both and wouldn't hesitate to recommend it to anyone for visual use. However, where the 130 is over-mounted, the 160 maxes out the G-11. We had some breezes and a 15 mph wind restricted the 160 to about 70x. The 130 could handle the same breeze to about 150x.
4) The 130 was just as bright. The 31 Nagler on the 130 delivered a 5mm
exit pupil which is a good match for my 48 year old eyes. The 160 delivered more light, but at a magnification that resulted in a 4mm exit pupil. It slightly "over-magnified" the images and resulted in a slightly dimmer view for wide field.
Bear in mind that this is strictly a wide angle comparision. The 160 was clearly superior on faint objects such as galaxies, or anything magnified over about 40x. The seeing didn't support any comparsion on planets, although I know that the 130 can give excellent planetary views.
The truth is that I can't say that an achromat wouldn't do just as well at the low powers I was using. But I am pretty sure that an achro couldn't do the wide field and the planetary the way the apo does; and it's easier for me to rationalize one apo than the two achros I'd need to get both views.
The conclusion is that I don't see any benefit for going over about 130mm for a wide field scope. As always, your mileage may vary.
Thanks,
Doug
Well, I thought the big & fast refractor topic merited its very own thread.
A few weeks ago I had the chance to compare a 160mm f7 and a 130mm f6 APO under very dark skies and found a few surprises. It looks like people might be interested in what I found, or at least we could compare notes.
The short story is that the 130 is the instrument of choice for wide field views. Here's why:
1) It has a wider field. I was using a 31 Nagler for just about as big a field as is possible in these scopes. The 130 provided about 3.3 degrees, the 160 just over 2 degrees. It doesn't seem like much difference, but it is and it surprised me. The wider field really opens up. Wide objects like the veil and the North American were well framed, star hopping was easier, and the Scutum star cloud was just astounding.
2) The 130 is easier to handle. The 160 is a good 50% bigger, heavier, and scarier. I breathed a sigh of relief every time I got the 160 into its case. It is as much as I'm comfortable lifting onto a mount. The 160 also put you in some awkward positions when viewing close to the zenith.
3) The 130 puts less demands on the mount. I used a losmandy G-11 for both and wouldn't hesitate to recommend it to anyone for visual use. However, where the 130 is over-mounted, the 160 maxes out the G-11. We had some breezes and a 15 mph wind restricted the 160 to about 70x. The 130 could handle the same breeze to about 150x.
4) The 130 was just as bright. The 31 Nagler on the 130 delivered a 5mm
exit pupil which is a good match for my 48 year old eyes. The 160 delivered more light, but at a magnification that resulted in a 4mm exit pupil. It slightly "over-magnified" the images and resulted in a slightly dimmer view for wide field.
Bear in mind that this is strictly a wide angle comparision. The 160 was clearly superior on faint objects such as galaxies, or anything magnified over about 40x. The seeing didn't support any comparsion on planets, although I know that the 130 can give excellent planetary views.
The truth is that I can't say that an achromat wouldn't do just as well at the low powers I was using. But I am pretty sure that an achro couldn't do the wide field and the planetary the way the apo does; and it's easier for me to rationalize one apo than the two achros I'd need to get both views.
The conclusion is that I don't see any benefit for going over about 130mm for a wide field scope. As always, your mileage may vary.
Thanks,
Doug
