ED100mm vs 8" Dob for planets

[Hammerman]

I just want to say that I am surprised you are selling it. I own the very same scope, and a 8" dob too. The dob outperforms on most dso's, but the 100ed blows away the dob on planetary and lunar detail. Open clusters are fantastic in this scope too!

Great buy for anyone interested!

[ausastronomer]

Hammerman,

If this is the case there is something not quite right with your 8" dob and I would check it out or get someone else to check it out. It is either not collimated properly, not cooled properly, some induced astigmatism is present due to overtight primary mirror clips or the secondary mirror holder, a not so great mirror and the scope should have gone back to the dealer for exchange.

Some of these issues I have mentioned above will not affect the in focus images at low power or even the views of some DSO's at medium powers but they are critical for top performance in a Newtonian at higher powers used for lunar or planetary performance.

A properly cooled and tuned 8" newtonian with good optics will outperform a 4" APO on everything including planets and the moon. A good 8" newt will do it by a fair margin.

CS-John B

[janoskiss]

This thread was split from Doug's for sale thread.

I agree with John's comments. Properly cooled and collimated Dob will do show a lot more detail on planets. Proper cooling may require more effort than you think. I never realised the full potential of my 8" Dob till I fitted a a 4 watt fan behind the mirror. Before I did that, small refractors often outperformed my Dob especially on planets (Saturn and Jupiter), to my constant disappointment.

[mickoking]

The best view I ever had of Jupiter was with a cardboard 200mm F6 Meade Dob.

[MarkN]

Damn you blokes!

Just as I had settled on a 5" achro for a second scope you come up with this discussion. My chief reservation with Dobs is the constant nudging required to stay on target and how they might handle the 17 mm T4.

Still inclined to go for the 5" achro but the 8" Lightbridge will get a look.

Mark.

[Robert_T]

I agree with John and Steve, re how a good well cooled and well collimated 8inch reflector should outperform any 4 in refractor on planets. This is particulalrly the case with planetary imaging where images are stacked and poor seeing frames removed... the larger 8in apeture is more susceptible to poor seeing effects visually, but when this is removed through sorting and stacking video frames the full force of the 8inches greater resolving power can be realised, revealing far finer detail than the 4inch can physically resolve no matter how good.

cheers,

[Dennis]

My experience with imaging the Moon and Jupiter with a 4" f9 Vixen ED refractor and a Celestron C9.25 verifies what Robert (and John & Steve) have said. The C9.25 leaves the Vixen for dead - in this case, aperture is most definitely king.

The views of open clusters in the Vixen 4" f9 ED look awesome due to the excellent optics - the classical diamonds sprinkled on a black velvet cloth analogy springs to mind. The C9.25 "appears" to show "bloated" stars, but often this is because the longer focal length means I am using a magnification of say, x235 compared to x90 on the Vixen using the same eyepiece.

I am also a confirmed equatorial mount person, so I am heavily biased towards the GEM because of its GoTo and tracking capabilities.

Cheers

Dennis

[janoskiss]

On the other hand, let us not forget why people (including myself) love good refractors:

- more aesthetically pleasing views than Newts or SCTs
- higher contrast (due to no central obstruction)
- no or little cooldown time
- never need collimation

just to list a few. The ED100 would also be well suited to deep sky photography. Here the lack of sensitivity to ambient temperature fluctuations and collimation problems would be a big plus also.

Stu's makes a strong case for the refractor here:
http://www.iceinspace.com.au/forum/showthread.php?t=6514
http://www.iceinspace.com.au/forum/showthread.php?t=6703

:)