33 replies to this topic

[gnowellsct]

According to that chart, shouldn't the ST80 have the worst image of the three? The CA wasn't really all that bad in the ST80 (I actually didn't see much rainbow color) and none in the Meade 70. I have observed Venus at over 100x with the ST80 and have found that stopping the aperture helps (removing that cap in the middle of the lens cap).

If you just observe through the center hole on the lens cap then you have turn the 80 mm into a 50 or 60 mm and effectively changed the focal ratio. So Mr conrady will step into help you out.

If you are not measuring magnification in terms of magnification per millimeter of aperture, or alternatively describing your magnification in terms of exit pupil, it's really hard to keep track of what you have going on.

But it remains the case that the 80 mm is a smaller aperture than the 90 mm and this to some extent will help it out even though it is a shorter focal ratio. But all you have to do is look at its position on the chart compared to the 90 mm. Whichever one is closer to the green is going to rule the day.

[grif 678]

The 70mm should be sharper because of the longer focal length. If the conditions are not very good, the 90 will show more of the bad conditions, since a larger lens.

[Bearcub]

When I try observing Mars through a 90mm (f7) refractor and some of my other telescopes, it is messy and difficult to focus. But, I get a good image in my Meade Infinity 70mm (f10) all the way up to 140x (I paid $12 for the Meade at a thrift store).

 

Is it a combination of a very bright object with atmospheric turbulence? Maybe the larger aperture scopes are a bit too fast?

Same for my 10" dob, vs refractor or mak. Just not the same league. Quality over quantity.

 

Also, what is collimation - dont look for answers, think. Once you figure out what is collimation, then you understand why a dob is no good for planets, it can be amazing for dso, for dso it is most likely the best type of telescope.

[Napp]

Same for my 10" dob, vs refractor or mak. Just not the same league. Quality over quantity.

 

Also, what is collimation - dont look for answers, think. Once you figure out what is collimation, then you understand why a dob is no good for planets, it can be amazing for dso, for dso it is most likely the best type of telescope.

Once you figure out collimation you understand why a DOB is great for planets.  I have a 127mm APO.  It produces very nice, sharp views.  But it can't touch the resolution my 10 inch DOB can provide for planets.  And the 16 inch and 18 inch DOB's are better than the 10 inch DOB.  Granted, Florida has steady skies more often than most other places.  However, if a DOB is allowed to acclimate, collimation is spot on - not difficult at all - and seeing is good the bigger DOB out performs the refractor.  Even on nights of not good seeing the big scopes at least equal the smaller scopes.  One night I set up my 127mm APO and my 16 inch DOB for Mars.  Unfortunately I chose a night that ended up with poor seeing.  I could only push both scopes to about 185X before the image got mushy.  The images were almost identical in the scopes. A couple weeks ago I had missed a couple nights with excellent seeing.  The night was not as good as the previous ones but still not bad.  I enjoyed great views of Mars at well above 400X with my 18 inch DOB.  I just wish I'd taken advantage of the previous nights to go even higher.

[Dave Mitsky]

Same for my 10" dob, vs refractor or mak. Just not the same league. Quality over quantity.

 

Also, what is collimation - dont look for answers, think. Once you figure out what is collimation, then you understand why a dob is no good for planets, it can be amazing for dso, for dso it is most likely the best type of telescope.

I've observed planets through all sorts of telescopes over the years including some world-class apochromatic refractors like a 206mm Astro-Physics StarFire EDF. 

https://astro-physic...6edf/206edf.pdf

The very best planetary views that I've had have been through large truss-tube Dobs with premium mirrors.

[quilty]

I'd say, in the 70 mm at not too high powers Mars is nicely pinpoint :-)
The grief comes when you're after resolving things on Mars. No proper diffraction pattern but no proper surface image neither.
And then it's about seeing. When seeing allows just 100x any small scope shows more beautiful stars and similar.

But at Pickering 6 or above, when you're after detail on Mars starting with the icecap, you'll prefer powers exceeding 300x soon.

[BigC]

It has been about  15 years(where does time go) when my Zhumell 70x900 gave a tiny but sharp Saturn and Mars with ice cap and Syrtis major? .Later that year my 10" Dob showed 8 bands one night  on Jupiter. 

 

Anyway ,I think a 70x900 or even better an 80x1200  would be the best budget scope for planets,etc.

 For the best views observing LATE is better if the desired object is visible.

[JOEinCO]

Also, what is collimation - dont look for answers, think. Once you figure out what is collimation, then you understand why a dob is no good for planets....

 

This is absolutely NOT true. 

 

And for the record, a "Dob" is a mount, not a type of optics. 

[gnowellsct]

I've observed planets through all sorts of telescopes over the years including some world-class apochromatic refractors like a 206mm Astro-Physics StarFire EDF.

https://astro-physic...6edf/206edf.pdf

The very best planetary views that I've had have been through large truss-tube Dobs with premium mirrors.

Even not so premium mirrors can, simply by the size of the real estate, outperform smaller instruments.

I have seen 10 and 12-in skywatcher newtonians outperform my 5-in Apo. I have seen my C8 outperform a 14-in obsession.

The two skywatcher newtonians outperformed my Apo because their mirrors are good to excellent and they had the real estate to outperform.

My C8 outperformed the 14-in because it had been set up earlier and pre-cooled.

And you don't always get the option to try again later. The planets are moving and get into a position where the views are not optimal.

But overall I have to agree the premium mirrors on the medium and large apertures, properly set up collimated and thermally acclimated, it's not going to get better than that. But even cheap mirror real estate has its advantages.

Greg N