Intes 6" Mak vs GSO 8" Dob ...and GSO 12" Dob

[janoskiss]

Seeing was better than last night for a couple of hours tonight. Did a little head-to-head between the 8" f/6 GSO Dob and the Intes MK-65, a 6" f/10 Maksutov. Dob was collimated before setting it down outside with the fan running. The Mak was just put on the EQ mount and given time to cool down.

The MK-65 appears to be non-collimatable. Within limitations of seeing star test shows it to be collimated okay, but there might be some astigmatism in the optics (mirrors or diag). I could not push the power higher than 210x, because I cannot use a barlow with the helical focuser: not enough in-travel. I'll have to wait for better seeing to properly star test it anyway.

Jupiter. The Mak shows a quite strong yellowish tinge to the planet's disk. This was not unexpected, as I was aware that this is typical for Maks. (have also seen the yellow colour in my 102mm Skywatcher Mak.) The Newt produces a closer to white disk, where cloud detail is easier to see with stronger colours (shades of orange and blue). The image is also brighter has greater apparent contrast. Detail is clearly superior in the Newt.

Star clusters. Newt shows more stars. Lots more in fact. And they appear sharper.

The difference in both cases is quite dramatic. I suspect this is mostly due to the difference in aperture. I was not expecting such a big difference between 6 and 8" though. :/ I also suspect the Newt with its smaller central obstruction (Mak ~33%, Newt <25%) has the edge in contrast.

Every time I compare the 8" Dob with another scope, I am amazed how well the Dob performs. I have to pinch myself when I recall that this scope cost just $399! Absolutely outstanding value for money. (okay there might be another $250 worth of mods on mine: quickfinder, focuser, big fan; but the optics are the same as when I bought it.)

The ability of the Mak on the EQ mount to track is terrific though. Tracking is not perfect but a big improvement from the nudge-nudge world of the Dob. I seem to have to keep pressing the x16 speed-up button every now and again to catch up to the planet. I don't know if that's the motor slipping or due to the very rough polar alignment.

But the EQ mount is a real pain in the bum when I just want to casually cruise the skies hopping from one DSO to the next (the 6x30 finder is not much help either). The Dob clearly rules there. To be fair I did have the tripod legs way too short for viewing near zenith. OTOH I already miss tracking when viewing Jupiter with the Dob. Yes there is more detail but it whizzes past quickly and you cannot relax into the viewing like you can with a tracking mount.

[ballaratdragons]

All you need now Steve is the best of both. EQ mount & track the 8".

[Dennis]

Nice write up with some very interesting results. Thanks for posting.

Cheers

Dennis

[iceman]

Interesting results, Steve, and nice report. The difference in aperture seems very noticeable. Would be interesting to read your reports when viewing with the 8" and 12" side by side..

I think a lot of people looking to buy new telescopes (or those with telescopes getting aperture fever) would be very interested to read what the visual difference is when looking at objects, between scopes of different apertures.

We always say "X% more light gathering", "Y% more surface area", etc, but to see it converted into REAL visuals and what you see versus what you can't, is very valuable.

Thanks for the report.

[janoskiss]

I don't think this EQ mount would do well with the long tube and increased weigth of the 8" Newt. That's the great thing about the Mak. It's v short and not too heavy so it's very stable on the mount.


I've done that a few times too.

On planets there is not that much difference in detail seen unless the seeing is very good. The 12" is quite a lot brighter but in average-to-good seeing it shows no more detail. The increased brightness is obtrusive in average seeing: just more light for the atmosphere to smear around the image. E.g., planet's limb or Saturn's rings look more blurred. I suspect this is why people often comment that smaller refractors give better views of planets than large Newts. A crossed polariser or appropriate ND filter can fix this (better than aperture mask), and the 12" can always give as pleasing a view as the 8".

With a 15-20 degree temperature change, the 8" is close enough to thermal equilibrium to start observing in 5 minutes, the 12" takes 15 minutes or more. Best views are obtained much later after at least 15 min for the 8", and 30-40 min for the 12". This is with baffled fans running.

The 12" is always a big improvement on stars and DSOs. Under dark skies the 8" pulls in lots of DSOs though and showpiece objects will certainly impress newcomers equally in either scope.

The 12" is more cumbersome to collimate mainly because springs sag so all 6 screws are needed in a push-pull arrangement unless you replace the springs. The 12" needs collimating more often.

The 8" is a lot more comfortable to use due to its size. Sitting with your feet on the ground makes a big difference to standing hunched over the EP or sitting on a tall stool high off the ground, especially when you have trouble finding close to perfectly level ground, as I usually do.

The 8" is truely a grab-and-go once you attach carrying handles to the sides of the base (diagonally opposed). The 12" is far from it: base, OTA, and fan+battery+counterweight(s) need to be carried outside separately, so 3 trips at least. (I have to carry everything down a flight of stairs to the backyard.)

The 8" secondary never dews up if the fan is running. (Unbaffled stock fan might not be good enough for this though.) The 12" needs a dew shield for extended observing otherwise the secondary will dew up.

[Starkler]

I found this also when i put the ed80 on an eq mount. I couldnt wait to convert it to an alt-az. Dobs are so much more intuitive to point.

[ving]

what sort of EQ is the inest on steve? isnt it a GS skyview2? if thats the case i think you will fin that if you put the 8" on there it will be fine for visual work... the GS SV2 is just a tad smaller (if not the same size i am lead to believe) than a HEQ5...

interesting review btw, the 8" really is a great scope :D

[janoskiss]

David, the mount is an older Japanese mount. The previous owner did not know much about it, other than it was sold under the name "Polaris". It appears to have been supplied originally with the scope by JMI scopes when it was new in 1994.

I think the vibrations would be excessive with an 8" f/6 on it.

[ving]

ok, yeah probably... why dont you sell a couple of scopes and buy a HEQ5 or EQ6 for the 8" then ;)

[janoskiss]

Nah, it would just get too heavy and cumbersome. If I had more room in and around the house, then maybe. But anyway, I don't have a couple of scopes to sell. I need all my Dobs! :D

[Dave47tuc]

I would prefer a 6” Maksutov be compared to a 6” Newt. or 6” sct.

A 8” Maksutov be compared to a 8” Newt or 8” sct.

Also a 12” Maksutov compared to a 12” Newt. or 12”sct.

A 4” Apo to a 4” Newt. I think you know what I mean. :poke:

My mak fits behind the front seat of the car:whistle:

EQ mounts are not hard to use once you learn how:whistle:

Which button do i push now:poke:

[janoskiss]

I think so. ;) Aperture rules. Everything else is secondary. But so often people do not listen. "Get the 8" Dob, you will see lots more more." "But at 4x the cost the whizz-bang goto high-tech looking tripod mounted 5" Meade/Celestron Mak/SCT must be better!"


My 8" truss Newt and Dob mount fit in a suitcase for international flights economy class. :D


It's not really the EQ-ness of the thing. It's getting under the thing, having to lock it or it moves, but while I'm locking it, it moves anyway, and the ungodly EP and finder position near zenith. I know all this is fixable but I'm lazy. :P What isn't fixable is Dec and RA axes being diverging (i.e., it's not a just a tilted alt-az mount). That takes some getting used to.

[janoskiss]

I've attached a couple of pics to go with the story. As you can see the Mak OTA is a lot smaller than the Newt. Maks are very compact scopes. They are much happier and sturdier on EQ mounts than Newts.

Was out with both scopes again tonight but much earlier, right at sunset to catch Jupiter while it's well placed high up in the sky. Seeing was much better, some moments were very good (could have used the resolution of the 12" but that was too much work). Conclusions are the same as I reported above, but better seeing helps both scopes see better.

The Dob gave excellent high power views at 200+x after 10 minutes of high-flow fan cooling the mirror. The Mak really needs a good 30-40min to settle in.

I spent more time with the Dob than the Mak, but I could see quite a bit of detail in the Mak too during periods of good seeing. Funny thing was that after observing with the Dob - watching Jupiter drift along for 20-30 seconds, nudge, repeat - and then going over to the Mak on the tracking mount, I could see the planet drifting across the FOV at the same speed as in the Dob, but in the opposite direction! Of course it was staying still, just my brain playing tricks on me! But it did it every time! :lol:

[janoskiss]

btw. Dave, hopefully someone with a 6" Dob will come along to Snake Valley. Then we can do a fairer comarison between Mak and Newt. ;)

[ving]

so your mak was tracking too fast steve?:confuse3: