which planet killer?

[catburglar]

Any thoughts regarding 3 scope options for lunar/planetary observing- specifically visibility of low contrast details. 

7 inch Mak, 6 inch F/8 achromat or 5 inch ED doublet

Cooldown and mounting issues aside, does the obstructed aperture of the larger Mak beat the others, or does the smaller ED have the edge in contrast that trumps the larger aperture scopes. The 6 inch F/8 is in there just because they regularly crop up and I've never tried one.

[Peter Drew]

Unless you can take any of them to locations South of the UK you are unlikely to get "planet killing" performance whilst the planets are so low for quite a few years now. The 7" Mak is capable of outperforming the 5"ED which in turn should outperform the 6" F8 achromat. The 6" F8 achromats seem to be of variable quality due to their mass production, a good one is surprisingly good for the outlay.   

[nightfisher]

I used to own a 7" Maksutov, and had some of the very best planet views ever but...........cool down is an issue, a long time is needed to get the best, and its a heavy OTA, so needs a good mount, and as Peter correctly posted, the planets are a bit of a damp squid for the next few years, a 150 pro is a good scope, capable of a bit more than just planets and a bit lighter

[dweller25]

I have a 7" Mak and it is a superb planetary scope BUT.......

It's an Intes Micro 715 with active cooling - which makes a massive difference

Highly recommended.

[Moonshane]

My best planetary views have been with my 12" and 16" dobs at full aperture and good seeing. In less good seeing find masking the big Dob down to about 170mm works a treat. That said i use my 120ed more than either of them currently and always with binoviewers for solar system objects.

[Pete Presland]

I had a 7" Mak180, its was fine planetary scope. The mirror slop was rather irritating though, easily solved obviously with a Crayford. Super long Focal Length: 2700mm (f/15) only sold because I wanted a bit more aperture/versatility with the C9.25.

It saw plenty of action when I owned, cool wasn't a problem with it in an observatory.

I didn't have a heated dew shield in those days. I had finished an imaging session pop in for coffee, when I came back later....

[Stu]

5 minutes ago, Pete Presland said:

I had a 7" Mak180, its was fine planetary scope. The mirror slop was rather irritating though, easily solved obviously with a Crayford. Super long Focal Length: 2700mm (f/15) only sold because I wanted a bit more aperture/versatility with the C9.25.

It saw plenty of action when I owned, cool wasn't a problem with it in an observatory.

I didn't have a heated dew shield in those days. I had finished an imaging session pop in for coffee, when I came back later....

That looks familiar Pete , I recall trying to use my OMC200 at the Peak Star Party a good few years back, I had it cooling for ages, fans on, successfully pulling damp air into the scope, I took the end cap off and the whole thing was completely fogged up inside and out! It took ages the following day with a hairdryer to clear it!

That said, the views were great when it was on song. However, Shane is right in that his 16" gives fabulous planetary views at full aperture and more stable, nearly as good when masked to 170mm.

[catburglar]

Thanks for the replies so far......I should say I'm visual only. The planets are a long way south for a few years, but the moon is still here, and there's lots to see.

I already have a 7 inch mak and it does seem to do pretty well- cool down, mounting and mirror flop issues are all sorted. But..... I keep hearing and reading that for low contrast details, unobstructed aperture is best.

I've never been in a position to do a side by side test, and I've got one of those itches that might need to be scratched.....I note Moonshane's fix of an off axis mask for a big reflector- but don't really fancy a newt...

 

PS- that mak looks cool.......

[Stu]

I love refractors, but can't see how a 120ED for instance would better a good 7" Mak for planets, although it will cut through poor seeing better.

[paulastro]

I think your point about a small (comparatively) high quality refractor cutting through poor seeing better than larger telescope is very important Stu  Really and truly, how often do we get better than one arc-second resolution seeing? - not often round here.  If you use a large telescope on a bright planet when the seeing is poor you generally just get a bright blob.  (by large, I mean too large for the prevalent seeing conditons, and this can be surprisingly small)

Of course, the answer is to have a large selection of telescopes to suit all objects and seeing conditions .

[catburglar]

I've been rereading Gerald North's book- Observing the Solar System.....some of it is a little dated, but he seems to rate a smaller refractor over a slightly larger mak for lunar / planetary visual observing.....although there's some room for interpretation in his comments.

I'm no optics expert, but I understand that the obstruction in the mak lowers the mtf- particularly at higher spatial frequencies. Whilst this can be compensated for by increasing the magnification there aren't too many nights where the seeing is sufficient to allow magnifications much over 250x. and if the mag is constrained to 250x then the 5 inch frac can handle that with it's higher mtf........

But, it looks as though the collective wisdom of SGL says stick with the mak.....

[Moonshane]

Probably the case to be honest. I have done what Paul suggests though and bought a range of scopes! Doesn't stop me hankering after a 100mm Tak though!

 

[nightfisher]

Cat burglar, as you already have the 7" Maksutov, keep that, but if you have a few quid spare and fancy a second scope look out for a used ED100, as said a lot of the time the smaller scope cuts through Britain's murk, i now use a 127 Maksutov as my main scope.

And to answer your point on central obstruction, yes it does take a little contrast away but in the Maks its less of a problem than in SCT

[spaceboy]

I totally agree with Jules regards smaller scopes can cut through seeing. I think I am in a part of the country where seeing is particularly poor. I don't think the industry around here helps much either. I do get those nights of superb seeing from time to time and having more aperture comes in to it's own but for the most part my collection of scopes are now mostly sub 5" as I get to see more, more often than I would often see using a larger scope. I know there are members more than happy using larger scopes and getting fleeting glimpses of good seeing but for me this strains my eyes and gives me a headache. I'd rather less resolution with slightly higher magnification / contrast that doesn't seem so effected by the poor seeing conditions I sit under.

[Stu]

Let's be clear, I am and always will be a huge fan of smallish (4") apo fracs.

It's a strange thing, and very personal taste I'm sure, but the scope I enjoy viewing planets with most is my FC-100. There is a purity and stability to the images  which I just really enjoy; Because of the smaller aperture it is less affect by seeing, and it just takes magnification very well when the conditions allow.

Now, when put side by side with a large scope it has lower resolution, shows less colour and shows up my floaters far more (the last two due to exit pupil). I've observed with the Tak next to a C8 Edge, and all the above apply, but the Tak image shows most of what it has got to offer most of the time, with the larger scope you have to wait longer for the periods of best seeing to get the best views. Add that to the cooling, mounting and dew issues seen with a large compound scope and I have gravitated towards the smaller scopes. But you will still see more with a good big one!

[mikeDnight]

I remember using paulastro's 7" Intes Mak Cass about ten years ago and felt at the time that it gave an obvious resolution advantage over my Tak FS128. But the Tak was sharper and gave a more pleasing view. I no longer have that 128mm fluorite doublet but now use a 100mm fluorite doublet instead. The views are stunningly good, especially when using a binoviewer on the moon and planets. The drop in aperture doesn't phase me one bit because the optical quality is so high. It also seems to cut through the seeing. Attached are just a few examples of what a good 100mm fluorite can do. 

 

[spaceboy]

Those are some amazing sketches Mike !!!

[paulastro]

That's a marvellous portfolio of drawings Mike, though I've seen them all before it's great to see them all together.  I seriously (and I don't use that word often! ) think you should consider publishing a book (portfolio?) of your drawings with text describing your observing experiences.  I think you shouldn't leave it too long so that I can be chief editor and ensure the book turns out as I'd like to see it .

[Peter Drew]

I've said it before, if the seeing isn't good enough to use a decent size telescope, i do something else.     

[paulastro]

25 minutes ago, Peter Drew said:

I've said it before, if the seeing isn't good enough to use a decent size telescope, i do something else.     

Peter, I expect that's why your telescopes and eyepieces etc are all so shinny and clean - cus you spend so much time polishing them!! 

[timwetherell]

My experience is that the range 4" to 8" is the steepest part of the aperture/performance curve when looking at planets. Below 4" they're little more than dots. Above 8" on most nights in the UK the atmosphere limits what you can see but in between those two extremes a little more aperture does add significantly to the definition. A 7" Mak is an excellent planetary telescope and very good value for money. ED apos are also great but unless your pockets are deep enough to get one a bit closer in aperture to the mak, probably won't give such good results. A 6" apo might outperform a 7" mak, a 5" might match it and a 4" would be probably struggling to match it on planets. But of course it would give great wide field views which the Mak can't. Classic achromats can be superb too but only if the focal length is sufficient for the chromatic aberration to be well controlled. the rule of thumb is the f ratio needs to be 3x the aperture in inches so a 5" would have to be f15 and a 6" f18. Modern f8 achromats are really nice allrounders but there will be noticable CA and it will take the edge off its performance on planets. 

 

[mikeDnight]

1 hour ago, paulastro said:

That's a marvellous portfolio of drawings Mike, though I've seen them all before it's great to see them all together.  I seriously (and I don't use that word often! ) think you should consider publishing a book (portfolio?) of your drawings with text describing your observing experiences.  I think you shouldn't leave it too long so that I can be chief editor and ensure the book turns out as I'd like to see it .

If I pop my clogs you can have my sketch books Paul, as I doubt my wife or sons will place much value on them. To them I'm just an eccentric nut who sits out at night in 6" of snow, and in subzero temperatures with tears in my eyes because of the cold breeze, to draw detail visible on a tiny planetary disc. Surely only a like-minded nut would be interested in a portfolio? 

[rwilkey]

6 hours ago, Stu said:

Let's be clear, I am and always will be a huge fan of smallish (4") apo fracs.

It's a strange thing, and very personal taste I'm sure, but the scope I enjoy viewing planets with most is my FC-100. There is a purity and stability to the images  which I just really enjoy; Because of the smaller aperture it is less affect by seeing, and it just takes magnification very well when the conditions allow.

Now, when put side by side with a large scope it has lower resolution, shows less colour and shows up my floaters far more (the last two due to exit pupil). I've observed with the Tak next to a C8 Edge, and all the above apply, but the Tak image shows most of what it has got to offer most of the time, with the larger scope you have to wait longer for the periods of best seeing to get the best views. Add that to the cooling, mounting and dew issues seen with a large compound scope and I have gravitated towards the smaller scopes. But you will still see more with a good big one!

Hi Stu, I tend to agree with you here, I get a lot of pleasure out of my 100ED, easy to put up and sharp views, less cool down time etc. 

[Stub Mandrel]

18 hours ago, Pete Presland said:

I had a 7" Mak180, its was fine planetary scope. The mirror slop was rather irritating though, easily solved obviously with a Crayford. Super long Focal Length: 2700mm (f/15) only sold because I wanted a bit more aperture/versatility with the C9.25.

It saw plenty of action when I owned, cool wasn't a problem with it in an observatory.

I didn't have a heated dew shield in those days. I had finished an imaging session pop in for coffee, when I came back later....

I thought the man said Maks have along cool-down period

[Pete Presland]

1 hour ago, Stub Mandrel said:

I thought the man said Maks have along cool-down period

It was quite long