137 replies to this topic

[Voyager 3]

The requirement of being 15x as accurate with collimation has me worried. I'm not certain my collimation is good enough even without a coma corrector.

I agree though that if I were to have f5 as a lifetime scope, I'd seriously want to try a CC.

Even if you upgrade to a 20" F/5 which may become your lifetime scope , you may still use the coma corrector .

Edited by Voyager 3, 05 January 2021 - 09:03 AM.

[Jon Isaacs]

I think that there are two streams of thought here.  First, there those who want to study Airy Disks across the field and others who want to see astronomical objects.  I used to be in the latter category but as I became increasingly infirm, I belong to the former.

 

If one has a tracking mount, then correcting the coma is less important because high magnification objects can be studied in the center of the field and the fact that the image degrades off-axis is not so important.  

 

But for those like myself who manually track, correcting the coma is very important.  At F/6, the coma free field of view is 4.75mm, in an 8 inch, it's 0.23 degrees.  

 

What that means is this:  If you start with a perfect mirror, it is diffraction limited over a field of 0.23 degrees.  That near perfect Zambuto mirror, it's a at least a 1/4 wave over that field of view.. That's a 55 second drift time.  

 

What a coma corrector does when combined with well corrected eyepieces is turn the sweet spot into the entire field of view at essentially any magnification.  The other night I was splitting some doubles with my 13.1 inch F/5.5, it has a Royce mirror., I was using type 6 Naglers which are sharp across the field.   But I wasn't using my Paracorr 2.. 

 

I was quite easy to see the coma as the double drifted across the field, I was using magnifications from 233x to 420x.  It wasn't all that bad but it was definitely there to be seen. 

 

With a coma corrector, the sweet spot is the entire field of view.. 

 

That's pretty sweet.

 

Jon

[Jon Isaacs]

Even if you upgrade to a 20" F/5 which may become your lifetime scope , you may still use the coma corrector .

When I bought my 25 inch F/5, I was thinking I might be able to get by without my Paracorr. That idea lasted approximately 5 minutes.

 

When I later bought my 13.1 inch F/5.5, I was thinking I might be able to get by without the Paracorr, that idea lasted about 5 minutes.  

 

I do use it occasionally without the Paracorr, in part just because of threads like this one.  Without the Paracorr, the views are quite nice but the Paracorr definitely cleans things up.  It's like buying a tool that would correct the chromatic aberration in any refractor you ever bought.  $500 and any achromat would have apo color correction.. 

 

That would be a no brainer.. 

 

Jon

[CHASLX200]

When I bought my 25 inch F/5, I was thinking I might be able to get by without my Paracorr. That idea lasted approximately 5 minutes.

 

When I later bought my 13.1 inch F/5.5, I was thinking I might be able to get by without the Paracorr, that idea lasted about 5 minutes.  

 

I do use it occasionally without the Paracorr, in part just because of threads like this one.  Without the Paracorr, the views are quite nice but the Paracorr definitely cleans things up.  It's like buying a tool that would correct the chromatic aberration in any refractor you ever bought.  $500 and any achromat would have apo color correction.. 

 

That would be a no brainer.. 

 

Jon

Don't leave home without it.

[icomet]

Too tall!

 

How about 12.5” f/7?
 

Just two short steps to the zenith.

 

Now that is my kind of scope. Always wanted a 16" F/6 Dob to a 20" F/6 Dob for planets.

Started work on this one. This 12.5" f/8  1967 Cave mirror was sent to an "eye doctor".

 

Clear Skies

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[stargazer193857]

I know people who really don't like 6" under dark skies, and are not too impressed with 8". They want 10-12". That is getting a bit long at f6, though those particular people were ok with f5.

Even in the 10" f6 camp, some people want tubes, others trusses, some light weight, others want heavy.

[stargazer193857]

I know people who really don't like 6" under dark skies, and are not too impressed with 8". They want 10-12". That is getting a bit long at f6, though those particular people were ok with f5.

Even in the 10" f6 camp, some people want tubes, others trusses, some light weight, others want heavy.

I just read the thread again:


Started with 6" f6 as a 2.4 deg wide field scope, with OP saying that longer than 36" married the field too much.

Jon said 6" f6 dobs are too short and belong on a tripod.

OP stated that above 8", f# needs to decrease.

Others chimed in that 10" f6 would be perfect.

A few said f7 has easier collimation. They were rebutted by people saying tube stiffness becomes an issue and that f7 has a more narrow view.

Don agreed that at 6", f6 is more appropriate for visual.

Jon and Don defended the coma corrector as the saverior throughout, while others argued it is not needed at f6.

Those preferring f5 said the coma is detracting but acceptable for their uses.


10" f6 maybe can be carried out a door when held by the poles. I don't know. It does require a higher chair, and is heavier and more expensive than 10" f5. But it has just enough aperture for deep sky.

A few wanted f6 above 10", but above 10" was the real tipping point where more people rebuttaled back it would be too tall for them to enjoy as much.


There are wide field lovers out there, and they like 130-150mm f6 as their apo substitute.

[Jon Isaacs]

10" f6 maybe can be carried out a door when held by the poles. I don't know. It does require a higher chair, and is heavier and more expensive than 10" f5. But it has just enough aperture for deep sky.

 

 

I don't recommend carrying a scope by the poles..

 

For deep sky, the argument again a 10 inch F/6 is a 12 inch F/5. Ergonomically their very similar and thev12 inch is significantly more capable.

 

Jon

[Starman1]

I find the eyepiece height on the 12.5" f/5 to be nearly optimum for my 6' height.

 

Translating that up, it's a

16" f/3.9 (I've seen more than one 16" f/4)

20" f/3.1 (wonder if that explains the 20" f/3 scopes out there?)

24" f/2.6 (hmm, didn't Mel Bartels build something like that?)

 

Translating that down, it's a 

10" f/6.3  Jon is right, though.  If going here, might as well get a 12.5" f/5

8" f/7.8 I used to occasionally see this, but modern scopes are f/6 or a bit less, and I almost need to sit on the ground to use one of those.  The alternative is a high box and a scope with a lot of shimmy.

For a 6", though, mounting it on a Stellarvue M2C or an Explore Scientific Twilight II or DiscMount DM-4 and using the scope in an alt-az configuration means an f/5 or f/6 is just fine.

[stargazer193857]

I find the eyepiece height on the 12.5" f/5 to be nearly optimum for my 6' height.
...
Translating that down, it's a
10" f/6.3 Jon is right, though. If going here, might as well get a 12.5" f/5
...

For me at 5'8", a 30" stool is about as high as I can comfortably sit without a foot rest. Foot rests are great in the winter, but maybe less safe from falls. Or maybe safer from centipedes. So assuming 30", then 60" is my best viewing fl.

But for grab n go, I found a 47" fl, if optimized as low as possible, allows me to use just a camping chair, which is lighter, less expensive, and more multitude than a starbound chair. Also 47" fl can be carried through a doorway.

Without a chair, 60" is better for standing, whereas 47" is good with kneeling on knee pads.

The problem though with 47" is 8" becomes the largest mirror that can be f6. 10" has to be f4.7. 9" f5.22 sounds nice.

Schott sells borofloat in 67"x51" sheets. That makes 48 8", 35 9", or 30 10", or can be 48 8.2", 35 9.3", or 30 10.4".

My experience is 8" is not bright enough on M13. 10" is plenty. 9" might be just right, but 9.3" would have f5 coma.

I wonder which aperture and f# would be best for introducing beginners to astronomy and getting them hooked.

[stargazer193857]

8" f6, or 8.2" f5.75, would be the most square inches of aperture per dollar. I just don't know if it would be bright enough to satisfy people. Maybe not as their biggest scope. Maybe they'd be happy as their grab n go that gets them to buy a bigger one.

Also have to consider which scope kids and teens can carry out.

[stargazer193857]

I think 8" f6 is an easier sell a 10" or 9".

Many people plan to some day get a 12" or 14". They will think a 10" is redundant to the 12", but that the 8" leaves more money for the 12", is better for grab n go, and is more complementary.

I like the fact that younger teens can carry an 8" easier, and collimates it.

Jon says his 10" is much more capable than 8", and that he never kept an 8" since he had the 10". He also is stronger than most and has 5+ scopes.

[Starman1]

1 magnitude jumps in reach: 5" >> 8" >> 12.5" >> 20"

1 Magnitude jumps in reach: 6" >> 10" >> 16" >> 25"  

Putting them together yields 0.5 magnitude jumps between sizes

5" >> 6" >> 8" >> 10" >> 12.5" >> 16" >> 20" >> 25"

All are commercial sizes, and perhaps that explains why 7", 9", 11", 13", 14" are not usual sizes.

 

There was a time that 2.5" changes in size were more common, i.e. 10", 12.5", 15", 17.5", 20"  but that has fallen out of favor.  15" seems to be disappearing, as does 17.5".

There was also a time when newtonians attempted to emulate the SCT sizes, and we got 8", 10", 12", 14", 16", 18", 20", 22", etc.

That may have depended on the availability of glass blanks.

And with so many scopes imported from other countries, now, metric sizes are becoming more common, like 200mm, 250mm, 300mm, 400mm, 500mm etc.

 

For full-tube dobs, 10" is about a reasonable practical limit for transport and carrying.

I myself would make an 8" a truss scope to gain the advantages a truss scope has over a full tubed scope, but truss scopes below 12.5" seem to be the province of the ATM.

 

But, going back to a 6", I think f/6 is practical for most people and even works ok on an EQ mount.  As a dob, I'd like to see f/8 just to keep my rear end off the ground.

Though a lot of people look down their noses at it, the venerable 6" f/8 makes a very nice first scope, handily beating the typical entry-level small refractors.