Can you use .... ??

[FlashDrive]

Can you use a f/4 Mirror in a small Dob ( 6" to 8" inches ) for ' visual use ' successfully ... I dare say, collimation needs to be spot on .

Any ' faster ' than that ....things could get ' tricky '

Col.

[Camelopardalis]

Why not? Would give you a nice wide view :D

For the average eye, I'd imagine collimation wouldn't be half as tricky as for imaging...

[FlashDrive]

Thanks Dunk :)

[JeniSkunk]

A 150mm/600mm f4 Newt, what eyepiece barrel diameter would you use?

[skysurfer]

F/4 aperture has coma issues on most eyepieces showing stars like 'comets' near the field borders, even when using the $$$ Naglers / Ethoses. A coma corrector (https://www.bintel.com.au/?post_type=product&s=coma+corrector&v=322b26af01d5) will help.

[FlashDrive]

Your right about Coma .... !!
Maybe not a good idea ... !!

[Atmos]

Col, I think it’s time for you to step up and get a F/3 :lol:

[FlashDrive]

Nah ...!! A 30" inch Dob I would ...!!:thumbsup:

[JeniSkunk]

Col, to do a 30inch f3 Dob, you're looking at a focal length of 2286mm.
But wouldn't such a scope be even more impacted by coma, than the proposed f4?

[Camelopardalis]

Unless you've got 20:20 vision, Col, you'l be right :thumbsup:

If you have, get a coma corrector ;)

Or not...isn't that part of the fun?!

[FlashDrive]

I'm giving the whole idea a miss.

Col...:)

[Startrek]

Even f5 newts have coma , my Televue Paracorr type 2 is a game changer in my Skywatcher 12” Goto is concerned, pin point stars at edge of field
In regard to imaging I use f6 and f5 newts and my Baader Mk3 once again gives pin point stars at edge of field

Never even considered an f4 scope , to damn hard to collimate and a coma galore

[mental4astro]

I have two 8" f/4 dobbie's. Bloody awesome rich field scopes. With a 30mm 82° eyepiece I can get a 3° FOV - that's the whole of the Veil Neb :)

These two Newts are also the only Newts I use a coma corrector with. Been using the Baader Mk IV for years as it adds no magnification factor, weighs bugger all and the focus shift is very little. I onlly use the corrector with with my 30mm & 24mm 82° EPs, and nothing else. Image is stunning with my 13mm Nagler without it. Coma probably is there, but along the very edge of the FOV, but who cares - you don't observe there. You really don't.

At home my solid tube 8" is my grab'n'go scope as it is always ready. Also a great outreach and sidewalk astronomy scope as it so light and easy to move about and for novices to use.

From a dark site, it is a beast of a rich field scope! 8" aperture with a 3° TFOV.

It is also no slouch when I push it to 400X on the planets.

The solid tube scope I also put onto an NEQ6 for photo.

Large secondary? Yes. And, so what? Think it's like looking through what? Pea soup or a brick? Not at all. Lovely crisp views, gorgeous massive Eta Carina views complete with dark pillars even from my home in Sydney, through to wee white ovals on Jupiter. Funny thing is I don't hear the same criticism of a large secondary in f/3.5 and faster monster dobs :shrug: Not with the same fervour.

So, yes, an 8" f/4 scope makes for a great visual instrument. Not tried one? Don't dismiss it until you do. Perfect all in one scope? No. Let me know when you find it though...

Alex.

Edit: I added a pic of each 8" f/4 dobbie.

[mental4astro]

A few examples, including a little sketch animation I did of a Europa shadow transit using one of my push-pull 8" f/4 dobbies, at 320X, observed over the whole period of the transit from start to finish. No tracking, all push-pull, nudge, nudge, nudge.

https://www.youtube.com/watch?v=FVpxeM7Tj8U

The sketches below were mostly done from my backyard in Sydney, with the one of the Veil Neb done at the Qld Astrofest a few years back.

[mental4astro]

Nearly forgot.

Collimation - no more difficult than any other Newt I use. But I have modified the secondary assembly of the solid tube OTA as what comes out of the factory is poor in design and materials & the primary mirror cell is also my own build, and the other dob is totally my own construct. I check the collimation of all my dobs/Newts every time I use them, & find I need to do minimal tweaking of the collimation. No scope any more tweaking than any other.

f/4 any more difficult than say f/5? No. I find no difference.

f/4 any more critical than f/5? Again no. Initial collimation is always tricky with all Newts, but from then on no problem.

The BIGGEST problem is as I said earlier the design and materials used in mass production scopes is poor, and it leaves the secondary mirror poorly supported in the spider assembly and liable to shift as the scope moves about, and the primary cell is a compromise too. THIS is where I would say the problem lies, and not because the scope is f/4 or f/5. These movements can be small, but that's enough to cause problems, and if not resolved will prove a constant pain in the neck, and even people to say that these scopes are problematic. They are not problematic, just poorly built (mass production scopes) - there are thousands of pages written on "fixes" for these mass production scopes for a reason. This is a shame as the optics can be mighty fine.

Alex.

[FlashDrive]

Thank You Alex ....I may re-visit the idea of this type of Scope sometime in the near future....but 1st..I will concentrate on the 10" LB.

Col....