John

Which screws James ?

Can you post a picture ?

Nothing wrong with any of those

59 minutes ago, wheresthetorch? said:

Wow - that's a lot for an 8" dob! 😮

It sure is. The original Revelation 8 used to be £199 new from Telescope House. The lowest cost 8 inch dob at that time, and since I would think.

8 minutes ago, johninderby said:

I’ve found the Bresser mirrors to be pretty well the same as the SW although they seem to perform a little bit better as they come with a proper mirror cell. A six point cell in the case of the 8 inch so the mirror is better supported and also has a proper anti-reelection coating on the inside of the tube and not just a thin cost of paint like the SW so a bit more contrast.

I have owned a few SW dobs and really liked them but now there is more choice.. 

Do we know what company produces these scopes for Bresser John ?

Skywatcher are obviously made by Synta and Meade used GSO for their Lightbridge dobs. GSO also produced the Revelation and TS dobsonians as well.

I was just wondering about these Bresser dobs ?

I had the 4.8 Nagler T1 as well at one point. That and the 7mm were my first Naglers. Eye relief was not a strong point I agree. The 4.8mm was very sharp though. Apparently Roland Christen used the 4.8 T1 Nagler as his principle testing eyepiece for his refractors for many years.

I didn't hate mine at all but I was happy to move to the T6 Nagler 5mm and 7mm to replace it in due course because they proved to be better eyepieces

Then the Pentax XW's and Ethos's proved to be even better .... for me at least

Nice scope - I used to have an earlier version of the Revelation 8. My first dobsonian !

Yours is better turned out than mine was though

15 minutes ago, miguel87 said:

I have an f5 scope and with my widest eyepiece of 70°, there is fairly significant stretching of stars right at the edge of the view. I would guess this would worsen with 90 or 100 degree eyepieces. An f8 scope would cope much better if you want crisp, wide angle views. Although many amateur astronomers are more than happy with f5 or faster scopes.

Actually, the 100 degree eyepieces that I have used have been well corrected even at F/5. Look at the prices of such eyepieces though  . With such eyepieces another aberration called coma becomes more obvious but that is produced by the primary mirror of the scope, not the eyepiece.

Astigmatism (eyepiece produced) looks like this:

Coma (scope mirror produced) looks like this:

You can have a combination of both of course !

These were the sharpest eyepieces that I've used - a University Optics HD ortho 5mm and a TMB Supermonocentric 5mm. The latter is probably the best high power eyepiece that I have ever owned in terms of sharpness, contrast and lack of light scatter. With a 30 degree AFoV and about 2.8mm of eye relief it was hard work though. Vixen have done very well to produce their HR range that deliver excellent performance and provide 10mm of eye relief.

From my experience, eyepieces optics start to be challenged (ie: show aberrations if they have them) from around F/7 and faster.

From F/5 things can get really messy in the outer parts of the field of view if the eyepiece is not well corrected.

I have not used one myself but it sounds like something an observing tent might be the sort of thing you are after ?:

https://www.astroshop.eu/instruments/omegon-tent-observatory/p,12278

13 minutes ago, Sprint said:

ahh ok so with a 70deg eyepiece as a constant an F6 scope will show the same area as an F10 but the latter will remain sharper to the edge of the area??

No. The eyepiece will show more sky (a larger true field of view) with the F/6 scope. If it is a low cost eyepiece then there may well be some distortion of star images towards the edges of the field of view at F/6 though. 

I'm not sure that you actually have any gaps. Moving from 12mm to 8mm is quite a natural progression as is 20mm to 15mm.

I think you have all the eyepieces you need to make the most of your scope

If you can start to put some £'s aside towards ultimately moving to, say, a 200mm aperture scope, that will make far more difference than investing in more eyepieces I reckon.

The true field of view is derived, roughly, from the apparent field of the eyepiece divided by the magnification that the eyepiece gives in the scope.

If you want a larger magnification and still to retain the same true field of view, you can use an eyepiece with a larger apparent field and a shorter focal length.

This online tool can be played around with to find what different combinations of scope and eyepiece can show in terms of field of view:

https://astronomy.tools/calculators/field_of_view/

M81 and M82 are some of the targets available in the above tool.

Interesting Sky & Telescope piece on "test" double stars and pushing the limits of your scope here:

https://skyandtelescope.org/observing/pushing-limits-a-spring-sky-double-star-romp/

A 1 degree true field of view is large enough to see both M81 and M82 with a little bit of room left around them.

The combination that gets a 1 degree true field varies depending on the scope and eyepiece used.

As an example, a 102mm F/10 scope with used with a 20mm focal length plossl eyepiece shows a 1 degree true field of view and 50x magnification - enough to fit both galaxies in the view at the same time.

Larger apertures show the galaxies more clearly but they are visible in quite small scopes.

This sketch was made by Michael Vlasiv using an 205mm aperture newtonian, under a dark sky, and a 25mm eyepiece giving a 1.25 degree true field of view. As you can see, both galaxies fit comfortably within that:

1 minute ago, niallk said:

....  Think I'm done purchasing EPs though

I think I've reached that point as well. Probably a while back really but I've had the occasional lapse since 

I have used quite a lot of orthos and they are very good in pure optical performance terms. I do prefer longer eye relief, wider field of view and a larger eye lens these days so I don't find myself reaching for my remaining ortho very much.

I think viewing comfort as a lot to be said for it, particularly using undriven, alt-az mounted scopes as I do.

M81 and M82 can be seen with very small and low cost scopes. They were the 1st galaxies that I ever saw through a scope - a cheap and old 60mm refractor. I was so thrilled to see them, I kept staring at them for hours !

You can pick them up with 10x50 binoculars but they are defined better as a galaxy pair with a scope at around 25x magnification.

From my back yard, where I have some light pollution though it's not too bad, I find that I get better results using a slightly higher magnification even if that is at the expense of true field of view breadth.

On the largest objects (eg: the Veil Nebula) it is nice to have the largest field you can get but most targets fit comfortably within a smaller field.

The Panaviews are a popular choice of first 2 inch eyepiece and work pretty well with the 200mm F/6 dobsonians. They are not perfectly corrected all the way to the edge of the field but are not bad at all for their price.

The 1.25 inch size eyepieces of similar focal length simply can't show as wide a field as those in a 2 inch fitting because the inside diameter of the eyepiece barrel is what determines how wide the field can be. A 32mm 2 inch eyepiece can show 40% more sky than a 32mm 1.25 inch.

The Aero ED eyepieces are more expensive but are a bit better corrected than the Panaviews as John says above. I currently have the 30mm and 40mm in that range.

Of course, when you are thinking about filters, the question of 1.25 inch vs 2 inch comes up when you have a 2 inch eyepiece

Sounds like you have got a good range now.

Paying more would certainly take you into the realm of diminishing returns.

Its getting a taste for very wide field views that costs the money 

The Skywatcher HEQ5 has a touch more capacity than the Celestron AVX. The NEQ6 is better still but my goodness they are heavy !

I guess you will know of this Italian website ?:

https://www.stelledoppie.it/index2.php?section=1

4 hours ago, Ciaran Meier said:

Hi Mark

Just want to check with you what was split here.  Was it the A B pair at 1.1" sep.  I've an 8 inch OOUK newt and I reckon 4" is about the best I'm getting with good collimation.  Doubles folk on here seem to go for the frac for this job.  What secret magic are you using to get down to 1" with the 150P 😁.

Ciaran. 

Can you split the "double double" Epsilon Lyra with your 8 inch OOUK scope ?

Those pairs are 2.3 and 2.4 arc seconds apart respectively and should be a straightforward split for your scope at around 120x magnification.

If you can't split them then either the seeing is poor or the collimation needs adjusting.

Another good "test" double that is better placed right now is Porrima - Gamma Virginis. That pair has a separation of around 2.5 arc seconds.

1 hour ago, Captain Magenta said:

John I apologize for quoting you 6.5 years on, but seeing as you have the gear, that would be a fabulous side-by-side comparison to do (please?)

M

Actually, since then I have done that comparison on a few targets. The image of, say a tight double star, looks pretty much identical through the 12 inch dobsonian stopped down using an off axis cardboard mask with a 10cm aperture. The aperture can be positioned so that it avoids both the secondary vanes and the secondary support so it is an unobstructed 100mm scope.

I would expect the mirror system to transmit a bit less light than the objective lens of the eyepiece - perhaps 90% rather than the refractors 98% so the performance on DSOs might be a little less with the stopped down newtonian.

1 hour ago, Don Pensack said:

The observer who wrote that comparison is almost exclusively an observer of planets and double stars and uses exclusively small scopes.

That doesn't invalidate his comparison completely, but his results might have been different had he used larger scopes or evaluated the diagonals on ultra-faint objects.

Not to mention that some of his comments might have been eyepiece or scope-dependent.  He has also reviewed the Baader BBHS silvered star diagonal and never even mentioned......

...... so take such comparisons with a grain of salt.....

 

Will you tell him on CN Don, or shall I ?

I have the Baader T2-Zeiss prism, an Astro Physics Maxbright and a couple of TV Everbrites and I honestly can't tell the difference between them.

Some folks apparently can though