Second Time Around

2 hours ago, Don Pensack said:

Human vision has a sharp center of 2-5° wide.

With an apparent field up to 68° or so, you can simply move the eye to look at the field, providing the image forms far enough away from the eye.

With anything wider, moving the eye sufficiently to look at the edge will move the pupil of your eye off the exit pupil of the eyepiece.

You have to roll your head over a bit to use direct vision to look at the edge.

And, the wider the apparent field, the more you have to roll the head to look at the edge.

If you stop to think of it, the edge of the field in a 100-120° eyepiece is closer to parallel to the scope than it is to perpendicular.

Some people don't mind moving the head (I'm one), while others simply hate it (maybe they have arthritis in their necks?).

 

Now, your peripheral vision extends to about 145° horizontal and 125° vertical, so seeing the edge of the field with peripheral vision when staring at the center of the field is easy.

It's looking at the edge with direct vision where you have to roll your head.

I have a damaged neck and, having tried wider field eyepieces, find I much prefer 60 to 70 degree AFOVs.

My favourite eyepiece of all is the 7.7-15.4 mm APM Superzoom.  Unlike most medium power zooms this has an almost constant AFOV.  This is 66-67 degrees, that I find ideal.  If I want shorter focal lengths I Barlow the zoom and so keep the same AFOV.

The usable eye relief is very much less than the quoted figure. In fact it's only just enough for some people who wear glasses to see the full FOV. However, it has more eye relief than the ES 24/68, and even more than the Panoptic 24/68.

An 8 inch Dobsonian is often recommended as the ideal starter scope.  In fact, they're often lifetime scopes.

However, in my experience one of the most common reasons beginners drop out, indeed probably the single most common, is that they struggle to find objects.

Yes, Go-To will help find objects and track them, but there's an even simpler way.  It's a type of "push to"  called Starsense Explorer made by Celestron.

It doesn't move the scope by means of a motor.  Instead you push the scope to where arrows on your mobile phone direct you.  The app works with most modern mobiles - there's a list on the Celestron website. 

The app is really easy to use.  Indeed my 7 year old grandson had no problem at all.

As mentioned above, you will need to "nudge" the scope to keep an object in view, but this is something that's easy to learn.

It's a little more expensive than a fully manual Dobsonian and comes with just one eyepiece, but you don't need to buy an extra finderscope as most of us end up doing.

First Light Optics stock it, together will all the Starsense Explorer range that are any good.  Go to https://www.firstlightoptics.com/celestron-starsense-explorer-series/cel_starsense_exp_8_dob.html

Ah, OK. I've learnt something there - thanks!

I have the excellent Farpoint 2 inch Cheshire.

I also have a  couple of Parallizers, plus I have a 1.25 inch Cheshire on the way from Farpoint,

Oh no!  I had a lot of respect for John and learned much from him as I'm sure others did.

Condolences to his family and friends.

How does the altitude adjustment on the Ursa Major compare with the spring type on the Stella Lyra?

Here in East Kent so many forecast clear nights over the last couple of months have been spoilt by thin cloud.  It's been so much so that I've been losing typically 2-3 magnitudes.

It's been showing up on infrared satellite images, and sometimes looking through my night vision device shows otherwise invisible broken cloud rather like a thin mackerel sky.

Has anyone else found the same this autumn/winter?  If so, whereabouts are you?

Yep, I'm going up to London on the Saturday.

Here are some thoughts on the zooms I've used so far, excluding the expensive APM Superzoom that is now my most used eyepiece:

Hyperflex 9-27mm.

I compared it with my Baader during the day on my shed. Even though the focal lengths are lower, the actual field of view at 27mm is about the same as the Baader 8-24mm at 24mm. Nor was it as sharp as the Baader in my f/6 refractor. I was planning to try it with my Quark, but there were no sunny days or clear nights before I passed it on to two of my grandkids to use with the 70mm f/10 I bought them for Xmas. Not surprisingly, it was better at f/10. Not Dioptrx compatible.

Svbony 7-21mm.

I tested it on a very low down Saturn, and surprisingly found that on my f/6 refractor it was as good as the Baader on axis. However, sharpness fell off somewhat off axis plus the field of view was quite a bit smaller. However, trying it on prominences with my Quark it was way behind the Baader, seemingly because of lower contrast. I bought this firstly for outreach rather than risking my Baader zoom. Secondly, it's very small and light, even compared to other Svbony models, so I thought it might therefore be useful in binoviewers. I need to test it further at night when my binoviewers come back from repair, but will almost certainly keep it for the reasons given. Dioptrx compatible with O-ring.

Celestron 8-24mm.

The version I bought was the spotting scope one, so it might be different to the astro model. The first thing I noticed was that it was even less parfocal than my Baader. In fact, none of these 4 zooms are parfocal to my aged eyes, but might be to someone younger. The field of view was also smaller. What disqualified it completely though was that it wasn't threaded for filters or screw in Barlows. I therefore returned it without further testing. Dioptrx compatible with O-ring.

Baader 8-24mm.

This is more expensive than those above, and not surprisingly outclassed all of them, both in sharpness/contrast and field of view.  Until I bought an even better but more expensive APM Superzoom it used to be by far my most used eyepiece, although I also own a selection of quality fixed focal length eyepieces. Dioptrx compatible with O-ring. The only downside, that's really important only for binoviewing with a Dioptrx, is that unlike most zooms the eyelens rotates when zooming.

I'm also a great fan of Barlows, as long as they're not the horrible plastic ones packaged with some cheap scopes.

I'd recommend one of the dual power 1.5x/2x Barlows.  These will effectively triple the magnification of each of your eyepieces.

For those of you who aren't aware, a Barlow lens multiplies the magnification of any eyepiece it's used with.  It goes in the focuser before the eyepiece.  The multiplication factor varies but 2x is most common.  Some of these 2x Barlows can also be used at 1.5x, although it's not always mentioned in the blurb. These dual 1.5x/2x Barlows allow the black lens cell to be unscrewed from the body of the Barlow and then screwed into the filter thread at the bottom of an eyepiece to give approx 1.5x. 

Another tip with most zoom eyepieces is to use the lowest amplification Barlow possible.  This is because most zooms have the widest apparent field of view at the shortest focal length.  For instance, if you want the magnification of a 6mm focal length then using a zoom at 9mm with a 1.5x Barlow will give you a wider field of view than using the zoom at 12mm with a 2x Barlow.

I'd very much recommend a zoom eyepiece as your workhorse. One zoom eyepiece will cover multiple focal lengths and so is really excellent value for money.

Despite having high quality fixed focal length eyepieces, I use my zooms a lot more often. The zoom plus a Barlow lens and a low power, wide field eyepiece is often all I use the whole evening.

Fixed focal length eyepieces may be slightly better corrected when compared with a zoom at the same magnification. But that's not always a fair comparison as that magnification may not be the optimum for a given object. This is because one of the many advantages of a zoom is to be able to dial in precisely the best focal length. For instance, this may be 13mm or even 13.1mm, which may actually show more detail than shorter or longer fixed focal length eyepieces - even better quality ones.

I particularly like the ability to increase the magnification to make use of brief moments of good seeing (a steady atmosphere). It takes more time to swap out an eyepiece, and the opportunity may then be missed. You can't see anything if you haven't got an eyepiece in the focuser!

Zooms also enable the field of view to be varied to frame an object to get the prettiest view. For this reason I particularly like them for clusters.

Many of those who post here and advocate fixed focal lengths are experienced observers. It's so easy to forget what it was like as a beginner! A zoom eyepiece enables beginners to easily learn what difference a change of magnification makes on all the various classes of object. It also shows them what focal lengths would be most useful to their eyes, their telescope, and their observing conditions. They then have the option of buying/not buying the most appropriate fixed focal length eyepieces for them. For these reasons I'd always recommend that beginners buy a zoom as their first eyepiece.

Good luck with whatever you decide, and do let us know how you get on

28 minutes ago, Alan64 said:

Alas, I wish the primary mirrors were parabolic...

https://www.telescope.com/Orion/Orion-SkyScanner-BL102mm-TableTop-Reflector-Telescope/rc/2160/p/134763.uts

FLO says both their Ursa Major ones are parabolic.

Don: re your reply to my point 9.  Rather than "finder eyepiece" I meant "finderscope eyepiece."  I've edited to say this, so thanks.

The finderscope eyepiece is one of the many 20mm/70 degree ones with a crosshair.  I've found you can replace the eyecup on these with a Dioptrx.

Before I bought my 8 inch Dob I examined several makes in showrooms to get a hands on feel.  (The Stella Lyra is FLO's own brand version of one of the two GSOs I tried).

As pointed out above, the quality of the optics is very similar.  You'll probably add different accessories to all of these models to get your ideal scope, and some of the supplied accessories you may not keep anyway. 

But almost certainly you'll keep the mount.   Unless you make your own, replacing a mount is expensive, even if you can find one.  So in the long run I feel the mount is by far the most important consideration. 

The mount that stood out was the Bresser.  As Keith posted above, the Bresser is better than the GSO and a lot better than the Skywatcher.  I'd add that the altitude rings also make it easier to carry the tube assembly in one hand.  Had I not found a secondhand OOUK Dob, I'd have bought the Bresser myself.

Quote:

"Perhaps, but without a lip similar to Tele Vue, you will lose the combination of free rotation with secure fitting. I wouldn’t want to knock the thing off the top of the eyepiece whilst clumsily adjusting it with cold hands.   

You could screw it tight and spin the whole eyepiece in the focuser I suppose - but that may be tricky when using eyepieces with screw up/down eyeguards, which is quite common in the longer focal length category. "

Many eyepieces have a 43mm screw top under the eyecup.  If you put a 42mm x 2mm o-ring or elastic band over the threads, not only is the Dioptrx held securely, but you can also rotate the Dioptrx to vary the angle.

It's even better with a 42mm (T2) thread; then you don't even need the o-ring or elastic band for the Dioptrx to rotate.

The first pic below shows such an elastic band attached to a Baader zoom; the second shows my holding the Dioptrx without the eyepiece falling off.

A final tip: put a tiny bit of glue or Blutac at the 12.00 position on the outer rim of the Dioptrx.  That way you can quickly and easily get the correct angle for the Dioptrx when changing altitude on a Dobsonian etc.

I think glasses v Dioptrx is very much a matter of personal choice.  Depending on one's eyesight there are pros and cons of each, and what's best for one person may not be the best for another. 

But making many eyepieces compatible with a Dioptrx is often pretty easy and leads to increased sales.  

However, I wonder how many have tried a Dioptrx?  And of those, how many have actually done a head to head comparison?

These are the reasons I prefer a Dioptrx plus some comments:

1) The coatings are better than on even the top of the range Rodenstock lenses on my glasses

2) The polish is almost certainly better than that of glasses - few people would pay for this on glasses so almost no demand, so no supply

3) My glasses have plastic lenses that scratch more easily than the glass lens in a Dioptrx

4) The angle of my astigmatism changes between my annual eye tests.  With a Dioptrx I can adjust this at the telescope

5) I find that, like many, I need an extra 0.25 correction for astigmatism at night

6) I wear my glasses on a cord and have no problems taking them on and off for looking at the sky - it quickly becomes automatic

7) I have no presbyopia and so don't need glasses at all for looking at close range objects such as screens or maps

😎 I have a Dioptrx on my finderscope eyepiece, but even if I didn't I don't find astigmatism critical on a finderscope as it's doing just that - finding

9) On the rare occasions I share my scope I wear my glasses

10) And most of all, and this is the clincher, having done head to head tests I found that I can see more with a Dioptrx than with my glasses

New prices have gone up a lot in the last couple of years due to COVID restricting availability plus the war in the Ukraine.

So I think that a percentage of the current price would make sense.  This will vary according to condition and what you're selling.  Others can chip in here, but I''ve sold quite a lot of gear over the last two years at about 2/3rds of the current price.

I'd add that you usually get a better price selling items separately rather than as a job lot.

Yes, I'd prefer 32mm for the maximum FOV and 12mm for greater eye relief and closer to the 2mm exit pupil that's generally considered optimal for DSOs.  However, the 32mm is £7 more expensive than the 26mm supplied and so would put the price up.

Since they're FLO's own Astro Essentials range of Plossls I wonder whether they'd allow us to swap them and pay the difference?

Whatever, the scopes as they are represent exceptional value for money.

Edit note. On reflection, a 32mm eyepiece would give an exit pupil of 5.1mm.  Although giving a wider FOV this might be too large an exit pupil where there's a lot of light pollution. The same applies to those of my age whose pupils no longer open as wide as 5.1mm.

If you do have a 26mm eyepiece then it makes sense to then add a 10mm as a 12mm would be too close to a 26mm plus a 2x Barlow.  Sure, there's less eye relief with a 10mm Plossl, but those wearing glasses to observe still wouldn't be able to see the full FOV at 12mm anyway.

The £7 extra may be too much in this price bracket, at least for the 80mm.  So on balance FLO may have got it right, even though I'd personally prefer 32mm and 12mm.

Browsing the latest new products from First Light Optics I came across new 80mm and 102mm table top Dobsonians with interesting and unique specs.  What's more, like all FLO's Ursa Major range they're excellent value for money.  These are the main features:

Parabolic f/6.3 mirrors.  The longer focal ratios give sharper images than the usual f/4 and f/5 offerings.  They also work well with less expensive eyepieces.

The fully multi-coated 26mm and 10mm eyepieces are 4 element Plossls with 52 degree FOVs compared with the poorer quality 3 element 45 degree FOV Modified Achromats or Kellners usually supplied.

Solid tube so no shroud to buy or make, but non-collapsible.

Rack and pinion focuser, that I prefer to the helical type often suppled with table top reflectors.

Red dot finder, collimation cap and red light keyring torch also included.

Perhaps the only downside is that there's no dovetail, but that may have added too much to the price.  Hopefully, when they're less busy FLO will let us know whether one can be fitted. It would also be useful to know the secondary sizes.

The prices are an exceptionally low £119 for the 80mm, and £139 for the 100mm!

https://www.firstlightoptics.com/beginner-telescopes/ursa-major-80mm-tabletop-dobsonian-telescope.html

https://www.firstlightoptics.com/beginner-telescopes/ursa-major-102mm-tabletop-dobsonian-telescope.html

I ordered an accessory from Olivon in Germany at 2pm the day before yesterday.  There of course it would have been 3pm. Because of being near to Xmas and the Royal Mail strikes I opted for express delivery via DHL.

The package was delivered about half an hour ago by DHL!

So a big thank you to both Olivon and DHL.

Paul, would a Celestron Starsense Explorer (CSE) scope work for you?  It's quicker and easier than Go-To.  It would also be more accurate for you as well as only a one star alignment is needed anyway.  This is because it uses plate solving to determine where it's pointing. Like you I just use a planet though.   

With the latest update it's got a huge number of objects in its database.  However, it doesn't track.

With a simple mod the CSE can be transferred from scope to scope via finder shoes.

Wow! I especially like number one.

Have you tried the APM 2.7x Barlow by itself without a GPC?  If so, what was the outcome?

Thanks.

22 hours ago, globular said:

Interesting idea. Please do let us know the results of your trial.

@Second Time Around I know you are a dioptrx expert - do you know if they would work earlier in the light path, or do they have to be close to the eye for proper correction?

I'm certainly no Dioptrix expert, witness I don't know the answer!

My gut feeling says it won't work but I'm probably wrong.  However, I too would be very interested to find out.

P.S. My first thought was wouldn't Televue have made a Dioptrx that screws onto the filter thread.  But then I realised that would be too easy to copy.  They want to sell eyepieces, so it would be better for their sales if a Dioptrx fits just their products.  Luckily for us a fair few other eyepieces from other makers will accept a Dioptrx.