John

Apologies to @Captain Magenta / Magnus for diverting this thread onto my particular issues.

Thanks again for your interesting report

28 minutes ago, runway77 said:

God bless < 70 degree eyepieces and f/6 and slower scopes

Fair point but up until now I was really delighted with the 100 degree Ethos's in my F/5.3 scope.

Come to think of it, nothing has actually changed with either the scope, the eyepieces or me so I guess I'll go on being happy with them

Well, it was clear when I put the scope out before supper.

Now solid cloud and looking like rain at any minute. Just bought the gear back in

Silly hobby !

9x50 RACI optical and Rigel Quikfinder for me. Telrad would be fine instead but I wanted something lighter.  RDF's are OK but the defined diameter circles of the Rigel and Telrad can really help with star hopping.

50 minutes ago, Don Pensack said:

Some reasons:

--you haven't looked for it and/or you don't look at the edge.

--your eyes  have too much astigmatism to see coma.  Comparatively, coma is a lesser issue than astigmatism.

--the outer part of a comatic star is fainter than the inner part.  If observing in light pollution, the outer part of the comatic star might not be visible.

--your optical instrument doesn't deliver sharp enough star images to notice it, either because of poor seeing or lesser optical quality.

--you eyepieces don't deliver sharp enough star images to notice it (not in your case, but often the case with many observers using low-end widefield eyepieces)

--You haven't trained your eye to see it  (this might be a blessing)

--You see it, but you ignore it (this is often the case, I find, with observers who aren't very fussy with image quality--probably not your case).

 

 

Thanks Don.

Rather depressing though 

When I had a 90mm mak recently I tried both the stock RDF and a 6x30 RACI optical finder (separately) on the scope. The optical finder was more effective (plus they are a more regular tool for me) but I felt that it didn't go with the very portable / compact scope package that I was after so I got on and used the RDF making sure that it was well aligned with the scope optics and managed OK with it. I don't actually have a straight through optical finder of any sort now because I much prefer RACI's.

It's somewhere between FPL-51 and FPL-53 in terms of potential for CA control but remember that the glass used in the mating element plus the figure and polish of the lenses is equally important.

So why am I not noticing coma ?

I've been using Ethos eyepieces with this scope for over 5 years now. Without coma correction.

I guess I thought I was a reasonably good observer. I'm beginning to wonder now .......

Generally, the lowest power eyepiece that I use with my 300mm F/5.3 Orion Optics dob is my 21mm Ethos. The field stop on that is 36.2mm.

17 minutes ago, JeremyS said:

Did I miss something, John? New Barlow?

@Dantooine has very kindly loaned me an Antares 1.6x barlow to try out

10 minutes ago, Captain Magenta said:

..... I hope I haven’t caused you to “once you look for it you can’t again miss it”.

Well it might be there but at a level which does not intrude, for me at least.

I've often let targets like Saturn and double stars drift right to the field stop and the detail seems to stay sharp right until the object disappears behind the field stop edge.

Sounds good to me !

Nothing but clouds here for the past few nights. I could do with a short non-technical stargaze

I've got a nice barlow to try out ....

3 hours ago, Captain Magenta said:

Funnily enough the OO mirror I ordered during Lockdown has just been completed and is about to be delivered. It is 1590mm apparently and is intended to replace my SW 1500mm, turning my f/4.9 into a f/5.2 - the comparison will be interesting. Though I'll need to do a bit of tube-surgery to install it. In time I'm going to order a custom carbon tube for it.

Mine is an OO 300mm with a focal length of 1590mm so F/5.3.

I'm sure there is still coma present of course but at lower levels than at F/4.9.

That said, I used to own an OO 250mm F/4.8 and happily used Naglers without a coma corrector in that scope so perhaps I'm not particularly sensitive to coma ?. I don't like astigmatism though.

The past week or so have been poor but over the last few months I've managed to do plenty of observing. It has been one of the better things about what has otherwise been a very challenging period as we all know.

I don't image though and I've kept my setups to those which are simple, quick to deploy and tear down. That seems to have helped to capitalize on the clear skies when they have been around.

If I had to setup complex gear each time I wanted to participate then I think it would be a much more frustrating hobby !

Edit: I've just realised that this thread was in the imaging section so please ignore my post - it's not really relevant.

Very interesting report - thanks for posting it

It is interesting to note the differences in coma between an F/4.9 newtonian and an F/5.3 (which I have). I guess there will be some present at F/5.3 but even with my 100 degree AFoV eyepieces I don't find that it is noticeable.

Good to know that the Paracorr will do the job if I do start to notice it

2 hours ago, markse68 said:

what was the reason for doing it this way and not staying a bit longer?

I think it's not so much a case of landing as it is moving alongside and docking with an object as small as Bennu. It's gravitational pull is so low.

They would have had to find a way to anchor the craft to the loose surface. Not easy I suspect.

Astonishing !!!

Thanks for posting this Andrew

I collimated a mak-cass 90 recently. I used an artificial star. It was a bit of a fiddly process with very small adjustments needed but once you get it, it stays put. I then checked it at high power (180x) on Polaris at the 1st opportunity.

I'm not sure that the cheshire can really help with a mak-cassegrain ?

I've never found filters really help with galaxies. Nebulae, yes, but not galaxies in general. Dark skies are the key for galaxies.

The black anodizing goes gold when exposed to strong sunlight for long periods. It may have been stood in a window or a conservatory for some time during it's life.

I've seen this quite often with Skywatcher scopes and others. I have a "gold" Meade 26mm plossl

54 minutes ago, parallaxerr said:

Thanks Ruud, that's how I understand it. One negative I'm picking up on is the effort required to hold the specific position to achieve the effect, which may be less than ideal for longer sessions.

I'm not a fan of "floating" my eye over the top of an eyepiece personally. I like a nice eye cup to rest my eye socket against and also to keep stray light off the eye lens.

I had that problem with the ES 92 12mm and the Tele Vue 32mm plossl before I fitted the eye cup extender to it.

These are personal preferences though. Some folks seek out these characteristics.

I nearly mentioned the Pentax XW28 in my earlier post but I'm aware that they are like hens teeth to find and do have some mild issues of their own. Probably better than an erfle though.

Yours is very similar indeed to the 90mm F/13 that @Lockie posted pics of in the thread I linked to earlier. This is the same view of the 90mm F/13 and you can see that small nut in the same position. @Lockie did manage to get the dew shield off though - I wonder if he could tell us how ?

I reckon that little nut and bolt are what is holding the cell / objective / dewshield assembly onto the tube. Sometimes there are three of those at evenly spaced intervals but perhaps they figure that one is enough for the relatively small aperture.

With the dew shield glued into the objective cell it's not going to be possible to get at the other end of that bolt as far as I can see. 

It's not aimed at owner maintenance !

I had one for a while. It worked well in my slower scopes - F/8 and F/9. In faster ones it still worked of course but showed some edge of field astigmatism. Quite a nicely made eyepiece I thought. Similar in build quality to the Nirvana's and William Optics ranges.

Might be worth a "wanted" advert if you are not in a hurry. You might be able to pick one up for quite a bit less than the retail price.

I used to like Saxon. Good solid British rock band