Giles_B

2 hours ago, Louis D said:

Unless you're dead set on the last 6mm of field stop diameter and 9 degrees of AFOV, I'd get some variation of the 30mm APM UFF.  I much prefer it to my 30mm ES-82 for many reasons (lower weight and bulk, greater eye relief, near perfect edge to edge sharpness, lack of ring of fire, etc.).  That, and you'll save a nice chunk of change.

Hum. My reasoning for that particular item was that I love my 20mm T5 Nagler, and I really don't enjoy my 34mm ES 68. Admittedly the reasons are a bit superficial - ES eye guards are flimsy and I get frequent kidney beaning, also compared to my Naglers I find the ES relatively low contrast in local bortle 5-6 skies. All things I can live with, but would rather not. The big Nagler seemed an obvious, if expensive, choice. I did consider the Panoptic.

It's moot now, the cash is spent on the s50. It'll be a good six months before I'm anywhere near the next big astro buy - hopefully I'll see the s50 before that!

Just taken the plunge and preordered an S50 from Flo.

Was saving for a 31mm Nagler - but family and work pressures, plus the endlessly cloudy weather seem to be conspiring to keep me away from the night sky at the moment (even solar is a bit dicey).

I'm hoping a easy eaa setup will let me scratch the astro itch a bit more regularly.

13 hours ago, PatientObserver said:

I most recently bought the Pentax XW 30. Makes scanning the sky so much easier. Might not have needed the finderscope afterall. Will likely get the 40 mm eventually, but I need to build up the astronomy savings account again.

How do you build up an account? I started an astro savings account, but I keep spending it before gets into 3 figures!

Yep, fully agree I'm just looking for trouble. That's what these short cloudy nights have driven me to!

1 hour ago, vlaiv said:

One of distances is related to resolving the small opening of artificial star - your being 50 micron star.

Other has to do with spherical aberration.

You can calculate first distance quite easily - look into what is resolving power of your aperture and then look at which distance star presents angular size that is smaller than this value.

For spherical aberration - things are not quite as easy as you need to understand quite a bit of optics to calculate that one - but it boils down to this:

Point source "radiates spherical wavefront" around it. When this wavefront hits aperture - it is slightly bent. When we look at objects that are far away - they are effectively at "infinity" as far as numbers are concerned - and that sphere has infinite radius and thus is almost flat - we get flat wavefront. This is what we want, and what the scopes are optimized for.

If our point source is not at infinity - it will introduce some level of spherical aberration to the wave front. When you examine image in telescope - you won't know how much of spherical aberration comes from telescope itself and how much is from the fact that source is close and not at infinity. Closer the source - more spherical aberration there will be (ratio of distance to aperture changes - so part of sphere changes - this is because aperture is fixed but we change the distance).

at some distance this spherical aberration induced by closeness of the target gets very small and can't be detected and won't affect results of optical tests. That is the second distance at which you need to place artificial star if you want to check for spherical aberration of your telescope and want to get accurate results.

That is the clearest explanation I could wish for! Amazing

certainly the image looked similar to the aberration caused by the spider @pipnina - and i'm reassured by vlav's point that it is unlikely pinching is even across the mirror. However I will take another look with these answers in mind - if nothing else it gives me something constructive to do with scope when the cloud comes in!

I have become the proud owner of a Geoptik artificial star, and got a chance to try it out a few days ago.

Testing the collimation of my 10" dobsonian from a distance of 23m away from the (50 micron) star showed a good pattern of concentric rings, but unexpectedly the circle of the airy disk was not perfectly round, but a bit more like a 20 pence piece.

Somewhere in my mind the cogs began to turn and I eventually remembered (a few days later) that an off-round airy disk of this sort was the result of the mirror being pinched. In the last 9 months I've changed my old 3 point mirror cell to one of the newer Orion Optics 9-point cell.

Now I've been wondering if I pinched the mirror when fitting it to the new cell - although I have generally felt the mirror has some play in the cell, rather than being tightly held (I can hear it shifting slightly when I take the scope out of the dobsonian base and lay it flat - and vice versa)

However the artificial star has two distances quoted for testing for a 250mm aperture - 21m and 1km - the smaller distance is for collimation and the latter for testing optical quality (or so I believe - I'm a bit sketchy about why there are two distances!)

So before I strip the scope down again to test the mirror, I was wondering if what I am seeing is an artefact of using a very close light source, rather than a genuine aberration in the optics?

Wise thoughts and explanations would be very welcome!

Apologies for this very belated follow up to the original post.

It took a long time to get the part, the original shop going incommunicado to my emails, more shopping around and finally finding a second well reputed local engineers who specialise in making custom parts, but at a much higher quote than the original one, which meant further saving and delaying.

After some discussion they recommended keeping thing simple with a connector that would attach to the M68 adaptor. This meant that the adaptor could stay short and achieve maximum robustness at minimum length. I was happy with part when it turned up, and it mated the two parts well - the little movement on the telescope side disappeared after painting the part:

I've now had the part about eight weeks. Initially had to return it for a skim of the inside diameter so there was better side to side movement of the focuser to allow collimation. As you see on the photo the collimation is not quite there when checked with a laser, but there is no splitting of the laser as it passes through the objective, nor is any miscollimation visible to me on a star test. In any event my experiments with realigning using the adjustment screws on the focuser have shown it is tricky to achieve a better result than the one shown.

The scope is well balanced despite the extra weight of the Steeltrack focuser, and the most important result - to achieve more in-focus with this excellent scope, has been achieved very nicely - I have a massive 35-40mm more in focus and am able to achieve focus even with my Naglers (although they aren't well suited to the usual seeing conditions of solar observation). After painting the part with blackboard paint inside and out, I am also very pleased with the visual appearance as well as the performance

I think there is a false dichotomy here between using push-to / go-to to assist finding astronomical objects, and learning the night sky.

I got into the hobby around lockdown, so a couple of years ago, and from the start have used the Celestron StarSense tech to get around, unless I am looking for something very obvious like the bright planets. However, I do find that I am also gradually learning the sky despite this reliance on technology - perhaps this is not in the immersive, all or nothing, sense that comes from being under the skies exclusively with a red light and a star map, but for me the star map comes on the journey as well, and is part of the overall experience. I'll spend time at the eyepiece, and time sitting back and looking at the sky in its entirety.

These are not mutually exclusive activities.

My biggest impediment is having a garden that faces west - the view south has trees and houses close by, so I have a very narrow window for observing Venus before it gets into the tree tops and yesterday was my first opportunity this year. I used my 80mm refractor with a Baader contrast booster and a variable polarising filter. Venus presented a well defined disk at roughly 3/4 phase, and with the contrast booster added there was just a hint of the cloud tops - but very little time for my eyes to acclimatise before it got too low

The positioning would be very awkward, but I'm tempted to mount my 10" Newtonian on the tripod to see how it performs in contrast on the next opportunity.

I agree with @SuburbanMak that Baader Orthoscopics are a good place to start. You could also consider the BST starguider eyepieces https://www.365astronomy.com/index.php?route=product/category&path=146_244 - they give a wider field of view and are more comfortable to use than Orthos, and fit a reasonable budget. I got along very happily with them for a while, but moved on to more expensive eyepieces that dealt better with coma at the edge of the field (not a problem for planets). If you are also looking at filters, I'd recommend a Baader Neodymium - expensive but in my opinion really improves the contrast on planets. On the expense side, I'd recommend checking out sites like UK Astro Buy and Sell, as well as the forums here for second hand gear.

What magnification to go for depends on the seeing you expect to have -  seeing is the limiting factor for planetary work. I usually observe from a city location in the evening before midnight - it's exceptional for my 10" dob to give useful magnification on planets beyond about x170 (i.e. a 7mm focal length eyepiece). A few times a year the seeing is better at that time and I might get down to 5mm or even 4mm. On the rare occasions I've been out at 4am, the seeing is better - but my productivity at work that day is pretty rubbish!

Great video, thanks for uploading. I've been so tied up with work I've not managed to get any time with the telescope during the daytime, despite some brilliant conditions

Hope I'm in a better position to see the next one.

Thanks Mark, that's good advice. I appreciate having talked it through. I  think I'll increase the wall thickness by another millimetre at that end of the connector.

The lead time from the engineers is about 5 weeks on the part, but I'll post back with a full report once I've received it.

Thanks Mark. The baader comes with m5 grub screws to fix to the dovetail. It also has a locking screw - so long as this is loose the part mounted on the dovetail can rotate, so the m68 adaptor can be reoriented.

One of my worries was that the dovetail would be secured by a 2mm thick piece of aluminium in design A, and this wouldn't be strong enough - but this doesn't seem to be a concern you have, so I'm reassured on that front.

Thanks Mark  - the dovetail part of the sketch is confusing, because the steeltrack has an integral dovetail:

Moreover the photo on astrosurf doesn't seem to show a dovetail - or am I missing it?

I really can't see how the dovetail in the sketch would connect to the steeltrack (unless the steeltrack is of an earlier design than the one above)

I'd like to connect a Baader Steeltrack to my ScopeTech STL80A-L - I've come across this thread on astrosurf - a french astonomy website - which suggests it is feasible http://www.astrosurf.com/topic/137491-changement-porte-oculaire-critères/ although I can't quite work out what is going on with the Baader side of the connection from the design sketch.

I've also found an engineering shop nearby who are willing to take on the production of a one-off part.

As the Baader side of the connection is not clear in the thread I can think of two possible design options, sketched in section below, but I'm very new to doing DIY on the telescope and I'd be interested which you thought would be the better design (or if an alternative would be better).

The options are

A - Connect to the Baader steeltrack using the steeltrack's integral 88mm dovetail. The custom part slides over the outside of the steeltrack and is screwed into the dovetail with 6x M5 screws

B- Connect to the Baader steeltrack using the M68 connector it comes with. The custom part is machined with a M68 thread and screws on.

In both cases the connecting piece slides inside the Scopetech tube, and is connected with 3x M4 screws.

Apologies for my sketch, but hopefully the options are clear:

On 06/01/2023 at 11:26, doublevodka said:

Definitely join your local Astro club, lots can be found on Facebook, that's how I got back into astronomy and purchased my first got from a club member. Bargains can be had from Facebook marketplace, but it certainly can be like the wild west too. There is a for sale section on this forum, but it won't appear until you've made enough posts on here (25 I think) or sites like https://www.astrobuysell.com/uk/index.php where everyone is generally an astronomer and helpful

Goto does add to costs, absolutely and to an extent anything electronic will devalue, but if you are buying used then someone else has already taken the initial hit. Smartphone apps like Astrohopper will get you in the general area

As for tabletop dobs in the garden, if you have an Ikea nearby - https://www.ikea.com/gb/en/p/kyrre-stool-birch-60416925/ 3 legged stools like this give decent stability like a tripod even on uneven surfaces, I use 2 with my starblast, on to sit on and one for the dob to sit on

Given an example of used - https://www.astrobuysell.com/uk/propview.php?view=193136 Skywatcher 130 Dob, pretty much half retail price and an honest ad with plenty of pics, it's really worth a look

Also worth updating your location on your profile, there may even be someone here on the forum nearby willing to help if you do end up looking at used equipment, on a tight budget it really helps you get more for your money 👍

Just to add to the tips for second hand equipment, members of this forum are very helpful if you want to talk potential purchases through and identify any pitfalls. My experience of buying second hand from here and from Astro Buy and Sell is that people are helpful, and equipment is well looked after.

With the caveat that I'm relatively new to this (got my first scope a couple of years ago - I'm sure more expert voices on the forum will have more insight) the optical quality will be good. A 150mm aperture will give decent views of the moon, Jupiter and Saturn, and deep sky objects, although the best views of the latter will need dark skies (and would be with any scope).

It won't be ideal for photography, but you should be able to get some pictures at the eyepiece. It has tracking capability so this will help a great deal.

I'm not familiar with the "astroseeker" part of the setup, but it looks Iike some kind of go-to system. This will probably need alignment with 3 bright stars to get started, but will be helpful navigating the skies.

the general recommendation on this forum is that it's a good scope (discussed under its skywatcher branding).

Overall it sounds like a decent telescope, so go for it!

1 minute ago, Cornelius Varley said:

@Giles_B Saxon telescopes are rebranded Synta telescopes, just as Orion are in America. This one is the Star Discovery 150 under another name. Sky-Watcher Star Discovery 150i WiFi | First Light Optics

Thanks @Cornelius Varley - that's useful to know, and my apologies for any aspersions about the quality. I guess the question becomes the standard: what does @Jackal1258 want and expect from this scope?

Other than second hand examples on ebay, this is not a brand I've heard of particularly. What are the specs (can you post a link?) My suspicion is that it's an inferior scope, and you'd be better served by a reputed brand such as Skywatcher, Bresser, Celestron etc. etc.  These forums will give you plenty of buying advice, but it would be useful to know what you are after and why you think this is a good scope for you.

Where are you thinking of buying it from? I'd suggest buying from a proper telescope dealer like First Light Optics, Rother Valley Optics, 365 Astronomy etc. rather than Amazon and suchlike.

As @Philip R says, you will need to turn the other end of the telescope (the OTA - optical tube assembly) upwards to whatever you want to observe.

Do you have any eyepieces? It looks like you have only a Barlow Lens in the eyepiece holder (which increases the magnification of an eyepiece) - you won't see anything without eyepieces https://www.firstlightoptics.com/eyepieces-barlows.html  Most people start with some Plossls - if the price new is daunting then second hand these will cost around £15-£70 depending on the quality - whichever way you will get what you pay for I'm afraid. I'd recommend UK Astronomy Buy and Sell for second hand equipment if you can't access the classifieds here (you will need to be on SGL for a little while before you can). 2nd hand astronomy stuff from enthusiasts is generally a good buy, but I'd generally be careful about stuff from ebay - the quality can be more variable.

Your telescope is a Newtonian - so you will need a collimation cap to line up the two mirrors ("collimate") - they are cheap new - https://www.firstlightoptics.com/other-collimation-tools/astro-essentials-125-collimation-cap.html - there are loads of online guides about collimating, it's pretty easy once you get used to it.

I'd recommend the reviews on BBC sky at night website, or member equipment reviews here for buying advice - or you can just post a query to the forums - people on SGL are always ready to recommend great ways to spend your money - but will give good advice in the process!

Good luck!

10 hours ago, Captain Scarlet said:

At risk of insulting (for which I apologize if so), are you absolutely sure you’re getting closer to focus as you rack in to the minimum? I’ve been fooled by that before. IE actually you can’t get enough out-focus to catch your closer targets. Pull your eyepiece away from the tube at max out to see. My only other thought is that those OO cells do have quite a lot of travel in the mirror “height” (I have two, an 8” and a 12”, I think they’re extremely good). Perhaps push the mirror up as far as you can with the collimation adjusters.

Magnus

Cripes you were right (no insult taken!) - physical reality is restored, at the cost of some embarrassment to me.... 😳

Shifting the mirror higher did not correct the problem. Once the weather dried out enough to get the scope out in the dark I was able to point the scope at a bright light source. It was much clearer than it had been by daylight that the image was resolving as i racked the focuser out. So it was back focus, and not in-focus that was the problem.

Huge apologies for spreading my confusion, and many thanks for prodding me in the right direction!

No, same primary. It doesn't seem to make sense. But if the problem is lack of in focus it indicates logically that I must somehow have set the mirror lower with the new cell. I don't understand how. But I have now raised it, so will check it tomorrow, and see if it resolves the issue.

The old - The old back was 10mm lower.

But I wonder if I somehow overcompensated...

Okay - I feel less of a lemon now this has got other people baffled. There is definitely a lack of in-travel for the focuser - I can see the image coming more into focus as the focus tube travels down.

Unfortunately there is no way I could mount the cell any lower  - and, while it's a good suggestion, I did try to rack the collimation screws back - but thank you @AstroKeith for the idea of a spacer.

However... I do wonder , given the opinions here, if the mirror is actually lower than it was rather than higher - I'd assumed I just didn't understand how a Newtonian worked - but as @markse68 says I could have made a mistake here - my conclusion it was higher was based on a bit of dead reckoning  - checking the relative position of the mirror in the cells against the position they attached to the scope.

If the mirror is lower than it was it would be quite easy to raise it, because there is a second set of attaching holes in the cell - so this is the first thing I'll try tomorrow (unfortunately my astronomy time is out for today)....

When a 9-point OOUK Mirror cell came up on ABS a few days ago I jumped at the chance to replace the sealed 3 point cell on my OOUK VX10 - a 10" f4.8 Newtonian - at a much more modest cost than buying new.

I've just spent half the day very carefully drilling the tube to install the new cell (this is the first time I've dismantled the tube), but now I've reassembled it I find I can't reach focus on the usual chimney pots and sundry that I generally test focus on in the day - I don't have enough in-focus at 34mm which is my lowest magnification eyepiece.

The position I needed to install the new cell in the tube meant that the mirror is about 10mm higher in the new cell than in the old one. I'd wrongly assumed this might be within a certain tolerance - the way I understood it I might move the focal point slightly, but I would just have to use more back focus, which is not a problem with my focuser. I don't understand why it's in-focus I am lacking.

The only ray of hope that I can see is that the view from my garden is quite restricted by trees and buildings - the most distant objects are only a couple of hundred metres away. So my hope is that I might still be able to reach focus on astronomical objects - this is the only reason I haven't reinstalled the old cell straight away - but it might be a while before clear skies will put me out of my misery.

Has anyone else ever had this problem, and am I likely to need to go back to the old cell?

Could anyone explain to me what I've done to the optical path and what steps I have available to remedy it - the old cell wasn't a big problem, but it did lose collimation regularly, so I'd rather upgrade if at all possible.

Photos attached to illustrate what I've done - on the second photo you can see the old set of holes are too low for the new cell, which would only fit into the scope if mounted a little higher.