markse68

What are the implications of field curvature? Is that the same as loss of focus at the edges?

Also if the moon is too bright (it was in my Tal-1 which is a similar spec) then you probably need a moon (nd) filter which you can pick up quite cheaply - really helps tame that bright disk and see more detail more comfortably

Still evaluating it tbh- just had a nice moon session and it was quite fine. The slight sepia cast wasn’t offensive and the image was sharp. I did get a very slight green fringe at the edge of the lunar disk but not horrendous. The moon went quite egg shaped as it moved toward the edge of frame but no worse than the TV WFs. 

On bright stars there’s definitely some slight coma (edit- astigmatism- little seagull stars) at the edge of field but I was also seeing that with the mil ep- not so the TV WFs.

The slight loss of focus at the edges was more noticeable in daytime than at night- most objects you’d be looking at would fit in the central area I guess so unlikely to be an issue- it didn’t bother me anyway- didn’t notice it unless looking for it.

It’s still the most comfortable eye placement wise of these eps but I think I prefer the wider afov of the mil ep. 

I think it’s a good ep- not great and not without minor issues but quite useable and sharp and it’s got the wide fov.

Mark

Maybe we could “borrow” one and do a shoot out 😉 I’d be happy to loan my TV WF 40mm 😂

58 minutes ago, Lurcher said:

I'll have to go back and have more of a look around the site myself.

Sorry for sidetracking from your beautiful work Mike. This one is wonderful 😉 Id love to subscribe to a site like that

13 hours ago, Lurcher said:

hoping this link takes you to it. https://astronomysketchoftheday.wordpress.com/2014/12/31/lunar-crater-maurolycus/

Hi Lurcher, what a fantastic site that is- some really wonderful images! Do you know anything about it? The archive stops at 2015- is it defunct? Shame if it is 

I am now fairly convinced that the plastic tube of my Fullerscope is a bit of a problem. Ultimately I'd like to replace it with an aluminium tube but that will have to wait- I'm thinking of making a lighter weight aluminium structure dob base for it too... There won't be much original left apart from the main mirror!!

The trouble is the cool down time and the thermals that seem to be present in the tube even after an hour in the cold that must disrupt sharp imaging- in fact I'm now wondering if they were contributing to the astigmatism I was seeing trying to split doubles at high mag.

This is what Polaris looks like slightly de-focused at 340x

Polaris thermal diffraction pattern.MOV

That's at first setup so a bit unfair- as it cools the diffraction rings become clear but are still swimming about.

So in the meantime I've decided to make a cooling fan arrangement for the existing tube. It'll primarily speed up the cooling of the primary but the air will then travel up past it and flush the tube with cool air.  Hopefully it'll improve things!

It'll be powered by 3 Li-ion batteries (I wanted to use a Makita battery but it won't fit) and a PWM fan speed controller will drive 3x 80mm quiet pc fans. It'll all be mounted on a perspex disk and will attach to the collimation knobs using velcro inside the 3 counter-bored posts- I need it to be easily removed as I always need to tweak collimation after driving to my observing site with it in the back of the car.  I've ordered the parts- just need to double check a few dimensions.

It should all sit just inside the bottom edge of the tube so I can still stand it upright for storage with the fans in place

On 03/12/2019 at 11:34, grjsk said:

I had forgotten my UHC filter, and I did not bother to go back for it at this time, but it might have improved things.

From my recent experience under Bortle 8/9 skies a uhc does make a lot of difference with Orion- really helps bring out the nebulosity. Also I was surprised how much magnification brings out of it- with my highest 320x the trapezium area was really 3D and I spotted a fifth star in there which was very pleasing! This was with my 8”dob though so a fair bit more aperture (I’ve read you should be able to see 6 with a 6” scope though- maybe under better darker skies)

On 18/11/2019 at 15:39, Second Time Around said:

Surprisingly, the red light on the Cosmo 225 goes dimmer than that on the Spot Lite 160,

Thanks for this review @Second Time Around

How much less dim is the 160- still useable? Prefer 2 batteries to 3. I’ve got the Petzl e+lite which has a lever for selecting the different outputs and from one end goes to red first but it’s not dimmable (but is pretty dim) it runs off cr2032s though which don’t last long- says 15hrs which is probably long enough but these say they last much longer 👍

7 minutes ago, Space Hopper said:

My main worry so far is getting them off. Be warned, once on, they are a devil to remove

That was one of my main criteria when looking- quick and easy to deploy as I hate walking boot style multi-lace hole pain in the.... I need something I can slip into from my normal shoes getting out the car and slip back out of easily and quickly. I missed the ones with Velcro and clips- they look ideal. The Sorel Caribou is ok on that front- just 4 big d-ring lace holes so easy to loosen and tighten. The other attraction was the sole tread which is little rounded spikes that I’m hoping will be easy to clear of mud before putting them in the car unlike a hiking type tread which is impossible without waking up all the locals banging them together with great force! 

6 minutes ago, Rob Sellent said:

@markse68 wheeling out his brand new boots for another chilly astro-session 😃

Very , very close haha- they’re  boats!

Lovely image. What are the names of the 2 on the left?

Same issue- just splashed out on a pair of Sorels on Amazon but they were pricey, are ridiculously huge and I don’t think I’ll be wearing them in daylight... 😂 Look warm though- haven’t tried them yet 

Thanks very much Oldfort- I didn’t find them in my search- will contact them about pricing

Ok but would it’s unstable nature cause the diffraction rings in its unfocused image to dance? Fairly sure that was tube thermals as I could distort them by putting my hand on the edge of the tube inside the aperture, and to double check it wasn’t atmospherics I pointed the scope straight up and found a star (🤷‍♂️) near zenith and it had the same effect 

I was having a go at Sirius last night to see if I could see the Pup- I couldn’t ... But in the process I came to a rather disturbing conclusion! I’d been out for quite a while- maybe 45mins so the scope should have cooled sufficiently though it had come from warm flat to icy cold night here in London. As I worked my way up through my eyepiece magnifications I was a bit disappointed in how not point sourced Sirius looked. Eventually I reached my highest power ep- my lovely Nagler 4.8mm type1. It was out of focus when I put it in and I saw something I’d not noticed before. Credit to my new  secondary which seems to have made a big improvement I could see for the first time beautifully sharp diffraction rings in the Airy disk- but they were swimming like a psychedelic light show at an early Pink Foyd gig! Quite mesmerising and I was enjoying them for quite a while but I’m fairly sure this was due to tube currents and not a good thing! In fact I’m wondering if they were contributing to the poor seeing and astigmatism I was getting at high powers that lead me to buy a new secondary in the first place 🤔

So to help remedy this I think I’m going to change the tube to aluminium. Aluminium must surely be the best material for a Newtonian tube- it can be quite thin and light and still be very rigid in tubular form but more importantly is an excellent thermal conductor with low thermal mass so should reach ambient quicker than any other material and offer less insulation to the interior of the tube- certainly a lot better than the pvc drain pipe I currently have! A fan would help too of course. 

Theres a company in the US called Parallax who make superb looking traditional newts and who will supply tubes to diyers but I hate to think how much freighting a tube that size to here would cost.

So I’m looking for a uk supplier of aluminium tubes- preferably seamless or at least welded rather than folded seam- anyone know of such?

I had a look for it with my 8.75” from my London skies last night with a uhc filter- not a hint of any nebulosity. Didn’t think to try an oiii but I read that Hb is good for bringing it out- which I don’t have yet...

The moon is a guaranteed wow object seen up close by the uninitiated- and me still 😉. Orion nebula  is spectacular right now- I’m finding a uhc filter really makes it pop. Andromeda galaxy, though only a grey blur from my skies, is still impressive to see and think about. Unfortunately the other major wow objects- the gas giants- are setting a bit early now. Pleiades if you can get wide enough- I can’t get it all in with my F7.3 scope even with a 50mm ep- it looks better through the finder 😉 And yes the double cluster is a favourite

Perhaps they absorb more heat from the environment than they lose through radiation countering the loss through the open aperture to sky- a sort of passive dew heater? I still think condensing moisture in the air must fall and they would shield from that if not pointing straight up

Very interesting- this is making me think a lot about heat and heat transfer and I think I’m starting to understand better- will give that a read- thanks

On a separate note I was thinking of making a small cordless unheated  blower for de-misting dewed up optics after Friday night when my primary got completely fogged. But I’m thinking that though it should do the job, the absence of added heat would lead to dropping the mirror temperature by latent heat of evaporation so it would likely dew up even quicker straight after 🤔 Probably not such a great idea

Ah- so the rate of radiance doesn’t change but the rate of absorption does?

16 hours ago, Carbon Brush said:

Ever wondered why sometimes in the morning your car windscreen is icy but not the side windows?
Side windows, being near vertical, are at surrounding temperature.
The sloping windscreen points to the sky, so cools towards sky temperature.

Hi Carbon, I don’t understand the physics of this and would have put that phenomenon down to the moisture in air condensing in cold temp and falling like fine rain into the car where it settles on the angled glass but less so on the vertical glass? I’m guessing you are meaning it has something to do with black body radiation which I don’t really understand but isn’t it to do with temperature differentials which shouldn’t be much different from sides of car to top of car?

2 hours ago, Stu said:

Keep the scope pointed away from vertical when not using it to avoid the mirror radiating too much heat and dropping below ambient.

Stu I read this in another thread and don’t understand the physics of it- would it really drop below ambient pointed up and is that because of convection currents cooling the air inside the tube more than ambient?

17 minutes ago, upahill said:

Out of curiosity do these suffer from the same delay problems as mentioned earlier?

They’re pretty fast acting I think as fuses go

6 minutes ago, fifeskies said:

wired inside the box is a tad inconvenient compared to a panel fuseholder on the outside where a few seconds lets you change a fuse

Not really- they’re thermal fuses that reset when they cool down so don’t need changing though they do degrade if activated too often. They don’t completely break the circuit but limit the current to a safe level