Captain Scarlet

On 12/12/2022 at 01:50, markse68 said:

Not in action but out of- but it has been snowing hard and it was dark so decided on a little maintenance ready for Wednesday- mirror clean and matt black tube respray- has made a significant difference that will hopefully improve contrast on Mars. Really lovely little scope to work on this Tal-150p. Also realised I had been collimating wrong as the secondary has no offset 🤦‍♂️ Should improve things too!

Before:

 

After

 

Mark

A quick question Mark … what is the mostly-hidden blue structure supporting your tube in the middle picture? An orange-crate cut into a V or curve? I might need to emulate it for similar purposes…

M

Thanks for the replies people. The small sensor size for a given price level for these cameras seems to be a deal-breaker for me. Even using a full-frame sensor a standard error (say 5 pixels) in estimating the exact centre of a star off an image translates into a focal length uncertainty of the order of 6mm for my 1800mm FL scope. A smaller sensor and bigger pixels will only amplify this. I might see if I can borrow somebody's mirrorless camera...

Thanks, Magnus

20 minutes ago, Elp said:

You'd have to check their specs. Many of the mid range to more zwo are 6.5mm, the cheaper ones this distance is a bit longer. You can make up the difference needed with spacer rings.

6.5mm would certainly work, probably anything up to 15mm.

1 minute ago, Elp said:

I would of thought a reducer lens may help with your short focal distance.

Unfortunately that would defeat my purpose, which is to measure the distance on the sensor between two known stars at prime focus so I can exactly calculate the focal length of the scope.

I'm not looking for a DSLR, I'm looking for a (perhaps old) dedicated astro camera to screw or mount directly on to a focuser flange. The brand name ZWO or QHY occur to me?

16 minutes ago, happy-kat said:

What sort of budget range. I wonder if a mirror less DSLR would fit in your flange-focus distance range. 

Ideally budget will be much less than the cost of a mirrorless DSLR. I already have a 6D and a 7Dmk2, so I wouldn't get any extra use from something like that. I'm hoping something inexpensive and perhaps a generation or two old might fit the bill and be cheap too.

I’m not an imager, nor can I see myself becoming a proper one. I’m too interested in Visual. However, I have been known to dabble especially for occasional wide-field MW style pictures, using my Canon 6D.

But for design purposes I do occasionally like to know and/or measure the exact focal lengths of my scopes, particularly Newtonians of which I’ve built two recently. I find that the >=44mm out-focus requirement for a DSLR is too much for my visual-only scopes.

So, my question: does there exist an inexpensive, say crop-frame sized, astro camera with a flange-to-focus distance of the order 15-20mm or less? It only needs to be good enough to capture focussed stars.

Thanks, Magnus

Meade Rainforest Pro get very good reviews. A friend of mine has them and they are very nice to look through.

Magnus

24 minutes ago, markse68 said:

It's back in I put the spot on when I got the scope so I'm fairly confident. Made me think about stuff though- obsessing about collimation using autocolimators like the Cat'sEye but that relies on the centre spot being perfectly central. And not central to the mirror blank but central to the paraboloid. How would you get that perfect?

Mark

👍🏼

Yes my thoughts too about obsessiveness with autocollimators and other techniques relying on that spot. Both my OO 1/10 wave mirrors were originally wrongly spotted, one grossly so.

As for the paraboloid not being symmetric, I think a grinding process more or less guarantees it must be symmetric, as accurately centering the position of a blank is easy. If it weren’t I think the mirror itself would be unusably bad in all sorts of other ways.

10 hours ago, markse68 said:

Not in action but out of- but it has been snowing hard and it was dark so decided on a little maintenance ready for Wednesday- mirror clean and matt black tube respray- has made a significant difference that will hopefully improve contrast on Mars. Really lovely little scope to work on this Tal-150p. Also realised I had been collimating wrong as the secondary has no offset 🤦‍♂️ Should improve things too!

Before:

 

After

 

Mark

Maybe take the opportunity to check the centredness of the spot while it’s out?

M

After First Light late afternoon/early evening in dark conditions, I went out later into the bright moonlight to resume. Tube sparkling in frost but optics seemingly clear. I couldn’t understand why when aligning on Capella I couldn’t get focus AT ALL, and Capella seemed much redder than it should. 🙄 stupid me. I’d pointed to Mars, which looked quite good high up at 141x.

I went to Pleiades with my Nagler 31 to reassure myself that coma correctors actually work … they do! What a difference. I also noticed that with the secondary now so close to the open end, any local bright light source, particularly at right angles to the scope, really impinges. Need to fashion a dew shield.

I looked briefly at Luna at 183x and noticed Lambda Cancri just off the Southern limb. I split Rigel but it was a bit wobbly and finished off with Sigma Orionis again, this time getting the C star but nothing really pin-sharp.

Brrrr, Magnus 

50 minutes ago, Stu said:

Looks great @Captain Scarlet. I know you’ve said it before, but it is indeed Trigger’s Broom 🤣🤣

Indeed it is 😁. All that’s left Skywatcher-wise are the spider and the rings. They must be quaking in their boots, but they’re both fine so safe for now.

I’ve spent the last few days completely dismantling and re-building my Orion-Helmerichs 12”, ridding myself of the old Skywatcher 300p main cell and installing a brand new Orion Optics cell (about which a new thread will appear shortly, avec pics).

Late afternoon and evening until just now was First Light for the new set-up. Focus was achieved - phew! My new holes were in the right place.

Having just come in to the warm to prepare supper etc, my fingers now really hurt, it’s about -4 outside. But it was a really good short session, I did more or less what @Stu did the other day, took in all the available planets, plus Almach for some extra colour.

Mars was too low really, but there was detail on view. Jupiter came and went with localized seeing, now mushy and now sharp. Saturn stole the show, Cassini Division easily seen plus Titan Rhea and Dione and possibly momentary glimpses of Enceladus but I’m not sure. Uranus was obviously a blue disc and Neptune a hint of greenish and just about discernible as a disc.

Fingers bloody freezing, dinner now and perhaps another session tonight on the Moon.

Very pleased with rejuvenated scope 

Cheers Magnus

Re apparent SA. I’ve had a particular phenomenon in the past, on two supposedly extremely well-corrected scopes, one a 105mm LZOS refractor, and the other a 1/10 wave Orion Optics Newt (with 1/10 secondary). It went as follows, on different occasions for each scope.

One night, even after plenty of cool-down (hours) the scope showed terrible SA, untidy and spiky one side, crisp and sharp the other. Thinks to oneself “sell this piece of rubbish”. Then the next night, perfection (relief)! This has happened as I say to each of those two scopes.

The difference between those pairs of successive nights was that the first night saw continuous and fairly rapid falls in temperature throughout the session, whereas the following-day “good nights” were stable of temperature.

The upshot being of course that on the variable-temperature nights the scope’s temperature was never catching up, always distorted therefore showing poor correction.

It was fascinating actually.

Magnus

I can see an easy solution. Flock the chimney tube! Not the way flock tape normally likes to bend but I’m sure it can be persuaded! 😁

Magnus

Great topic Mark. It would be really good if someone who knows their stuff on these lines might monitor this thread and impart wisdom. I’ll certainly post up some of mine.

My one main thought for star-tests for fast Newtonians for collimation purposes, is that one must not try to collimate away the offset in the doughnut to achieve perfect symmetry. That way lies guaranteed miscollimation.

M

33 minutes ago, HollyHound said:

That is an amazing location you have there 😀

I thought our farm location wasn't half bad, but that looks like it get's nicely dark 😁

At Easter when the Milky Way is away from zenith I generally can measure 22.0 on a good night. In summer, 21.5 ish. A definite factor when agreeing to move out here 🙂

The only drawback is this thing called Irish Weather. We get everything the N Atlantic can offer.

But on the good nights … my God.

M

This was my Orion-Helmerichs 200 newt on Skytee2, after Mars’ occultation but before re-emergence. Quite incredible the image-stabilization and quality of mobile phone cameras these days. They were 2-3 second handheld exposures!

Magnus

… and the re-emergence. I missed the actual moment this time as it was a bit further round than I was looking at 170x, but I saw it just after it’d lifted clear. The seeing much worse than the for occultation an hour prior. Back to bed now.

Magnus

1 minute ago, ED Splitter said:

I have reappearance worked out at around 5:53 for me, what time do you have for your location? 

Yes about the same, I m about to check up in more detail

Amazing! Mars occultation. Best ever view of Mars too. Orion/Helmerichs 200 newt with Delos 6 and PC2. Disappeared about 10 minutes ago, I await the reappearance!

Magnus

Just disappeared for me this minute. Omg incredible. Best I’ve ever seen mars too! One hour to re-appearance.

Magnus

Something that got overlooked from my recent sister-in-law cross-border delivery, so she simply posted it instead. I really like the Baader finder shoes:

Magnus

I use three types of focuser: a Feathertouch Crayford on my 105mm refractor; a Feathertouch SCT microfocuser on my Skymax 180; I have three Baader Diamond Steeltraks: one on each of my newts and one on my Intes 6" Mak. I've measured them all to be able to ascertain how many turns I need to defocus by, say, 10 wavelengths. Their linear-travel-per-full-turn are as follows, with the numbers given being in fully-reduced mode. The coarse modes are x10 ish in each case, I think.

Feathertouch Crayford: 1.7mm per fine turn

Feathertouch SCT microfocuser: effectively 3.0mm per fine turn (i.e. 3mm movement in focal plane per fine turn, as it's the mirror that gets moved).

Badder DS: 2.1mm per fine turn

Hope that's helpful, Magnus

Oh no! What is it about Helmerichs tubes? With mine I spent ages carefully calculating taping marking and measuring. And then I used one of the spider holes as centre for the 80mm focus hole 🤦‍♂️ . My solution was to change ends and use my new huge hole as a mirror access/vent.

Magnus