secondary mirror far down the tube?

silv (Annette) · #1 03-09-2012, 08:18 AM

TS has a lightweight 6" f/4 newt on the shelf.
Complete assembly weight is stated between 4.4kg and 5.3kg, depending on which language you are viewing the model's page in.

I will probably order this one for AP on my EQ5 - thinking that the low weight of max 6kg will be okay for my tracking. The low cost of 600 NZD (460 AUD) including shipping and NZ GST certainly is okay for my wallet and leaves room for 7x50 binos.

What do you think what the benefits and disadvantages are of mounting the secondary mirror so far inside the tube?

Disadvantages wouldn't be turn offs for me because I've seen "example images"

on the web and am really okay with what comes out of this tube.

But I'd like to understand this feature from a theoretical and practical point of view.

All input most welcome and much appreciated. Thanks

multiweb (Marc) · #2 03-09-2012, 08:46 AM

I regularly imaged with a F/5 Newtonian that had a dew shield extending nearly the full length of the scope so the secondary was in the middle of the tube like your photo. To me is it's an advantage because the secondary is encased in the tube so no dewing and no stray light. At F/4 I doubt very much you'd get any kind of vignetting with the aperture if that's what you're worried about.

PS: The only down side was it's more likely to get caught in the breeze with the extended front. But then again when it's windy there's not much you can do about it.

pixelsaurus (Mike) · #3 03-09-2012, 08:51 AM

With the secondary mounted down inside the tube, dew is less likely to form on it. It is like having a built in "lens hood". Practically, if you need to align the secondary, it could be tricky but then again, it is not something you would need to do very often.

silv (Annette) · #4 03-09-2012, 09:44 AM

yeah, the dew protection.
I very much like that effect. Thanx guys

Marc, the wind is a good point to be aware of.
On that note: does the stability support of the tube walls get reduced or enhanced regarding tube flexture by having the vanes in the middle of the tube?
and also regarding flexture due to the focuser and finder weight in that location in the middle of the tube rather than on the end?

Another aspect I can't quite understand is the 2ndary being closer to the primary should enhance the normal obstruction effect? It's also quite big: 60mm.

I'm not concerned, at all. I'm just curious

multiweb (Marc) · #5 03-09-2012, 09:49 AM

Quote:

Originally Posted by silv

On that note: does the stability support of the tube walls get reduced or enhanced regarding tube flexture by having the vanes in the middle of the tube?
and also regarding flexture due to the focuser and finder weight in that location in the middle of the tube rather than on the end?

I don't think you'll have to worry about flexure. If anything the tube wall is stiffer in the center than the edge.

Quote:

Originally Posted by silv

Another aspect I can't quite understand is the 2ndary being closer to the primary should enhance the normal obstruction effect? It's also quite big: 60mm.

I'm not concerned, at all. I'm just curious

I wouldn't think so because the light entering the scope is parallel so the obstruction stays the same.

silv (Annette) · #6 03-09-2012, 09:51 AM

The balancing on the mount should benefit from this weight shift down the scope, don't you think? the lever - or what do you call that - is shorter.
I imagine that this leads to the tube being mounted rather midway between focuser and primary mirror and the main weight resting closer to the mount head.

Of course, leaving the unsupported tube front hanging in limbo and being caught by Marc's mentioned wind.

Or am I thinking wrong? I don't have much spatial imagination.

multiweb (Marc) · #7 03-09-2012, 09:59 AM

Doesn't matter much in DEC as you can be North or South heavy. The only concern on a big newt is close to the meridian imaging close to the SCP when you have weight shifts with a heavy camera when the focuser is not pointing right up or down under the scope. You get off balance in RA as soon as the scope goes past the meridian and the camera adds to the weight westwards.

silv (Annette) · #8 03-09-2012, 10:02 AM

Quote:

marc: If anything the tube wall is stiffer in the center than the edge.

aha!
And I thought that maybe the frame at the end of the tube lends stability to the material - more so on the front. Cool, Marc

Quote:

because the light entering the scope is parallel so the obstruction stays the same.

I just went and held my hand inside my 8" tube and the reflected image on the primary mirror appears larger the closer my hand gets. But looking through the focuser it seems to not change its size? Funny optics.
I think I'll draw a sketch to understand this better. Thank you

silv (Annette) · #9 03-09-2012, 10:06 AM

Quote:

Doesn't matter much in DEC as you can be North or South heavy. The only concern on a big newt is close to the meridian imaging close to the SCP when you have weight shifts with a heavy camera when the focuser is not pointing right up or down under the scope. You get off balance in RA as soon as the scope goes past the meridian and the camera adds to the weight westwards.

AHA! So many little jigsaw puzzle pieces fall into place just now!

At this moment, I understand exactly what I have observed with my 8" and the bad tracking in those critical positions. I did read about those effects but it didn't really make "click" and I couldn't apply it during my sessions - until now.
Super!

silv (Annette) · #10 03-09-2012, 10:11 AM

What I like about the short tube and the location of the focuser:
I won't have as much trouble during 3-star alignment on stars in the west.

With my 8"/1000 f/5, when the mount had slewed to northwest/west, I couldn't look into the eye piece without turning the tube inside the rings . Because the focuser was then on top of the long tube - and even with the tripod legs fully drawn in, I would have needed a ladder. (I stand 159cm tall.)

Or twist the tube - which during alignment is not an option.

With the short tube and the focuser being even lower from the ground, I won't run into these problems, anymore, I reckon.
Maybe I can even let the tripod legs out a bit which should enhance stability. Or so I've read somewhere. Not sure about that, though, either.

silv (Annette) · #11 03-09-2012, 10:21 AM

Oh, and possibly being able to have the focuser more on the top of the scope than on the side - during balancing and three star aligning - and then, while imaging west/south west - will make it even easier for the mount to track the load.

This is all good news ! Thanks!

multiweb (Marc) · #12 03-09-2012, 01:21 PM

Quote:

Originally Posted by silv

Oh, and possibly being able to have the focuser more on the top of the scope than on the side

If you can clear the DEC motors on your mount I'd rather have the camera facing up. If you're facing down the cold air will fall onto the MPCC or nosepiece and the other side of the glass with ambient air will fog up. If you can't get a couple of counter weights (magnets) and stick them opposite inline with the focuser and camera. If you still can't then wind down your counter weight 2 to 3 turns when you shift West so your scope doesn't fall on the other side and your mount still pushes the counterweight up against the load pushing back East.

silv (Annette) · #13 03-09-2012, 03:52 PM

that's an interesting thought, oh.

hm. to me, it seems safer regarding dew drops if the camera looks down:
the body covers the focuser partially.
and everything that wants to run, runs downwards, away from the exposed sensor.
I imagine, towards the outside, the camera body is better insulated against spray rain and such. so I'm not that concerned about the buttons and little gaps around them being exposed to dew. the manufacturer has designed it a bit to be protected against anticipated liquid in those obvious weak spots, I hope.
What the designing engineer could not anticipate is a single dew drop running down the focuser barrel into the sensor. Uaaahhhhh!

silv (Annette) · #14 03-09-2012, 04:01 PM

but once I can do longer exposures, I should remember wrapping some clear foil around the top and side of the body. yes. thank you for your stimulating thoughts, Marc!

multiweb (Marc) · #15 03-09-2012, 04:03 PM

You don't have dew inside the camera body. If you did it would ice up straight away. You'll get dew on the outside of the glass because it's colder if you point down.

As an exercise take your QHY9 run it for 30min at -20c. Then put it face down on the table and check in another 30min. It's likely you'll get a dew spot on the nosepiece window. I used to shoot my flats by doing that on top of the EL sheet and wondered for a while why I was getting fogging on the glass and halos in my flats.

multiweb (Marc) · #16 03-09-2012, 04:04 PM

Quote:

Originally Posted by silv

but once I can do longer exposures, I should remember wrapping some clear foil around the top and side of the body. yes. thank you for your stimulating thoughts, Marc!

No need as there is an O-ring in the QHY9 body. Not like the old 8s that used to leak from everywhere.

silv (Annette) · #17 03-09-2012, 04:56 PM

oh, I use a cool Sony Nex-5N, not a cooled QHY