Prime focus camera newt

[furrysocks2]

From the bottom up...

Notes:

• mirror installed but slightly off center due to existing bolt holes in cell not being concentric
• remove M6 coupling nuts on the bottom to drop the primary out
• with no kit at top end, CoG is within the length or the 3x2 timber, but very close to the protuding nuts
• thing is too wide to sit in my current dob mount

By substituting something thinner for the 3x2, using T-nut inserts on outer face with the correct bolt length, I can mount my alt bearings as high as they'll go and mount counterweights in T-track towards the lower.

[furrysocks2]

Tools away, floor swept.

[furrysocks2]

Unless it was a trick of the light, for my first (and probably only) visual, I managed to find and split Mizar. I couldn't have done it without the observing chair I built a couple of weeks ago.

With any luck, Santa (or one his elves) will bring me a camera that can see stars. 

[furrysocks2]

I'm trying to work out the constraints for the top end configuration to ensure no vignetting of the 100% cone. I don't yet know what components I might use or in exactly what order they would be configured, so just trying to make a start with how far forward of the focal plane I can keep to a 1.25" barrel size.

From http://www.alpo-astronomy.org/jbeish/Newt_Sec_Mirror.pdf, I found the following formula:

Quote

100% illuminated secondary c = l(D - i) / F + i + (2OR)

where:

•  l = the distance from the secondary to the focal plane
• D = the primary diameter
• F = the focal length
• i = linear image size at the focal plane
• OR = overlap rim (ignore)

 

If I assume that having "a 100% illuminated secondary of minor axis width c" is the same as having no secondary but rather "an aperture of diameter c", and that "distance from secondary to focal plane" is the same as "overall optical length from focal plane", then I have produced the following graph for various linear image sizes, based on the width of potential sensors.

 (I appreciate I'm using sensor width and should perhaps be using diagonal.)

 

If I were to assume a 1.25" filter as the last component, ie closest to the primary for ease of changing, its 26mm aperture would limit me to an optical length of around 130mm, slightly longer for smaller sensors but I might design for 2/3" max. If I also used a 0.5x focal reducer on the camera itself, then I'm not sure exactly how that changes things or what I do to calculate it but I'll start with halfing the previous value, around 65mm. Without a filter at the end and instead simply a bare 1.25" barrel, then the slightly larger aperture increases the limit on optical length to 150mm without focal reducer, 75mm with a 0.5x reducer.

I'm also considering balance and perhaps the focuser could be below the spider, protruding through the center hole with camera/focal reducer above. A couple of potential microfocusers I have seen have a stated optical length of 45-56mm and 52-76mm.

If I'm designing the spider based on a 50mm central obstruction, then any light shield not much <50mm OD and of a practical length shouldn't be too much of a concern.

 

I'd be very grateful if someone more familiar with all this could please comment or confirm if I'm going about this the right way?

 

[furrysocks2]

Something like this might work...

365astronomy 50mm finder/guidescope

• remove mount and tube rings
• remove front shield (61mm dia)
• remove objective lens

That leaves:

• stated 43mm focusing range (33mm by drawtube, 10mm by helical focuser)
• stated 55mm outside diameter

 

To mount the thing, the challenge would be to not exceed 55mm central obstruction.

Consider a spider cut from aluminium, with a central disc of diameter 55mm and a hole in the center sized to clear the knurled microfocuser (taking dimensions from the image, 48mm). The hole would either be slightly oversized or chamfered on the bottom side so as to partically accept the taper between the drawtube and its lock screw (taking dimensions from the image, ~26 degree taper).

This would mean that the assembly would pass up through the hole from underneath, leaving the microfocuser (and camera, etc) free to extend above the spider. The taper of the barrel would partially nest, centrally, into the chamfered or oversized hole (no lateral adjustment then possible), and the drawtube lock screw should be aligned under one of the three spider vanes, leaving the main body to hang below. 

To collimate, with the top end nested in the spider, support brackets projecting from the underside of each vane could be used to position thumbscrews near the end of the tube and project keepers or springs over the lip of the end of the barrel to prevent the assembly from falling downwards.

I've sketched this out...

It seems to tick a few boxes in my head - perhaps a bit fiddly to fabricate.

 

[furrysocks2]

After receiving a much needed cash injection earlier today and with the kids in bed, I've been putting together a shopping list. 

As well as continuing on with my scope, I would also like a grab and go with a wider field of view - the Orion 70mm multi-use finder looks neat.

I still plan to try out a wee RunCam so taking it back to M12. I've only found one filter holder that is of small enough diameter to sit within the central obstruction of the scope. Anyway, I've come up with a potential list of parts...

There are possible variations within the above already, but I wanted to keep the diagram tidy. I've omitted extensions, too - kits containing a few different lengths are cheap enough for T and C, haven't found a cheap 1.25" kit yet.

That all comes to between £300 and £350 depending on a few choices or over ordering extensions, etc - plus a camera. Shopping around could save me £30 or so maybe but it still seems like a lot of money, particularly as the focal reducer(s) are cheap ones.

 

Any comments on the above?

 

Edit: and of course it's now I remember the offer of the 2nd hand Startravel 102T...

[furrysocks2]

Deliveries have started to arrive:

• 1.25" filter holder - aperture 10.5mm, my filters 11.2mm
• T to M42 adapter - diameter 59mm
• 1.25" focal reducer - reduces AFOV for visual but should be ok in front of a sensor
• [edit - and it only bodge fits into my EP so thread pitch may yet be an issue]

I've also acquired an LN300 camera which is max 50mm across.

 

I blew the rest of the seasonal budget on an ST102, diagonal and camera and I've still a decent mount for that to get. Still looking at a 50mm finder and have begun to prototype a collimating mount, but the cost for a couple of focusers attached to a tube is quite high.

I may resort to a single helical focuser and take up the slack with extension tubes but there remains the issue of a collimating mount - I can't quite work out how to get some of the weight below the spider with this.

[furrysocks2]

More bits arrived:

• 1.25"/T low profile visual back - nothing to say, nice item
• M12 to C mount adaptor - metal, with locking ring
• 1.25" LRGB filter set (thanks, Vicky!) - thankfully they fit in the filter holder
• Runcam Own Plus - tiny, and impressively large sensor for the size of it, remote for OSD

The postie has been delivering many small packets recently. Seemingly there was conversation on the doorstep yesterday morning between him and my wife about how all the bits were for my telescopes (plural), while there was still no side on the bath, etc. To which the postie replied "it takes some of us longer to grow up than others".

While I resent that a little, he's not wrong. 

 

Still swithering over the 50mm guide scope idea. In the meantime, I've started converting my newly acquired EQ3 to a goto system.

[furrysocks2]

While the EQ3 will be goto, this imaging newt (and my other visual newt) may end up mounted on an equatorial platform, perhaps with resettable slow-mo hand alt-az controls.

http://www.reinervogel.net/index_e.html?/Plattform/plattform_VNS_e.html

Plans:

http://www.reinervogel.net/index_e.html?/Plattform/Bauplan_e.html

 

Edit slow-mo:

http://www.cloudynights.com/page/articles/cat/articles/how-to/amateur-telescope-making-atm/atm-building-a-dob-and-refractor-from-parts-r1082

[AKB]

Wondered whether this might be of interest, if your chunk secondary camera support vanes cause too much in the way of diffraction spikes?

(How is your French?)

 

[furrysocks2]

7 minutes ago, AKB said:

Wondered whether this might be of interest...

Très intéressant, merci à vous!

 

I'm yet to try out the plywood spider with a camera - I've done visual once but most of my head was in the way. I picked up a SW150 recently with four vanes - that was my first ever view of diffraction spikes (my DIY dob has a curved metal ruler).

My understanding is it's a balance of spike length/intensity vs contrast. Curved vanes in my solid spider might alter their appearance but would cause a slightly greater obstruction. The masks that you link to look like they'd easily retrofit to any straight vanes.

Thanks for the link.

Matt.

[Mick J]

Nice work Furry, love it all 

[furrysocks2]

1 minute ago, Mick J said:

Nice work Furry, love it all 

Thank you very much. 

[furrysocks2]

removed

[furrysocks2]

I've tried out Maskulator on my spider design:

 

 

Here, I've subtracted the original from the revision, and then inverted - not quite sure exactly what it shows, but to me it looks like a dirty smudge when you try rubbing out pencil with your finger... ie it's spreading the diffraction around a bit more.

[AKB]

Yes, that's kind of what you would expect.  Still, just a thought, it was worth a go even just to find out why NOT to do this.

 

[furrysocks2]

12 minutes ago, AKB said:

Yes, that's kind of what you would expect.  Still, just a thought, it was worth a go even just to find out why NOT to do this.

 

But removable, so something to play with.

[furrysocks2]

RETIRED.

Will try something similar on my 12"