Thoughts on which imaging rigs to concentrate on

[Gina]

I've looked back through this thread and yes, I did have the 1600 camera working with the Esprit and FF BUT it was with the OAG and EFW2 in the optical path.  Strangely the focus seemed consistent across the field!  The back focus was 11mm out!!  ie. image sensor at 65mm from the optical face of the FF rather than 54mm.  Could it be that the back focus of the Esprit field flattener is really that tolerant?  Seems unlikely. 

Looks like there could be some night sky with just partly cloudy tonight so I might just set up the Esprit and just take a frame or two and see how good the focus is.  I can use SharpCap for this and eyeball the result immediately.

Edited September 15, 2016 by Gina

[Gina]

Just measured up the actual rig and the distance from OAG to camera face is 53mm.  Add 6.5mm camera back focus gives 59.5mm + 10 FF adaptor gives 69.5mm.  Specified back focus is 54.9mm so error is 69.5 - 54.9 = 14.6mm    That's even worse!!

[Gina]

Seems I need a FLO M48 to T2 adapter - that should do the trick.  A short test should show if the PA is good enough for unguided imaging with the Esprit though.

[wimvb]

Soap bubble please. This seems a real challenge that would put the camera to the test.

[Gina]

I'll see what I can do and the see what it can do

[Gina]

Now have the Esprit on the EQ8 with the camera, EFW2 and OAG plus FF attached and all connected up.  Everything is working except the remote focuser which is suffering from bad USB connection on the Arduino Nano.  If I wiggle the plug it will connect but it won't stay connected whatever I try.  These mini USB connectors are just not up to it!!   Think I'll see if I can sort out an alternative tomorrow.  Anyway, I cna probably manage with manual focussing tonight - if the cloud clears at all and doesn't turn into thunderstorms!

[Gina]

There's a hole in the clouds so I'm off out to the observatory to open the roof and point the scope at the hole

[Gina]

Changed to Lum filter and focussed on the moon - slewed to Alpha Cygnus, reduced the exposure to 2ms, adjusted slew to put star in centre of field, changed zoom to 200% and refocussed, changed zoom to 400% to show 4 pixels clearly and finally adjusted for fine focus.  Seeing was causing the star to wobble by 6-8 pixels.  Enabled cross-hairs and took a screenshot.  Left it for five minutes and took another screenshot just as cloud was coming in.

Here are the two screenshots separated by 5m.  The star drift is clear.  I estimate this as something like just over a pixel per minute.  This is at 3x the focal length of the lens - 135mm to 400mm.  Aperture from f2.5 to f5 or two stops and needing 4x the exposure for similar light on sensor.  So 30s becomes 2m and the drift at this position in the sky is about a couple of pixels.  This will be alright for reasonably bright DSOs.

[Gina]

Capture at 10:25pm or 23m after alignment of cross-hairs.

[Demonperformer]

10 hours ago, Gina said:

If I wiggle the plug it will connect but it won't stay connected whatever I try.

As a short-term fix, I don't know if this will help, but I used to have the same problem with the power supply to the SE mount. I was reluctant to start sticking screwdrivers into the socket to push the pins apart, and found that, if I looped the cable slackly over mount and then applied a couple of (separated) pieces of electical tape to hold the cable in place, it worked fine. Why I think that worked was that the slack in the cable and being held firmly in that position with the tape, meant that any movement in the system did not get transmitted to the plug and start jiggling it about - at least that was my guess.

[Gina]

Thanks for the suggestion but I tried fixing the cable but to no avail.  Think I'll try a new Nano but the long term solution is going to be more radical - either replacing the Nano with a UNO or better still a Raspberry Pi 3.  Have to see if I can revise my Python   Shouldn't be too difficult to translate my Arduino script in C++ to Python

[Gina]

Regarding the star drift due to imperfect PA, I guess I could improve this by further attention to accurate PA.  Maybe I'll try drift alignment.  The forecast for tonight has worsened with only an hour of clear sky forecast and by the time tonight comes the clear sky could have disappeared completely.  This leads me to think about further PA fine adjustment rather than imaging - if even that's possible.

Edited September 16, 2016 by Gina

[Gina]

I'm a bit confused about the Esprit field flattener.  Mine has a rear thread of about 53mm with a 10mm long adapter reducing to 48mm which fits the OAG I have but the FLO web site quotes the rear thread as 48mm.  I wondered if this was including the adapter but the photo doesn't show an adapter.  If the FF back focus of 54.9mm includes the adapter to 48mm, I'm in business.  They also quote the back focus as 75mm from the rear element so I'll go out and measure it.

[Gina]

Distance from rear element to mating surface of M48 adapter is about 20mm.  Take that from 75mm and we get 55mm or near enough 54.9mm, the quoted back focus

[Gina]

Right then, let's have another look at filter wheels and add up the dimensions.

EFW2 :- As I recall, the distance from front face to rear camera adapter plate is 22.3mm.  The M48 to T2 adapter is 10mm and the 1600 camera with 11mm adapter back focus is 17.5mm.  Total = 10 + 22.3 + 17.5 = 49.8mm.  Actually less than the 54.9mm back focus of the FF and needing a T2 spacer of 54.9 - 49.8 = 5.1mm     Workable without OAG.

SX MiniFW+OAG :-  Looking at the dimension drawing, the thickness works out as 4.3+28.2+4.3 = 36.8mm.  This is with a T2 front and T2 rear thread so we need the 10mm M48 to T2 adapter and camera back-focus of 17.5mm including adapter.  Total = 10 + 36.8 + 17.5 = 64.3mm.  OH! That's 64.3 - 54.9 = 9.4mm too much!    So the SX wheel will not work with the Esprit field flattener and 1600 camera!

[Demonperformer]

But if you remove the front bit of the camera that will reduce your 17.5 to 6.5 and so your total is 1.6mm too little - unless I am missing something?

[Gina]

Yes, but that gives you a male thread, requiring a female thread in the filter wheel.  With the SX wheel you can get either male or female adapter plates and these are the same both sides so yes the deficit can be eliminated but this means that the focus is brought forward and the OAG stalk becomes too long by about 11mm.  It might be possible to get the hacksaw out and take 9mm off the stalk allowing the SX Lodestar to reach focus (I think).  I might try this.  Having the filters closer to the camera will reduce the slight vignetting.

Alternatively, I can look into using the Atik EFW2 with separate OAG though the OAG doesn't have as much accuracy as I would like and there could be an error in the focal plane angle.  The EFW2 is extremely well engineered and made but all this lovely optical accuracy may be spoilt by a poor OAG!

The SX wheel with built in OAG should give better optical alignment and if I can get the guide camera to focus I think this would be the best way to go.  For the smaller DSO such as galaxies, I would probably use the MN190 and the focal plane is much less critical with a 1000mm FL so the separate OAG and EFW2 will be fine giving me all the filters I want in one wheel.

[Gina]

I've replaced the Arduino Nano with a new one and the USB connection is alright now.  Should last for a while and give me time to sort out a Raspberry Pi version   The earlier forecast couple of hours of clear sky tonight has evaporated and turned into partly cloudy all night   Might get the odd gap I can use - have to see.  OTOH there is now some clear sky forecast for tomorrow night but that might go when the time comes!

[Demonperformer]

Silly boy! Just because the camera end of my FF has a male M42 T thread, doesn't mean that they all are. Just seen that yours has a M48 thread, hence the adapter and the extra 10mm, so I agree with your figures.

Still, the 54mm front-to-chip distance, with the 17mm back-to-chip distance, would be just right for my optimal 55-57mm distance ... NO! CanNOT afford it ... this month!

[Gina]

I've had another idea to use the SX mini FW+OAG - turn a ring in aluminium with a 48mm female thread and about 75mm OD.  This can be attached directly to the FW with three screws like the T2 adapter.  That way I could save maybe 5mm.

[Gina]

Currently the sky is clear in the Cygnus area and I've been capturing images of the Crescent Nebula and may have the Soap Bubble in the frame.  Cooler tonight and I'm getting to almost -30°C.  This is 120s exposure with a gain of 600 with histogram correction in Ps.  Full frame saved in PNG format.

[Gina]

Decided to reduce the gain to 500 to reduce noise and increase exposure to 150s to compensate.  Here's a single frame at -28°C.

[Gina]

Another 150s sub with same gain and temp of -27°C.

[derrickf]

5 hours ago, Gina said:

Alternatively, I can look into using the Atik EFW2 with separate OAG though the OAG doesn't have as much accuracy as I would like and there could be an error in the focal plane angle.  The EFW2 is extremely well engineered and made but all this lovely optical accuracy may be spoilt by a poor OAG!

I'm confused about what you mean by this statement about a lack of accuracy. I use an Atik EFW-2 and Atik OAG combo imaging at 2400mm FL and a pixel scale of 0.45 arc seconds per pixel and get a routine guiding performance of less than 0.2 arc seconds RMS.

What have I misunderstood about your issue?

 

Derrick

[Gina]

Not the Atik OAG but this one.