Just to let you know not all is myth I have just received my 16" Skywatcher GOTO Dob from My AStroshop and although a light shroud for it will have to be home made it is a very nice and large scope and the views are great.Simple to put together and easy to operate.I'm not the most photogenic person however
True, but those extra rings are non standard even for Orion, which was the point of my comment.
James
Hmmmm perhaps I haven't given due attention to all the details throughout the thread......
Anyway, having used a 12 inch gso newt for imaging for quite some time, I would not have another go in the configuration it is sold in.... Try by all means, but having been through the process myself, the scope needs a LOT of work to make it a really viable imaging scope.
I just posted a Youtube video showing how I tested the OTA wall strength.
It's a lot better now with the 3mm aluminium reinforcing, but still not perfect. When the tube is vertical (i.e. pointing at Zenith) it hardly moved at all.
James
wow, did'nt realize tubes flexed so much. nice and simple test. would you know what the weight of the camera is?
i'm guessing it would flex a bit less when the camera is in the focuser as the load would be distributed across various points along the drawtube and there would be less torque at the top end of the focuser, for this test it is concentrated at the top edge of the focuser.
The camera is about 2.3kg.
It's a very sensitive test - there's a cool animated gif on this page that shows the relationship between what you see and the error expressed in degrees.
Remember almost all the movement is from the OTA tube wall, and not the focuser.
James
Well I use the qhy9 which is a DSLR size chip and the MPCC keeps it flat edge to edge. it doesn't matter if the light cone is bigger than the MPCC, as long as enough light is getting to the sensor, eg, if your MPCC sucks in say 80% of the 100% light cone you wont loose anything!
As for the Secondary on a 12" F4 if it isn't around the 90mm size you will probably loose out due to vingetting or your illuminated circle will be small hence light drop off will be a issue. My 10" is running a 70mm secondary with approximately 27% Central Obstruction and i have measured with CCD inspector about a 7% light fall off in the corners on the 40d it got to just on 10% so im pretty much using 100% of the 254 mirror's capacity.
If the focuser is anything like the Linear 3" or 2" focuser seen on most GSO scopes... bad news and you will most likely be upgrading very quickly so add a extra 400 bucks to that budget. once again its all good to have a f4 newt, but if it isn't collimated then your off to comet ville! Tube droop kills your collimation/ location of the MPCC! bad joo joos
My 10" has close to 120mm of back focus which is good for a OAG, FW, CCD, MPCC. As for collimation, the only way to do it isn't with a laser it only gets it rough, if you want to do it right Cats eye auto collimator, sight tube and collimated cheshire is the only way to be cirtain that your scope is bang on and preforming well and oh it doesn't have to be cats eye, but a Autocollimator is very advisable! The last thing is the collimating gear on the scope mirror cell and secondary. if its like the skywatcher standards for secondarys... if they have made them thicker and given them beef then Kudos!
As for flexure... All newts suffer! like Clive has already said you need OAG or SBIG tech to quell the angry mobs! hence what i did! OAG with a 3"FT focuser. the tube doesn't flex that much at all around the focuser due to the secondary vanes that are tight as a guitar string strengthing the circular hollow section!
Sometime soon i will be writing a little novel about my travels and findings for making a cheap newt work like a good quality astrograph.
Brendan,
The secondary is 87mm which is a bit on the small side. A bigger problem is the location of the focuser. It's too close to the primary - so the CCD is too far away from the secondary and this causes unnecessary vignetting. The 8 inch version had a similar problem.
Quote:
Sometime soon i will be writing a little novel about my travels and findings for making a cheap newt work like a good quality astrograph.
the vingetting is more than likely coming from the secondary being too small On the standard skywatcher 254mm dobsonian the fully illuminated circle came in a a whopping 6mm..... at 78mm for the 12" F4 like i said you would be looking a minimum of 90 odd mm to start getting close, but more like 100mm to do the job properly!.
Have a look at this image Just to give you a idea thats close to 400mm from the OTA surface to the sensor + about 150mm to the secondary! It missed out on focus with a Starlight adaptive optics unit + Starlight OAG + QHY9 and filter wheel! by about 5mm.
Im only talking with experience on my own rig but i have done alot of reading and looking at the designs for it. I have even designed a 8" F4 GSO mirror newt and that required a 70mm secondary to get a 20mm 100% illuminated circle! which doesn't even cover my qhy9 properly.
Brendan
Last edited by bmitchell82; 19-05-2011 at 12:00 AM.
The secondary is 87mm which is a bit on the small side. A bigger problem is the location of the focuser. It's too close to the primary - so the CCD is too far away from the secondary and this causes unnecessary vignetting. The 8 inch version had a similar problem.
Small update: The 3 inch Wynne corrector arrived and I got to try it out last night. Luckily it fits in the focuser like a glove. I had to add a few grub screws to the focuser tube to hold the corrector in place - but it wasn't too difficult. Because the corrector extends the full length of the focuser drawtube (and then some), it seems reasonably secure.
When focused on a star, the bottom of the corrector is almost exactly level with the OTA wall - i.e. perfect.
So even though the focal point **seems** a long way out, it is in fact ideal for the 3 inch corrector. To put it in perspective, the corrector is 137mm long and the focal plane is 58 mm out from the end of the corrector. So the focal plane is around 195mm out from the OTA wall.
So yes the only realistic option to improve the vignetting in this scenario now would be a bigger secondary.
My calculations suggest the nearest commercial size 4" minor axis secondary to get 100% lighting to the edges of the KAF 1100 chip, with the 8" back focus from tube wall. A good source would be Antares Optical. You will probably need to replace the secondary holder and spider as that supplied does not look adequate to support a piece of glass like this.
Are there pictures anywhere of what you have done to the OTA?
Not yet - I'll post some on my site soon. All I've done so far is:
- one strip of aluminum inside the OTA under the focuser
- a few tiny grub screws in the focuser tube to hold the coma corrector in position (I had to remove the compression ring becaase it was too small)
- dew heater on the secondary
- I **think** I put stronger springs in the primary cell? My memory isn't what it used to be...
- robo focus (after that picture was taken)
All in all not too many changes. Now I just need to spend more time on it. For example, last night I finally got Focusmax and all that v-curve stuff working properly after lots of mucking around.
The more I look closely at that image, the happier I am with it.
Just having a look at your picture full size your system seems to avoid those unsightly halos around brighter stars evident in pictures taken with the Orion AG12. I suspect that the halos are caused by poor ghosting analysis in the design of the corrector ( but I'm open to other suggestions) , which the Keller corrector seems to be free of.
Just having a look at your picture full size your system seems to avoid those unsightly halos around brighter stars evident in pictures taken with the Orion AG12. I suspect that the halos are caused by poor ghosting analysis in the design of the corrector ( but I'm open to other suggestions) , which the Keller corrector seems to be free of.
Well Mark there is more to it than that actually.
Firstly ghosting in a corrector lens system usually manifests in images as odd reflections that appear in the frame somewhere and not as halos around stars themselves...or perhaps you were referring to something else?
Halos around stars of various shapes and sizes can be caused by a number of things, including: the focuser barrel, adapters, chip cover slip, the CCD window, the filters or the corrector and the coatings on any one of the optical surfaces near the chip, or in some cases a combination of some or all these factors. As a result different combinations of scope corrector/flattener, filter, CCD window and chip cover slip give different results with different filter makes and even different filters within a single brand line.
The halos you are refer to in my images I have noticed are present most prominently when using the Ha filter and to a lesser extent the OIII and SII and they are weaker in the R G and B filters and basically non existent in the Lum. Now the intersting thing is that my Lum filter is a brand new not released Astronomik filter that Gerd Neumann sent me to try - this low halo effect is evident in my recent M7 image actually and I would say the degree of haloing in this bright star data set is no worse than many FSQ and other optical configuration images I have seen
If you also care to take a look at Adam Jesionkiewicz and Peter Shah's and other AG users work who use various combinations of cameras, adapters and filter brands (but the same corrector), you will see your theory about the Orion Optics corrector being the culprit is likely not accurate.
Mike
Last edited by strongmanmike; 24-07-2011 at 01:09 AM.
There was a lot of internet posting about reflections and halos about 2 -3 years ago when the first Apogee and FLI 16803 chipped cameras were released.
The STL11 chip is half the size of the 16803 so you are not comparing apples with apples there and the reflection debate started with the 16803.
It seemed to mainly be prevalent with scopes that use correctors.
To be clear that is all scopes with a 16803 chip. Even an RCOS. An RCOS probably could get away without using one but it would lose its tight stars further offaxis which the correctors/flatteners fix.
The usual fixes were:
1. A new range of antireflection coatings on filters (Astrodons were some of the worst offenders as they are mirrored type filters) So you now have several offering Generation 11 filters. I use Astrdon Gen 11 filters and those coatings work well.
2. Cameras had aperture masks installed.
3. Apogee and FLI offer no cover slip versions of their cameras as these can cause small halos around stars.
4. CCD Window needs to have antireflection coatings and be of good quality material. FLI addressed this very well.
5. Edges of filters need to be blackened. Baaders need to have the blackening extend onto the face of the filter 1mm as the coating ends just before the edge (a different manufacturing approach).
6. Most reflections seem to bounce off the correctors/flatteners and the filters and then bounce around anything not nailed down reflection wise. As Mike says any bright parts, anything anodised (highly reflective in IR).
But the biggest fix seems to be the choice of filters.
So if you are thinking of getting one of these cameras chose the filters carefully. They don't all perform the same.
Thanks for the detailed discussion on halo causes, guys. I wonder if surface roughness on the filters could be causing some near field scattering as well ?.
Mike: Proper ghost analysis requires an examination of both on axis and off axis ghosting. It seems clear from your results and from others that its a filter/ camera issue rather than the corrector.
I don't have many data points from personal experience, but I have looked at a **lot** of images online and I see halos or whatever you want to call them in images taken with ASA and Orion newts, including the people mentioned by Mike above. Guess which image in the attachment is from an ASA 12" and which one is from an Orion AG12?
I was fully expecting this when I ordered my coma corrector and I won't be surprised if and when it happens. So far, it hasn't.
if they have made them thicker and given them beef then Kudos!
