I was going to post this in the ATM forum but thought it would be better here. Well I have had this filterwheel and a QHY9 for a while now and two things really bothered me. Firstly was the M54 thread in the FW casing. This is not good for adaptors unless you want to use Baader M54 - T2 couplings (causes vignetting). Secondly the supplied nose piece has the M48 thread cut on the wrong side making it impossible to fit reducers etc(Theo is getting some new ones). If you have one don't panic for the fix is really easy and better then you may think. All you need is a hacksaw and file though access to a lathe is better and this is how I did it. If you cut off the 2" nosepiece back to the flange the M48 threads are exposed. Simply clean up the face with a file or lathe and you have yourself a 1mm thick (when fitted) M54 - M48 adapter. This allows all the normal bits and pieces to be added and it only adds 1mm of back focus. I have a number of 2" WO diagonals around which are converted to SCT threads so their 2" nosepieces are redundant. These screw straight into the modifed adapter and they have the M48 thread cut on the right side. Any nose piece with a M48 thread will fit though.
Pic 1 & 2 - The offending QHY nosepiece notice where the filter thread is.
pic 3 & 4 - Thats one slim adaptor but still has 5 mm thread to screw in M48 threads.
pic 5 - fitted to FW, only 1mm proud so does not affect backfocus.
pic 6- with WO 2" nosepiece.
pic 7 - TS-OAG- 9 mounted (48mm threads = no more vignetting)
pic 8 - All together with the TS field flattener attached (for refractor). It is a simple screw out the TS FF and Screw in the AP CCDT67 reducer (with spacer) for The LX200 as both have the same distance requirements to the chip and are almost identical.
That is one tasty looking OAG/Corrector solution too..
Very ingenious work I think Mark. It looks like you had a real lightglobe moment sometime recently and thought "Aahhh HAA! - Thats what needs to be done! " And a good job you've done of it all too..
And i just wasted $79 and postage for buying a converter
Not to worry Martin, I have bought and tried just about every Baader adaptor trying to find a way around this. It's only when I sat down and had a good think about it that the penny dropped. Grab a hacksaw and file and get cutting .
This is well and good, however, both my WO Flat II and my Baader MPCC have M42 threads.
What is really needed is two or three adapters, depending on your equipment and how you want to couple your QHY9 and camera together.
There is also the need for an adapter between the camera and the wheel to correctly position them for the best/most practical mounting solution. I will do some schematics and post them to the QHY site as soon as I can find the time.
Upgrade to what? The mpcc is the corrector for newt users. Are you saying you get problems with an mcpp in front of the wheel?
Robin I am saying in my experience anything with a T2 thread in front of the FW will cause vignetting. When I tried the Baader M54 - T2 adapter thats exactly what happened which is why I tried this method. You simply cut too much of the light cone to get full and even illumination. I do not have all the answers, I have only tried to show how I got around it.
Great thread, and fascinating reading. I'm sure there must be some vignetting having the smaller MPCC opening in front of FW, but you have to get the 55mm spacing to the sensor. No way I can afford an RC or other imaging solution for creating a bigger illuminated field that doesn't suffer coma any time soon.
I've used my MPCC with a 450D for about 18months before the QHY9, and can't see how the geometry is much different now for 8", MPCC and QHY9. QHY9 sensor pretty much same size as 450D was. I've attached an example flat from my set up stretched hard to try and show any vignetting. Can't claim to be perfect in any way....
One thing I do find is with Baader adaptor (which uses about 6mm, I have to take out the 12mm window adaptor on front of QHY9 and replace with a 10mm space in front of FW and behind MPCC to get 55mm spacing (hope that's not double dutch...)
One other thought - there are apps to model light paths. I wonder if the light cone on my F5 reflector is coming in at a tighter angle to the QHY9 sensor than a refractor at slightly higher F ratio might be
wow looking at that rob your getting massive vingetting . I recently looked at the raytrace programs and found that a increse in the secondary could virtually illuminate 100% the 40d sensor (58-70mm upgrade). but the MPCC causes some good vingetting.
In my flats now its kinda funny, i don't have a nice graduated gradient from the center to the outside. its a very abrupt line where it goes from 100% to 80% odd, this tells me that the MPCC is the culprit, i will do some tests removing the mpcc from the fray just to see what the difference is.
The only way around this is to go for a larger 2.5/3" corrector, they are made and they do cost! but they are quality objects.!
Myself i would like to go for a Wynne 3" 0.75x keller corrector they are about 1200 dollars but will take a F4.7 to F3.5 with a effective focal length of 895mm and effectively turning the scope into a newtonian apo with tight spot sizes! makes for some ultra wide field whooze action!
wow looking at that rob your getting massive vingetting . I recently looked at the raytrace programs and found that a increse in the secondary could virtually illuminate 100% the 40d sensor (58-70mm upgrade). but the MPCC causes some good vingetting.
In my flats now its kinda funny, i don't have a nice graduated gradient from the center to the outside. its a very abrupt line where it goes from 100% to 80% odd, this tells me that the MPCC is the culprit, i will do some tests removing the mpcc from the fray just to see what the difference is.
The only way around this is to go for a larger 2.5/3" corrector, they are made and they do cost! but they are quality objects.!
Myself i would like to go for a Wynne 3" 0.75x keller corrector they are about 1200 dollars but will take a F4.7 to F3.5 with a effective focal length of 895mm and effectively turning the scope into a newtonian apo with tight spot sizes! makes for some ultra wide field whooze action!
I think the marvel is in how well flats normally do correct so well for vignetting. Its rare I have trouble with objects on the edge of my field unless data collection has been problematic for some reason (e.g. dewing). Its all relative to how hard you have to stretch your objects during processing I guess. I pushed a fair way beyond usual with the flat above.
Agreed wider corrector would be nice Brendan, but its hard to justify for an OTA that's really only worth a few hundred dollars. I'm still happy with what the 8" lets me explore. I suspect if I did enlarge my secondary I wouldn't gain to much more illumination because of MPCC as you say.
Will be interested to see how this next generation of short F scopes and correctors works out though - watching with great interest. Onwards and upwards!
This is an interesting thread given most of the imaging systems seem to use T-threads. The Televue Paracorr has the full opening IS system however even this uses a T thread to connect to the camera.
I can't understand how a larger secondary mirror is going to reduce vignetting if the restriction is in the camera nosepiece or the focuser drawtube?
No it doesn't reduce the Vingetting introduced by the MPCC. but the difference is that the calculations i got when analysing my 10" Dobsonian from Skywatcher showed that on the 40d imaging sensor i only had a 8mm diameter spot that took full advantage of the 10" mirror. the edge had reduced to a mear 78% of the mirror's capacity.
With the upgrade i have gone from 8mm 100% illumination to 24mm diameter 100% illumination. so what i hear most people say?... it means that my image is copping 100% of the mirrors power right upto the MPCC to which mapping the vingetting with flats kills the rest of it. This allows fainter things to show up. not just in the middle of the image! but virtually right to the edge. The mpcc has a 22mm 100% though put corrected circle just for info.
look at this image and youll see what i mean, be sure to notice the faint blue nebulosity's extension to the right hand side, and the Ha loops in the bottom left, you don't tend to see those in M20 images at this kinda image scale and remember im only using a DSLR not a dedicated cooled Astro CCD...
Taken in the back yard of my mates house in Secret harbor WA 40km from Perth CBD
8 x 10min iso 800 10" guided with finder EQ6pro and modded 40d
No flats, darks or bias taken so it shows the extent to which the 100% illumination is illuminating. Do note that the center 100% illumination isn't 100% in the center hence the vingetting is slightly offset. but now with the cats eye collimation gear + the 3" focuser thats on the way ill sort that when i install the new focuser!
This is the best photo that i can find that shows vingetting. with the same setup youll notice that the noticable darkness happens alot earler and that is without the MPCC... so thats just open with no vingetting due to focuser nor adaptors. Ill do a test this week with the flats box to make a definate comparison in relation to the increase in secondary size to vingetting spot size.
Rob,
everything i do has a alteria motive! i don't upgrade these parts for the mear fact of trying to get more juice out of a already squeezed lemon (figure of speach). Im working towards a premium ultra fast newtonian... or as some would call it UWFW... (ultra wide field whooze). As i cannot see myself getting a dome for at least 4 years maybe more making high res long focal length very hard. So having premium bits will allow me to get the best i can from this 10" sw dob before i custom make a CF tube and drop a fast F3 - 3.5 10-14" mirror, then ill swap everything straight over. Almost like layby... plus im a OCD tinkerer
Last edited by bmitchell82; 05-05-2010 at 03:43 PM.