Anyone know where I can pick up a spacer for my MPCC locally?
I have the ST8300 and a SN 8 inch which suffers from reasonably serious coma. In the past with my Canon 350D I simply added the MPCC at the front of the camera as the CCD was set a reasonable distance back inside. The CCD in the new camera is a lot further forward - there's only 17mm to the front of the camera and 31mm to the front of the nosepiece. I need to stretch that out to 55mm to ensure the MPCC works correctly with the camera - at the moment I have odd looking stars at the edge of field.
I expect the filterwheel (when it becomes available from SBIG) will add a bit more distance to the MPCC but I am still likely to be 10 - 15 mm short.
The Baader spacer kit has a 14 mm spacer which looks about right for me (I hope).
I see that Optcorp sell the spacer kit separately from the MPCC but Andrews and My Astro Shop only sell them with the MPCC. Any suggestions on where else to try?
Pete
Last edited by pvelez; 23-01-2010 at 02:14 PM.
Reason: More accurate reflection of contents of post
I might be wrong but I 14mm finetuning ring in MPCC kit has m48 filter thread on both end. is that what you need? I thought you would need T thread M42 male to female.
For exact spacing you can follow the link Marc provided. He can custom make the adapter for MPCC.
You could go myAstroshop.com.au and combine
10mm + 3mm or
10mm + 6mm
As Marc said there is a tollerance of +/-1~2mm, but you need to be sure about your required spacing first. I believe you can confirm those from SBIG gurus.
I took a few shots of the Tarantula a few nights ago. The attached shows the results.
Now ignore the lousy focus and minimal Sidonioing and have a look at the star shapes at the edge of the shot.
I used a Ha filter screwed into the MPCC which was in turn screwed into the 2 inch nosepiece for the camera.
I understand that the MPCC requires 55 mm from focal plane to the MPCC. By my estimate, I have only 31mm to the MPCC ie I'm 24mm short.
So does this explain why I have stars at each corner that seem to be elongated towards the centre of the shot? And if so, will an extra spacer address this?
Just for your info, this is a median stack of 4 x 5 minute shots. Guiding was reasonably solid with PHD.
Yep. If you are too close you will get "reverse coma", Which I believe is referred to as Over-correction.
I've got a CFW on my ST8300 that takes up about 22mm, so I had a custom made adapter to suit the MPCC given the distance from the CCD to the front of the CFW, taking into account the width of the filters..
If you're not using a CFW at the moment, a 40mm Male to Female T-thread extension will get you pretty damn close if its screwed straight onto the front of the ST8300, and into the MPCC. It will give you 57mm spacing, which is within spec and should produce good results...
I have a spare one here.. If you want it, its yours.. PM me with your address and I can get it in the post monday...
Thanks Alex - not only is it good to hear that its not just me, its even better to hear that it has a technical name too.
Thanks also for the offer of the spacer. Once again, the helpfulness of fellow IIS is great and very appreciated. I've committed to take something similar once I'd verified that it was what I needed and it woudl be rude of my not to go ahead with it.
Out of interest, what FW are you using? I have been waiting for SBIG to release an adapter to the CFW8. I have a 50.8mm LRGB set ready to go and also 2 inch threaded Ha and OIII filters which both fit the CFW8. There hasn't been anything released as yet as far as I can see. It would be good to hear if there is a ready replacement available.
I am extremely happy with the camera so far - I hadn't appreciated that it has such a large dynamic range. Its as if you can stretch it from here to the next suburb.
I'm using an FLI CFW 2-7 7 position 2" filter wheel... It required me to also buy the FLI AD-2 adapter, that allows the use of the FLI CFW with T-thread based cameras.. This was expensive, however you could get something like the Orion 2" 5 position motorized wheel from Bintel for $650.. That is a T-thread wheel, so it will thread straight onto the ST8300 body, and presents a T-thread at the front too.. So you can then just buy the correct spacer for the MPCC and go for your life!
The SBIG CFW that you can use with the 8300 is the FW8-STL... Not the CFW8... the CFW8 is a 1.25" CFW.. The SBIG 8 position 2" wheel is a little more expensive then my FLI option, however, it has the added advantage of using the I2C port on the ST8300 for both data and power.. My FLI wheel requires its own USB and 12VDC power cables... The SBIG CFW FW8.. requires only the short serial lead that runs between the camera and the CFW... This is a great way of sorting out cable management...
I too am supremely happy with the ST8300..
Good luck mate... Looking forward to your images..
Thanks Alex - don't expect much from my camera for a while yet. Leaving aside the pesky clouds, I feel like an L plater who has just received a sporty new turbo charged Impreza but who cannot take it out of the driveway for the next few months.
I suspect the FW8-STL is the way to go. It will be easier if the FW talks to the camera and draws power from it directly. All I need now is to learn how to connect it up - I had understood that the adapter was still in the pipeline. Do you have further information on this?
Just for your info, this is a median stack of 4 x 5 minute shots. Guiding was reasonably solid with PHD.
Any thoughts?
Pete
If the MPCC spacing is too long you might be over correcting but in this particular case I think it's field rotation. If it was over-correcting your stars would look like "T" not "-" .
So if its field rotation, I need to improve my polar alignment - yes?
Pete
The tarantula is pretty close to the pole so you will get field rotation on 5-10min exposures if you're far enough yeah. Were you much far out? Try another target closer to the equator like a cluster if possible or rich starfield. They're good candidates to test coma.
Marc - I dont think its field rotation, as the direction of the star elongation seems to point from the outter edge of the field, into the center... This is either coma or curvature. if it were rotation, the stars would all be smeared in the same direction encircling the guide star.
Maybe under-correction? I dont know for sure, but when I used an MPCC at the wrong distance from my Starshoot Pro, the stars looked much like those in the images shown...
Marc - I dont think its field rotation, as the direction of the star elongation seems to point from the outter edge of the field, into the center... This is either coma or curvature. if it were rotation, the stars would all be smeared in the same direction encircling the guide star.
Maybe under-correction? I dont know for sure, but when I used an MPCC at the wrong distance from my Starshoot Pro, the stars looked much like those in the images shown...
Alex, I'll dig some pics to illustrate what I'm talking about and will post shortly. I have stacks in my "gallery of horrors" ... lemme check..
actually, it was pretty good - particularly as I was reasonably lazy when setting up as I don't have the FW yet. In the past I've used Alignmaster but the stars it defaults to are almost always bhehind the house, behind a tree etc.
I use EQMOD with Starry Night and a few markings on the back deck to set up the tripod. I aligned on Canopus, Avior, Alderbaran and Betelgeuse - with these the Tarantula landed almost exactly within the centre of the CCD frame when I started. It surprised me actually.
I might have another look at the shots I took in Orion the other night to see whether I had the same issue.
Re: the FW8-STL CFW.. The adapter is still in the pipeline yes, however if you contact Ashley at Preciseparts.com he will be able to make you an adatper to go from T-Thread to the FW8 in no time flat.. Or you could email SBIG requesting an adapter and see how you go? I dare say they are getting them sorted out, as there are a fair few people out there at the moment with ST8300's and no CFW to go with them.. The ST8300 wheel and the FW8-STL to ST8300 adapter will likely be high in the priority list.
Agreed - the FW8 is probably the better way to go... I'd not thought of it when I bought my rig... Having said that, I see no need for 8 positions, and my 7 position does the job nicely, and cost roughly the same.. so essentially, end of the day - I'm happy... Just... Happy with a few extra cables
Here's a couple of shots and my understanding of what's going on as I eventually fixed it.
1_ obviously field rotation. Everything rotates in reference to the guide star in this case in the keyhole vicinity. (10min exposures). Crisp arc lines. No coma, just field rotation. Combination of field rotation + coma makes stars on the edges look like squares. Still digging out an old tarantula shot. (that's why I said it's a bad choice to test)
2_ Over-correction. MPCC is too far. Stars are bright from the edge blurring inwards (inverted comet). T shape on an ED80 is more pronounced the further out you are.
3_ Collimation. Your field is like a horse saddle. Some of it is perfect some of it is worse. No uniformity. Primary/secondary/focuser axis offset.
4_ Coma (under correction). Stars are bright from the center blurring outwards (comet shape)
