SBIG ST8300 and MPCC spacer

[pvelez]

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

[multiweb]

specs are 55mm +/-1~2mm for the spacing. I don't know about local but you can get one from here (http://www.tan14.com/gears.htm).

[sadia]

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.

[pvelez]

I suspect you are right.

Any thoughts on where to pick one up?

Pete

[sadia]

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.

[pvelez]

Excellent - thanks to you both for the tips

Pete

[pvelez]

I had a further play with my new SBIG 8300.

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.

Any thoughts?

Pete

[pvelez]

Here is a larger file - it should illustrate the issue better.

Also attached are crops of the top left and bottom left corners respectively. You can see the elongated stars shaping towards the centre.

Any suggestions would be appreciated

Pete

[AlexN]

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...

Cheers mate, and good luck with the new camera!

[pvelez]

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.

Pete

[AlexN]

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..

Alex.

[pvelez]

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?

Pete

[multiweb]

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 "-" .

[pvelez]

So if its field rotation, I need to improve my polar alignment - yes?

Pete

[multiweb]

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.

[AlexN]

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...

[multiweb]

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.. ;)

[pvelez]

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.

Pete

[AlexN]

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 :D

[multiweb]

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)

[DavidU]

Whoeww Marc ! They are shockers LOL

[pvelez]

Marc and Alex

thanks for your time on this.

There are a few interesting ones there Marc - might make a good thread actually. We can call it Bring Out Your Dead. I have a few more to add believe me.

Looking at Marc's examples it seems my shot is closest to the fourth.

If I'm right, its a result of placing the MPCC too close to the focal plane. Happily that matches Alex's diagnosis.

I am having a motor fitted to the focuser in the next week or 2 so I'll have the Bintel boys check my collimation. And I agree that the polar alignment wasn't perfect which shouldn't be too hard to sort next time I am outside (damn clouds!).

I'll report back when I make some changes with the spacing.

Alex, on the FW, I'll email your fellow to see what he can do about an adapter and also contact SBIG direct. The yard certainly isn't big enough to satisfy the desire to have burn with the new vehicle as it is.

Pete

[multiweb]

:doh: You're absolutely right... Coma... The thumbnail is clearly labeled corner left. I misread it and thought the elongation was not in the direction of the guide star but aligned with the periphery. :nerd: When Pete confirmed it in his last post after I posed some shots I went back to the shot thinking "what are you guys smokin' ?...." then looking more carefuly realised I'm the one who needs help ;)

[AlexN]

Not to worry Marc - I do that ALLLLL the time... Misread/Misinterpret whats been said.. :) Usually I put my foot a lot further in my mouth though :D haha

In any case - at least its sorted and hopefully with his new spacer, Pete will see nice round stars! :)

Well... I gotta go... I have a date with some of the nicest looking seeing brisbane has had in a few months... just shooting darks/flats while the moon is up then its off to the target for some quality photon snatching time!! :) YAY for ME!

[allan gould]

Nice examples Marc of what can go wrong. Very helpful explanations.

[pvelez]

Out of interest Alex, where did you pick up your CFW 2 - 7?

Pete

[AlexN]

Bought it straight from FLI Pete... It cost $995 USD + 100USD for the adapter between the ST8300 and the CFW (FLI Part no: AD-2) and $25US expedited shipping with UPS.

$1120 USD total, I paid $194 import duty/GST through customs once it got here.. All up it was $1436AU at the exchange rate on the day of purchase, including the GST customs duties etc..

Wouldn't have it any other way! The FLI CFW is superb... its quiet, its very very accurate, repeatability is spot on... The only downside is the cost of 50mm unmounted filters... My NB filter set cost me $1850AUD.. I really wanted the 3nm Astrodon's but they were $1200AUD per filter...

The LRGB filters I managed to pick up 2nd hand off the forums for $375 - A bargain considering they are $895USD new..

The Baader LRGB / SII Ha OIII filters in the 36mm size for the new SBIG ST8300 CFW are actually very affordable... Much cheaper than mine in comparison...

[pvelez]

Thanks Alex

I have been waiting for some news from SBIG on thier adapter. It looks like they have the 36mm wheel sorted but the adapter for the 50mm remains a mystery.

I have a set of unmounted 50mm LRGB filters which I picked up with the camera. I also have mounted Ha and OIII filters. The FLI site suggests that the CFW 2-7 wheel will take both but I've sent an email off just to make sure.

I'd still prefer the SBIG wheel as it cuts 2 cables out of the equation (USB and power). With all this rotten Sydney weather I am not in a tearing hurry - even though I am keen to take it out for a test drive now.

Out of interest, what spacing do you have with your MPCC?

Pete

[AlexN]

for the MPCC I got a custom made adapter that goes between the CFW and the MPCC. Its 16.3mm long. This takes into account the width of the filters and their affect on the optical distance between the MPCC and the sensor. It cost me $200 AUD from precise parts in the USA..

Ashley at Precise parts would be able to make you an adapter to go between the ST8300 and the STL FW8 no worries.. Send him and email and he'll do up a quote.. . You don't need to know the specifics of the adapter required, just tell him "I need an adapter to attache the T-Threaded ST8300 to the SBIG FW8 filter wheel" He has technical drawings of all the latest gear, knows the setups and will get back to you within 24hours with a price...

Had I realised that at the time I probably would have bought an FW8... As you say, it runs of an I2C cable that plugs straight into the ST8300, the FLI wheel requires its own USB and 12v power.

[pvelez]

Email sent - thanks Alex.

Still cloudy here in Sydney of course - will be worse when the gear arrives!

Pete

[AlexN]

You might be lucky Pete...

I got the CFW about 2 weeks before the camera showed up.. My camera landed in australia on the 24th of December (Sorry kiddies, no playing outside on Christmas day... Its raining, and its my fault! :) ) It rained from the 24th to the 3rd of Jan, Then the camera arrived on the 4th of January. (The day I started back at work!! :() Luckily for me, it was still clear that friday/saturday night.. I went out bush and got first light on the first weekend I had it! :)

Fingers crossed you get the same luck!