How critical is the back focus on an MPCC?

[rfdesigner]

I'm looking to add both a filter wheel and a coma corrector to my setup: Coma Corrector - slimline SX-OAG - SX-FW - ATIK 383L+

The total CCD to flange distance for this little lot works out at 57.5mm, just 2.5mm longer than the MPCC is designed for. The scope is an F5 Newt.

Can I get away with this?.. does anyone have any numbers on just how good/bad this might be.

Thanks in advance

Derek

PS I've looked at the RCCI but it looks like it really cares that the OTA is good and stiff, and mine isn't THAT stiff.

[davew]

The distance is critical ( ish ) but not necessarily 55mm. Sounds daft but if you look around you'll see that many people get close to the figure but not the actual figure.

Things also change with a CCD and filter in place. Badgers found he needed a slightly larger distance. PM him may be ?

I use an 8300 chip and it works best around 1 to 2 mm longer with narrowband filters in place. I don't use a Newt though.

Dave.

[Earl]

With the filter you will probably be needing 56.5 it's trial and error really. This is one reason why I'm not planning on the reducer for my baby q as it won't have spacing issues native

[steppenwolf]

Close enough I'd say especially if you are using filters.

[badgers]

My f/5 newt is currently spaced at 56.5-57mm with the Skywatcher corrector and working perfectly right to the corners.

You have a larger sensor than I do so the tolerance is tighter.

Above 57.5mm I started to get problems in the corners, but again on a smaller sized sensor.

So I'd say you are possibly borderline at 57.5 for 'near perfect' correction, but possibly good enough for what you want.

You could look at the T-adaptors or M48 adaptors in your imaging train and see if there is any place you could eek back another mm by soaking up unused thread.

Anton

[rfdesigner]

thanks... I was hoping the F5 of my scope would buy me some wiggle room.

adapter wise, I'm not so sure what to do as I'm at the limit with my proposed line up with off the shelf parts, the only thing I can think of is to get a bespoke adapter made up.

Derek

[A320Flyer]

I have the SW CC and a 150 f5 Newt. The CC is set at exactly 55mm from the chip and I have had a remarked improvement on the shape of stars in the corners. I use a QHY8L which has a big chip. Attached is a screen grab of CCD Inspector showing the before and after affects of adding the CC.

One thing that might be important is that with the CC fitted and focussed, I only have about 10mm or less of inwards travel left on the focus tube. I dont use a filter wheel but this might not be enough to accomodate one.

Bill

[davew]

Close enough I'd say especially if you are using filters.

Agreed.

A OSC and a mono with Ha filter aren't quite the same.

Dave.

[rfdesigner]

Thanks for all the feedback.

I also contacted Baader directly asking what performance I might get with my proposed setup... their reply:

Bad ones.

There is no use in compromizing the 55mm rule.

Not anymore in the age of CCD-chips.

Best regards,

I don't know it they're 'being German' or not but it looks like I need to be much closer to the 55mm (or 56mm with filters as pointed out by several people here, thanks)

So I have two options:

1. get new slimmer adapters / modfy my ones to make them slimmer.

2. get a different coma corrector.

I've looked around and the next one up for back focus is the Astro-Tech one sold by Ian King "Altair Astro Coma Corrector and Field Flattener for 8" F4 Newtonian" with a 70mm back focus.

Problem is it lengthens the focal ratio by 10%, I wouldn't mind a fractional reduction but a lenghtening is going the wrong way for me.

Any other thoughts on ways to crack this little problem?

Derek

[badgers]

You could try and get a 2nd hand TS 9mm OAG... I know it's a pain, but they really are slim.... I'd not be able to reach 56.5mm without it.

You'd certainly be able to get a good price on your existing OAG....

With the Lodestar you'll not have issues getting guide stars, I certainly haven't had any issues yet.

I'm stuffed with my Triplet Apo Flattener, it requires 51.5mm or around 52-53mm with filters so I'm struggling to get close. Although the tolerance seems a lot higher at f/6 on a field flattener than at f/5 on a coma corrector.

Anton

[rfdesigner]

You could try and get a 2nd hand TS 9mm OAG... I know it's a pain, but they really are slim.... I'd not be able to reach 56.5mm without it.

You'd certainly be able to get a good price on your existing OAG....

With the Lodestar you'll not have issues getting guide stars, I certainly haven't had any issues yet.

I'm stuffed with my Triplet Apo Flattener, it requires 51.5mm or around 52-53mm with filters so I'm struggling to get close. Although the tolerance seems a lot higher at f/6 on a field flattener than at f/5 on a coma corrector.

Anton

The TS-OAG is certainly an idea, but the reports seem to be mixed on that one. Since I managed to debug my SXOAG I'm very loathed to part with it, it's so solid I don't need to bother tying up my cables (the whole scope get's pulled before the OAG shifts)

I've just been going back through all the options (including changing the camera for a 460EX.. ouch!) in fact it won't buy be much in performance terms compared to the slightly longer ATCC coma corrector option. Both end up with effectively 4.5u pixels, which for me is just too small.

Either I go TS-OAG or I get my adapters milled down and retapped. Seeing as I know someone who's recently purchased a mill/lathe thing (I'm sure there's a proper name for it) the easiest option may well be to buy him a few beers, I just checked and neither the MPCC or the 383L+ would clash with the adapter mounting screws, so I could buy 2mm at each end of the imaging train that way.

why is nothing ever straight forward?

Derek

[rfdesigner]

I think I have a solution.

383L+, Atik EFW2, SX OAG. I realised I could drill and tap the face plate on the EFW2 to take the SX OAG (I own a tap and die set and a set of 0.1mm increment drill bits)... So as there's nothing new under the sun, I surfed the world wide wait and lo!.. steve richards has beaten me to it.

http://www.progressiveastroimaging.com/viewtopic.php?p=44485&sid=36ea66fe1b24c00554f7345cca5e4c19

Good to know it's going to work.

Derek

[Tom Habaj]

That's how I connected my FLI CFW with SX OAG:

[steppenwolf]

Oooh yes, It works very well indeed!

[steppenwolf]

I've looked out a close-up image of the two devices connected together:-

[rfdesigner]

made for each other .. very professional

out of interest, what's the heaviest camera you've lung off the back?

Derek

[steppenwolf]

out of interest, what's the heaviest camera you've lung off the back?

389gm of SX SXVF-H9 !