Baader mpcc at f3.9

[alacant]

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Hi everyone

f3.9 Newtonian. Things are going well apart from the pinched mirror and the coma. The former I can fix but I don't know where to start with the coma.

 

I'm using the cc out of the box with the 3mm extension ring which evidently I simply attach to my Canon 1200d m48 t-type ring to get the correct spacing (55mm?). There's still coma toward the corners however. Whilst it's easily corrected in software, I'd like to be able to eliminate it. Here's a light frame:

https://drive.google...ZExtcHBkTmNyRFE

 

ES assure me that the only way to get coma free would be to use their own 4-element cc .which costs almost as much as the telescope itself. But then again, I suppose they would say that.

 

Baader advertise that their cc is good to f3.5 so, does anyone have experience of the Baader with fast f?

 

Any advice most gratefully received. 

TIA and clear skies,

 

 

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[Costas Soler]

Hi! I use a baader mpcc on my f/3.9 Newtonian. I get a little bit of coma around the edges as well, but almost all the objects image are small enough so that I end up cropping out the coma stars. I've used the baader mpcc for a while now, and I'm pretty happy with it so far. 

[alacant]

Hi. Thanks for the reply. How do you connect it? There seem to be 2 configurations out of the box: either the t2 m42 with the male-male thread and thin spacer .or. the wider m48 route with the 3mm spacer between the m48 adaptor and the cc. Confused... TIA.

[laser_jock99]

I presume you looked at the manufacturers data sheet for the recommended spacings? I tried them and found them to be 1-2mm out! Needed to buy a set of spacer rings though to find out.

The Baader is good - but being designed also for both visual and photo use it is a bit of a compromise I think with all the rings, stops and adaptors that come with it make it tricky. I actually found the Skywatcher F4 Aplanatic Coma Corrector better to use in practice.

 

[Costas Soler]

Hi. I'm at school right now, but can I send you some photographs of how I connect mine when I get home tonight?

[alacant]

18 minutes ago, Costas Soler said:

when I get home tonight?

Hi Costas. Yes, that would be great. TIA.

15 minutes ago, laser_jock99 said:

 

I presume you looked at the manufacturers data sheet

Mine is the version III but still with a myriad of spacers and rings! What spacing should I use? Was the 1 or 2mm difference doable with the kit they send with the cc? TIA

[laser_jock99]

21 minutes ago, alacant said:

Hi Costas. Yes, that would be great. TIA.

Mine is the version III but still with a myriad of spacers and rings! What spacing should I use? Was the 1 or 2mm difference doable with the kit they send with the cc? TIA

I had to buy a set of extension rings from e-bay and try different spacings till I found what worked by trial and error. I don't have my notes to hand so can't tell you exactly where I ended up at.          

[alacant]

Hi. Yeah, OK. So start at 5mm and work upwards (?) That will give me something to start with. Thanks.

[alacant]

Progress. I've now tried both Baader recommended methods for for connecting the cc.

1. dslr - m48 adaptor - 3mm spacer - cc - telescope

2. dslr - m42 adaptors - thin Baader spacer (the one with the writing on) -  cc - telescope

2 is better; the coma all but disappears but it's m42. Observation: 2 focuses around 5mm closer to the telescope than 1. What gives? Surely, both 1 and 2 should give the same cc to sensor distance and so the same focus position(?) Why must I refocus between m48 and m42? I checked the thickness of the adaptors. Both are the same, around 11mm. I'd like to use the wider m48. Ahhgghh! Anyone? Or is this new ground? TIA.

light frame from method 2 the m42 route

[richyrich_one]

On 29/01/2017 at 11:07, alacant said:

Progress. I've now tried both Baader recommended methods for for connecting the cc.

1. dslr - m48 adaptor - 3mm spacer - cc - telescope

2. dslr - m42 adaptors - thin Baader spacer (the one with the writing on) -  cc - telescope

2 is better; the coma all but disappears but it's m42. Observation: 2 focuses around 5mm closer to the telescope than 1. What gives? Surely, both 1 and 2 should give the same cc to sensor distance and so the same focus position(?) Why must I refocus between m48 and m42? I checked the thickness of the adaptors. Both are the same, around 11mm. I'd like to use the wider m48. Ahhgghh! Anyone? Or is this new ground? TIA.

light frame from method 2 the m42 route

If you measure the thickness of the spacer you used in method 2 it is 2.5mm not 3mm. By using a 3mm spacer with a standard t-ring it would give you 58mm and not the 57.5mm you need. Enough to make a difference to the coma? You say both t-rings are around 11mm. Do you have anything to measure them precisely? 0.5mm here 0.5mm there adds up.

[alacant]

3 hours ago, richyrich_one said:

Do you have anything to measure them precisely?

Hi. No, I don't so I just ordered calipers. That should help solve the mystery and point taken about all the 0.5mm adding. Thanks for the reply and diagram.

[laser_jock99]

On 30/01/2017 at 20:19, alacant said:

Hi. No, I don't so I just ordered calipers. That should help solve the mystery and point taken about all the 0.5mm adding. Thanks for the reply and diagram.

Calipers- every astrophotographer should have some!!

[alacant]

On 30/1/2017 at 18:03, richyrich_one said:

Enough to make a difference to the coma?

Hi again. Just in case anyone is dropping by, we established that the first diagram -the one with the 2.5mm spacer- is incorrect. The ring does nothing; it is not a spacer and the distance to the sensor is in fact measured not from the ring, but 1mm to its left as pictured. I wonder if that is the measurement which should be 55mm?

Anyway, I tested the t2 method again: this is a single light frame from my 700d. The coma is more pronounced at one side; tilt? Any/all ideas/things to try most welcome. TIA.

[richyrich_one]

3 hours ago, alacant said:

Hi again. Just in case anyone is dropping by, we established that the first diagram -the one with the 2.5mm spacer- is incorrect. The ring does nothing; it is not a spacer and the distance to the sensor is in fact measured not from the ring, but 1mm to its left as pictured. I wonder if that is the measurement which should be 55mm?

Anyway, I tested the t2 method again: this is a single light frame from my 700d. The coma is more pronounced at one side; tilt? Any/all ideas/things to try most welcome. TIA.

This is all starting to sound very familiar, I am having the same issues and my subs look very similar.

From what I have been reading about collimation, I think I'm correct in saying that the margin of error becomes much smaller when using a coma corrector so collimation really does need to be spot on. And at F3.9 you really are pushing the tolerances! I always thought I knew how to collimate and what I was doing was good enough. Well I have admitted defeat and have a catseye system on its way...hopefully I can use it to sort my tilt issues once and for all. It may be overkill but at least I'll know it's right and I can look elsewhere if it persists.

I hope you can get it sorted.

[alacant]

Hi. Thanks for taking a look. Just to get things in perspective, the stars 'recover' remarkably well after stacking. What tails remain can be corrected in the -rather excellent- StarTools lens module.

Just a heads-up: It looks like the grubs around the t2 adaptor are loose and there's one missing -should there be a screw in each of the three holes as I can see two would do it-on my cheap AliExpress version. Maybe I should use Araldite or wedge it...

Collimation: I believe it to be the same at 3.9 as anywhere else (?). Are you saying we need to do something different?

• focuser beneath secondary
• primary centre donut on sight-tube cross hairs
• Cheshire reflection on the same cross hairs
• done

In my case, the first two seem to stick. This is a stack of 12 light frames from the same series:

 

[richyrich_one]

10 minutes ago, alacant said:

Collimation: I believe it to be the same at 3.9 as anywhere else (?). Are you saying we need to do something different?

The process is the same, just the "sweet spot" is smaller so accuracy is key.

 

31 minutes ago, alacant said:

It looks like the grubs around the t2 adaptor are loose and there's one missing -should there be a screw in each of the three holes as I can see two would do it-on my cheap AliExpress version. Maybe I should use Araldite or wedge it...

The screws are important as they will pull the insert in and keep it all square, if you wedge it etc you may induce tilt.