Canon with new TOAG and coma corrector seems to have coma.

[SteveBz]

Hi Guys,

I just decided to upgrade my guiding to OAG from guide-scope.  I hope I've got the spacing and things right, but I seem to have a deal of coma that wasn't there before (and a bit of gradient).  Here's a picture followed by a full description of the problem:

I have a Newtonian C8-N with a low profile focuser so I bought a TOAG like the Orion TOAG.  Because I had a Nikon camera, I had to switch to Canon to make the T-ring adaptor work, so in fact there's a steep learning curve with all this equipment: Canon 450D, TOAG, QHY5L-ii m. Plus existing Newtonian and MPCCiii with light polution filter.

After a lot of faffing around in daytime looking at the local trees, I managed to get it to focus on both cameras and two nights ago took about 90 mins of NGC6888 (see above).  I was quite pleased with the focus and guiding and it looked OK apart from the gradient and coma.  This post is really about coma.

The TOAG comes with a number of spacers/ adapters and the MPCCiii comes with a couple of small spacers and adapters.   With the Nikon I had both adapters in place and it worked well.  As I recalled these were for the Nikon only because of it's slightly different flange-depth.  I thought the Canon didn't need then so I took them out.  As you can see the Bottom right is especially coma filled, but to some extent all the way round.  I imagine it's worse in one corner because maybe my collimation is a tad off, but the bigger issue is coma.  My main optical train is Canon 450D DSLR->Canon DSLR thin profile T-ring adapter->TOAG->MPCCiii->LP filter.  On the TOAG side, it's not yet quite right and gives strange shaped guide-stars, but that goes: QHY5L->c-t adapter-> TOAG (I have a 5 mm spacer on order).

I hope if I can improve the coma in the main cam, that will improve the guide-cam too.

Does anyone know anything about the relative spacing of the MPCC iii in a Canon/TOAG environment?

Regards,

Steve.

[SteveBz]

49 minutes ago, SteveBz said:

Does anyone know anything about the relative spacing of the MPCC iii in a Canon/TOAG environment?

I've just remeasured the distance from the sensor mark on the camera to the TOAG/MPCCIII junction as 54.5mm.

Edited September 7, 2020 by SteveBz

[Tommohawk]

Canon flange distance is 44mm, your OAG is 10.5 mm I think, and the coma corrector requires 55mm. Can't quite see how the OAG connect s, but your measurement of 54.5 mm seems correct, and it follows you have a 0.5mm error. I wouldn't have thought that would matter too much. 

The "coma" seems more marked on the right side... Maybe something isn't quite square, is slight tilt? 

[SteveBz]

On 08/09/2020 at 22:57, Tommohawk said:

Canon flange distance is 44mm, your OAG is 10.5 mm I think, and the coma corrector requires 55mm. Can't quite see how the OAG connect s, but your measurement of 54.5 mm seems correct, and it follows you have a 0.5mm error. I wouldn't have thought that would matter too much. 

The "coma" seems more marked on the right side... Maybe something isn't quite square, is slight tilt? 

Hi Tommohawk,

Thanks for replying.  Yes, I think there must have been a slight tilt.  I hope I've fixed that now, but the most important thing was I inserted the OAG m48-m48 spacer, which is about 2 mm between the OAG and the MPCCiii and amazingly it works fine.  I have no idea how, but really it's beautiful.  I have also inserted a 5mm CS-C connection spacer and the OAG focus is also much improved. I now have very round stars in the OAG and lots of them and the main image looks good.  

Looks like you are based near to me.

Regards

Steve.

[alacant]

Hi

The Baader cc needs 58mm backfocus. Stick a 3mm m48 spacer -or a low profile 2" filter ring without the glass- between the OAG and the cc and you should get you as close to coma free as you're gonna get over aps-c.

Cheers and HTH.

Edited September 12, 2020 by alacant
ove over

[Tommohawk]

Well the Steve says he has added 2mm, if I understand him correctly, and it seems to have done the trick. So that more or less fits with 58 mm back focus.

But most websites seem to say it's 55 mm back focus, and the baader datasheet doesn't seem to say anything about back focus. Seems odd. But if it works, I guess that's what matters!

[alacant]

3 hours ago, Tommohawk said:

websites seem to say it's 55 mm

Hi

The op is -wisely- using the m48 connection. So he needs around 58mm, not 55mm.

I copied Baader's diagram, here.

 

Edited September 12, 2020 by alacant

[SteveBz]

5 hours ago, alacant said:

Hi

The Baader cc needs 58mm backfocus. Stick a 3mm m48 spacer -or a low profile 2" filter ring without the glass- between the OAG and the cc and you should get you as close to coma free as you're gonna get over aps-c.

Cheers and HTH.

Hi Alacant,

Thanks for that info. It clears up the problem. Actually I had 2 spacers to choose from. I'll remeasure in the light of this information. Maybe I can even improve it some more. I can certainly improve the focus a bit, and that might give me more stars when I'm pointing away from the milky way.

I've used lockdown to make huge strides in my equipment and software.

Next I really need to look at that dec backlash. It's becoming the highest priority by quite a margin.

Regards,

Steve.

[Tommohawk]

3 hours ago, alacant said:

Hi

The op is -wisely- using the m48 connection. So he needs around 58mm, not 55mm.

I copied Baader's diagram, here.

 

Doh. I didn't look at the diagram properly! 

[SteveBz]

17 hours ago, alacant said:

Hi

The Baader cc needs 58mm backfocus. Stick a 3mm m48 spacer -or a low profile 2" filter ring without the glass- between the OAG and the cc and you should get you as close to coma free as you're gonna get over aps-c.

Cheers and HTH.

Hi People,

I measured the new spacer and it's about 3mm which would put me exactly on the 57.5 line. However, the ridging on the mpcciii plus the compression ring cause the whole camera to slip out by a further 0.75 of a mm or so, putting me just over. However it's a great improvement.

[alacant]

2 hours ago, SteveBz said:

the compression ring

Yeah, the undercut on the Baader is a pain. Use a thin 2" parfocalising ring so that the compression ring misses the lip of the undercut. Or remove it altogether and have the screws bear directly on the cc. 

HTH.

[SteveBz]

37 minutes ago, alacant said:

Or remove it altogether and have the screws bear directly on the cc. 

I did think about that.  Have you done it?  Are there any repercussions?  Sometimes it just makes things worse!

If I use a woodworking G-clamp (with rubber feet!), it does actually hold it in place. But then it doesn't look quite so lovely...plus the question of additional weight.

S

[alacant]

41 minutes ago, SteveBz said:

Have you done it?

Yes, everything. Apart from the clamp. In the end though, we got a GPU. Which just works. Boring eh?

Good luck anyway; it's what ap is all about!

 

 

[SteveBz]

10 minutes ago, alacant said:

we got a GPU

GPU?  Like a computer card?  What does that mean?

Steve.

[alacant]

5 minutes ago, SteveBz said:

GPU?

https://www.teleskop-express.de/shop/product_info.php/language/en/info/p6706_TS-Optics-NEWTON-Coma-Corrector-1-0x-GPU-Superflat---4-element---2--connection.html

 

[SteveBz]

9 minutes ago, alacant said:

https://www.teleskop-express.de/shop/product_info.php/language/en/info/p6706_TS-Optics-NEWTON-Coma-Corrector-1-0x-GPU-Superflat---4-element---2--connection.html

 

Wow, twice the price of an mpcciii for a smooth tube.

So I take it removing the brass compression ring didn't work.

Did you try an angle-grinder on the offending shoulder bit? A bit offensive, I know, but if the slope went the other way it would pull it in rather than push it out. Maybe a slow drill would be better.

Steve.

[alacant]

On 13/09/2020 at 15:53, SteveBz said:

for a smooth tube.

It's far more than a smooth tube;)

On 13/09/2020 at 15:53, SteveBz said:

removing the brass compression ring didn't work.

It works fine. You just need to fit new M4 screws.

 

On 13/09/2020 at 15:53, SteveBz said:

angle-grinder

Maybe a lathe rather?

Cheers

Edited September 14, 2020 by alacant