Coma on a Takahashi 130 - How can this be?

[DIYman (Doug)]

Hi all. This is a bit of a rant.



I bought this Tak 130 from the dealer here in Australia (interesting experience to say the least) and was surprised by the amount of coma that is evident in images taken. A search of the net failed to find one comment that criticizes the scope's performance in that area amid all the advertising hype. Well here is one.



For the price of these instrument one would think coma would not exist to the degree it is so easily visible. Takahashi answer is to expect customers to purchase a field flattener at an exorbitant cost instead of improving their optics. This I had to do from overseas much to my chagrin.



I found I have less coma in my images taken using my old newtonian with a self ground 8 inch - f6.7 mirror. (Hey! perhaps I should get a job with Takahashi as a optical consultant.)



Not likely would I buy another Tak product based on what I now know. I wonder how Astro Physics and other good quality scopes stack up regarding coma. If you are a refractor owner I would like to hear your views.

[SteveInNZ]

Which 130 ? Epsilon or refractor ?


Steve

[JA]

Originally Posted by SteveInNZ

Which 130 ? Epsilon or refractor ?


Steve

Probably refractor: refer the last sentence of his post.

Best
JA

[gregbradley]

Do you have any photos taken so we can see the coma?

Flatteners have a spacing they need to be set to in order to correct for coma to the corners.

How were you measuring the coma? Was it imaging or visual?

What size focuser? What camera?

As you can see there are several variables here.

Even though its a refractor its possible that it is out of collimation. TOA optics I believe are quite exact in their spacing and collimation.

Greg.

[Atmos (Colin)]

With a triplet refractor the level of coma is, for the most part, determined by focal length. This is because the amount of field curvature is largely determined by the focal length. The longer the focal length the more field curvature. So, a Tak 130mm F/7.7 will have less field curvature than an AP 130mm F/6.

The TOA-130 has excellent chromatic correction and should have very little spherical aberration which is all you can expect from from a well built triplet. If you’re doing imaging with a doublet or triplet, you MUST use a flattener to correct for field curvature.

[casstony]

I assume you're seeing aberration in the outer field. Provided the on-axis view/star test is good there's probably nothing wrong with the scope. It's normal to add corrective optics for imaging unless you buy an astrograph like the FSQ106.

[CeratodusDuck (George)]

It is a well known fact that EVERY refractor apart from specially built Petzvals will need a flattener or reducer/flattener for astrophotography. Tak, AP, TEC - they all require it. It’s the nature of the design. It is mentioned in all their manuals. Even for visual many suggest a flattener (though most observe central objects, not peripheral)

If you want flat field coma free out of the box you buy a petzval quadruplet like the Tak FSQ 85, 106 or 130 or others.

I hate to say it, but it seems you did not do research prior to purchase. It’s 101 research that stops a lot of these misinformed posts. Apologies if that offends but it is the truth.

[Peter Ward]

Likely field curvature, not coma. There is nothing defective or odd about that.

Some images through the 'scope would help....

[CeratodusDuck (George)]

Yes, FC not coma. Got drawn in

[DIYman (Doug)]

Thanks everyone for you input - much appreciated. I have attached a small jpg showing the extent of the coma. BTW I used a Canon 6D.

[Atmos (Colin)]

Yeah, that looks entirely normal for a FX sensor on a refractor without a flattener.

[Slawomir (Suavi)]

Looks very good to me - apparently no tilt which is great. Just add a flattener at the correct spacing and you should have a nicely corrected imaging circle.

[gregbradley]

I would say there very definitely is tilt. The top 2 corners are a lot worse than the bottom 2.

What size focuser is this? Tak scopes need 4 inch focuser to correct for a full frame sensor.

Greg.

[Peter Ward]

Not Coma. Not really helpful to keep calling it that....

Field curvature plain and simple.

Nothing too tragic either, as CCDinspector indicates the system is well centered.

If you get the Taka field corrector (they have a very good one) the focus problem will go away.

[DIYman (Doug)]

Originally Posted by gregbradley

I would say there very definitely is tilt. The top 2 corners are a lot worse than the bottom 2.

What size focuser is this? Tak scopes need 4 inch focuser to correct for a full frame sensor.

Greg.

Greg.
I have a 4 inch focuser.

Cheers

[Bart]

If you are using the 4 inch focuser then I think you can use the TOA-67 flattener or the TOA-645 reducer to correct the field curvature.

[Paul Haese]

Definitely field curvature there. You will need a field flattener to correct that problem. You do have a field flattener on the scope before the camera don't you? You can't just put a camera on the back of the scope without a field flattener. Putting a flattener on it will remove this problem.

[DIYman (Doug)]

Thanks all for your input and advice. It looks like the bank account will take another hit. Clear skies.