Lunt CaK module option?

[snorvich]

I am planning to order a Lunt CaK B1800 filter. The plan is to create an aperture mask for my TSA120 to create a 90mm aperture and hence an F10 focal ratio (per reading a thread here). Is it better to get a straight through or a diagonal filter? I am old and not likely able to see well enough for visual. Any comments or suggestions would be most welcome.

[swisswalter]

Hi Steve

I would go with a straight through wedge.

[astrodanco]

Get the straight through. There seems to be little to no point in getting the diagonal. You'll have one less optical surface (the diagonal mirror) in the optical path.

[snorvich]

Thanks. I hear there may be reflection issues when imaging the limb with the diagonal? Is that correct?

[sullij1]

I have noted reflections with the diagonal and posted them in the past. Not just on the limb. They are ever-present but subtle at center. I Second the straight through recommendation.

[snorvich]

Thanks. Since Lunt has both options, I will go with the straight through. I do not want to be fighting reflections.

[thesmiths]

Straight through is undoubtedly better. The optical path is also shorter so there is less chance of having problems reaching focus. The only issue is the waiting time on the straight through version is typically longer. They seem rarely to be kept in stock by distributors.

[PEterW]

Why f10? I use a b600 straight through unit in an 80mm f6.8 and the results are none too bad. Forget visual usage, go with a camera. I tried visual with a guy shill had had cataract operations and he couldn't see more than a pink purple fuzz ball that I can at half his age (or so)!

Calcium is well worh getting into though!

Cheers

PeterW

[snorvich]

Yes, I have no intention of visual with CaK and for imaging ordered the straight through. Of course, the lead time is months but that is not really a problem. (I have had cataract surgery plus I am no longer a spring chicken)