Coma-correcting eyepieces can provide a lighter, cheaper, more convenient solution that can optically outperform a coma corrector plus standard eyepiece.
Below some of the drawbacks of using standard eyepieces in a coma corrector that are remedied by coma-correcting eyepieces.
1. Size and weight
Popular coma correctors like the Televue Paracorr 2 and Explore Scientific HR are bulky devices; they are typically about 140 mm long and add around 0.5 kg at the eyepiece.
If a telescope has been optimized for small secondary size with a low profile focuser mounted as close as possible to the optical axis, the coma corrector will intrude in the secondary cage and prevent using a filter slide on the telescope.
It's worth emphasizing that well designed coma correcting eyepieces do not have to be bulkier than standard eyepieces!
To make the point, the image below compares the Televue Paracorr + Nagler 31 combo to a 28 mm 86° coma correcting eyepiece design. We superposed the 28 mm 86° lens layout on a photograph of the Televue combo.
While the compact 28 mm would weigh less than half of the 1.35 kg of the bigger combo, it produces a 15% larger true field with excellent edge correction, comfortable eye relief, and minimal distortion and curvature.
2. Cost
Televue Paracorr 2 pricing is around $550/€700 in US/EU, Explore Scientific HR around $400/€350 in US/EU, the integrated Feather Touch SIPS adds about $850/€1000 in US/EU.
Money that could have been spent on eyepieces!
3. Ease of use - Focusing
The Televue Paracorr contains a helical focuser (a.k.a. tunable top) to keep the coma corrector at the right position. When changing the eyepiece, the eyepiece has to be moved at its approximate focuser position with the tunable top. Then you can finetune with the actual focuser of the telescope.
While many observers have got used to doing this, it still is a hassle compared to normal focusing, and adds some mechanical flexibility to the optical chain.
The Feather Touch SIPS product page explains the advantages of not needing a tunable top:
“Once it is set-up, there is no need to fumble with locking screws, turning your flashlight on to check its position, etc. This saves time and hassle. Also, the assembly is more rigid and flexure is greatly reduced. The SIPS Coma Corrector totally eliminates the need to re-adjust/calibrate when switching eyepieces. You will simply adjust the focus knobs of your Feather Touch focuser for viewing. Because of this, the SIPS Coma Corrector will save you valuable observing time, less hassle and ease of use.”
Obviously, living without a coma corrector altogether provides the same benefits, and more!
4. Focus in-travel
Coma correctors require some extra focus in-travel, typically 15 mm but sometimes more (the lower the Barlow effect of the coma corrector, the larger the extra focus in-travel will be). This moves the focal plane farther from the main optical axis, requiring a bigger secondary mirror or reducing the fully illuminated field.
5. Number of optical elements - Transmission and scatter
The coma corrector inserts 4 or 5 elements in the optical path, adding to the 6 to 10 elements from premium eyepieces. Every element generates some light loss (glass transmission and surface reflection) and creates additional scatter.