If you're talking about a Daystar Quark for the etalon and if you mean the Baader 7nm HA filter and if all the elements in your scope are air spaced (no Petzval or oil spaced elements, I don't think they're commonly used in achros), you are close. The only problem, as pointed out to me by icecreamcat, is that the Baader 7nm HA filter does not adequately reject UV/IR, whereas their 35nm HA filter does. Baader's UV/IR filter will work as well.
From other threads,
1. "I spoke to John at Televue today and the answer is "No [front ERF required]". Apparently they have used the Daystar Quark with 2" IR/UV block filter with no issues on the NP127. The reason being that the rear elements are air spaced and 5 inches away from the focal point so all that heat reflecting through is of no issue to the telescope itself. He said that they had it out imaging for at least 4 hours with no ill effects." -cloud cover
2. "Hi Marktownley, I e-mailed DayStar about reducing the ES 5" Apo to 4" and then putting a 1.25"
Baader UV/IR filter on the end of the diagonal with this was acceptable. The response by DayStars
representative Tiffany is below.
Thanks for your note. We have been discussing these ES triplet scopes lately.
We were talking with Greg Bragg and Scott Roberts at the Winter Star Party about them specifically.
The ES 127 triplet does not use oil in its APO lens spacing.
With just air spacing, there is no risk to any of your telescope's components to gain heat or degrade under the UV light of the sun.
Greg and Scott at Explore Scientific use their Quark regularly on these telescopes with the rear mount UV/ IR cut filter alone without any heating issues.
So I would actually suggest there is no reason NOT to use the UV/IR cut filter on your ES 5" triplet.
We would worry about you using any RG610 colored glass after the objective.
Colored glass absorbs energy, where the UV IR cut filter is reflective.
So a concentrated beam will absorb all that heat and expand, breaking.
Just a UV/IR cut filter before your diagonal should be fine.
If you want to stop down, you can, but it is not necessary.
Tiffany" -De Lorme
3. "I have been use a Quark and the IR/UV cut filter with my Williams Optics 132mm with no problem.
I setup with a 2" extension , 2" diagonal , UV / IR filter, Quark and Denk Binos. I have to have the extension to reach focus.
I mentioned this somewhere else here. I bought my Quark at Heart of America Star Party last summer . Daystar was there demonstrating the Quark and I was impressed. I spoke with Jen Winters , Daystar owner, in length about he scope I planed to use the Quark with and she insisted that the IR / UV filter would be ok on the 5" scope , no full appature EGR filter needed.
My scope has an Air Spaced Triplet Apochromat lens. I have never noticed any heat build up anywhere through the system.
I don't use a tracking mount, I have a Disc Mount 6 , don't think there would be a problem with a tracking mount either." -bigdob24
To my understanding, there is not a safety issue, but there may be an image quality issue for imaging, due to the potential for thermal air currents in the OTA.