Full aperture D-ERF vs sub-aperture D-ERF for TS 152mm f/5.9

[Petrus]

I would like to ask more seasoned imagers about their thoughts and experience about choosing full aperture D-ERF for TS Optics 152mm f/5.9 scope or sub-aperture D-ERF installed at another side of the focuser (inside the tube) similar to the option Tecnosky offers as a product (Quark Chromosphere, Tecnosky 152mm with pre-installed 75mm D-ERF).

I would choose between Baader 160mm (full aperture) or 75mm D-ERF (sub-aperture).

I have so far experienced using 115mm aperture with Quark Chromosphere, which gave me bit of skeptical thoughts about will my local seeing be too often good enough for using an even bigger aperture. I don't have permanent setup, so everything must be taken out and it every time for imaging.

All thoughts are welcome!

Photo from Tecnosky website:

[MalVeauX]

Hi,

If you plan on using the whole 150mm aperture, go full aperture. Especially with short focal length. The disc image is tiny with that focal length and so all the energy of the 150mm is concentrated into a smaller area, it will be very hot in that tiny area, so a sub-aperture D-ERF is less effective there. It would be easier if the focal length was much longer so that the full disc image was a larger area and so the concentrated heat was covering that larger area (less heat per area), allowing a smaller D-ERF to function better. So if using F6, I would absolutely suggest full aperture instead. Look into Altair Astro's full aperture, you can get dual or triband perhaps. They're excellent optically too (flat).

Very best,

[Petrus]

Thank you for your detailed answer Marty!

[marktownley]

I have gone with a 160mm full aperture Baader with my recently purchased 152/5.9 frac.

[Petrus]

Nice to hear you are getting experience with this setup too! I decided to go with full aperture D-ERF too when it becomes possible.

[pawlik]

I used Baader CCD-R as a sub ERF with my TS 152/900 and it worked fine, but I always kept in mind that was only 2" filter placed on concentrated light beam, especially on tracking mount. Now i use full aperture Baader D-erf so no worries about safety, heating etc.

[pupak]

The inner DERF needs to be positioned so that the diameter of the sun disk is close to the diameter of the filter, i.e. close enough to the objective and tilt it slightly. If the internal DERF is close to the eyepiece, the solar disk is in a small area with a large heat intensity. I know several people who have been running an internal ERF Lunt 2" in a suitable location for several years without the slightest problem.
But I am not one of them.

[marktownley]

Any etalon that uses a soft coated (blocking) filter in a large aperture, fast focal ratio system with a sub aperture ERF will suffer premature failure. Fact.

I'm talking about the soft coated filters fsiling here.

[pupak]

Any etalon that uses a soft coated (blocking) filter in a large aperture, fast focal ratio system with a sub aperture ERF will suffer premature failure. Fact.

I'm talking about the soft coated filters fsiling here.

How early is too early? The manufacturer of the filter does not specify the lifetime, so it is difficult to define what is premature.
The older LUNT ERF 50mm has been working in the AR150/F8 for at least 4 years. It is regularly visually inspected and shows no signs of damage. As a second protection, a 2" UV/IR cut is placed in front of the telecentric element.
I thought about it myself, but ended up buying the Baader DERF 160 and 180mm. However, I also have a 35nm H-a filter in the set, which has proven very useful for me by increasing the contrast of the image. It obviously filters out any ambient reflections well.

[MalVeauX]

Hi,

Realistically all that is needed for virtually any aperture that is portable is an optically flat film or optically flat glass that is IR reflective and then use an internal sub-aperture selective filter (such as a red or h-alpha centered imaging class 2" filter). The bulk of the thermal issues are handled with just the IR removal itself. UV barely is even part of this, not even worth worrying about. The rest of visible spectrum carries significant thermal energy, but it's manageable with just removing IR really and the selective sub-aperture filter will help with the remainder of visible spectrum.

I imagine a large plate of optically flat glass or film with IR blocking coatings is cheaper to produce/buy than a full on D-ERF with H-alpha and/or other bands coated for. It would greatly simplify the DERF question and work for all frequencies under IR.

Very best,

[Dennis]

Red glass full aperture ERFs (Daystar / Lunt) are simple but cost the same as a full aperture d-erf because the figuring of the glass.
The "bulk" energy is not only in ir but also in the visible spectrum.
With internal secondary filtration you just bring a good part of energy into the scope again, not much gained.
Spending so much money for a h-alpha equipment and then saving on internal seeing improving equipment is kind of like using a takahashi apo with a standard china diagonal / barlow.
There is no cheap solution when it comes to this unless somebody invents D-ERF film.

[OlegLviv]

I bought TS 150/5.9 a few mounth ago and am happy and bought full aperture Baader Erf 160mm and the parasitic rays and various bad reflections disappeared!

[Ivan_V]

I also bought D-ERF 160mm from Baader for my TS152 f5.9 several years ago. It works very well and absolutely safety.

[Petrus]

Nice to see these! I wish things go well and I have the opportunity to get this full aperture D-ERF during this season.

[Rusted]

At the risk of repetition: D-ERFs still pass some heat energy. Particularly with larger apertures.

Do not put your eye where you cannot hold your hand at focus!

[Valery]

At the risk of repetition: D-ERFs still pass some heat energy. Particularly with larger apertures.

Do not put your eye where you cannot hold your hand at focus!

Even if one can hold a hand, do not put your eyes there!

[marktownley]

Here's mine

2023-04-09_02-51-56 by Mark Townley, on Flickr