To ERF or not what really is correct ?

[Ewan]

I know this topic has been touched on a few times in this forum but please bear with me as there has been so many posts giving conflicting opinions I am really not sure who is correct.

I have a Quark Chromosphere I use with my 80mm F6 APO, at present I use a Baader 2" UV/IR as an internal ERF.
The UV filter sits 35mm inside the rear of the focuser inside a threaded WO 2" extension tube, the Quark then goes into this, so far so good & no issues that I know about.

Now I am looking at buying an Altair Astro 152mm F5.9 Achromat for solar & other use's, I thought at first I would need a front mounted ERF for this setup but having seen various comments is this strictly true ?

Recently I was advised by Rupert from Astrograph that I could use a 2" Ha 35nm filter as an internal ERF in the same place & position as my UV filter I have been using.
I was a little surprised by this as that filter only costs £109.00 as opposed to approx £900 for a front mounted ERF.

1) How does a 50mm ERF on the focuser end compare to a front mounted ERF for the 152mm ?
2) Baader 35nm v's the 2 above ?
3) I presume as the 152 is a lot more aperture thus brighter I will have no problems having to use high Gamma or any Gain using any of the above filters ?
4) Why does Daystar insist a front mounted ERF IS needed for 120mm & over if you can use these 'other' filters ?

The thing is after searching the forum for post's on this subject I find some older replies say 'You will need the front mounted ERF' then now it would appear we can use a cheaper option, is this because of new advances in filter design ?

You could reply & just say 'well save up for the ERF then', the funds are not an issue but as I see it spending £109.00 instead of £900 odd makes good sense ONLY if it is 100% safe for my eyesight & equipment.

I would like to see what people's own views are & how they came to their decision on what they bought.
This is an important issue not just for me but for anyone who does or is thinking of buying into solar viewing / imaging, the Quark is a fantastic piece of equipment & is very tempting for the cheap cost but not worth your eyesight.

Thanks for taking the time to read this.

[marktownley]

Interesting questions. Simple answer is both are correct. However which suits you best is going to depend on what exactly you are trying to achieve.

The 'ERF' on the Quark is the little silver filter approximately 12mm diameter when you look at the end of it. Regardless of what sized scope you use the energy of the sun via the objective is concentrated into a spot size of less than 12mm, now common sense tells us the larger the scope the more energy this little ERF has to deal with. The little filter is going to be specified to operate at a 'maximum' temperature, as you get to this temperature a couple of things could happen - firstly the lifetime of the filter will reduce as the coatings fail quicker, secondly the filter could catastrophically fail and crack. This is why Daystar recommend using a UV/IR cut ahead of the quark on scopes above a certain size to reduce the effects of thermal loading on the filter system, and then hence an external ERF on scopes above a larger diameter.

Now consider what an internal / sub aperture ERF actually does - it is reflecting the energy of the wavelengths we dont want back out of the OTA - a UV/IR cut is going to be reflecting wavelengths below about 400nm and above about 700nm, which means anything in between is still reaching that lil' ERF on the Quark, which then reflects anything other than 656nm back out of the tube. The 35nm Baader is a better option than the UV/IR cut filter as this reflects more energy; everything other than a 35nm peak of light centred on 656nm, so, with larger scopes means there is much less thermal load hitting the quark, especially important say with a 152mm scope for example. The 2" 35nm Baader is an excellent very cost effective way of using the Quark with a larger scope.

So why then is there conflicting views with some people saying an external full aperture ERF is the best solution? It all depends on what you are trying to achieve...

If you are using the quark for high resolution closeup imaging then generally your aim is to produce a picture of maximum clarity and detail for what your scope (of whatever aperture) is capable of producing. To do this we use a number of techniques to maximise our chances of achieving this; firstly we may image at a time of day when the seeing is at it's best, say earlier in the morning. Secondly we use 'lucky imaging' when we take our pictures; we take a large number of individual images, 100s-1000s, and then we let stacking software pick out the best / sharpest from this initial run of images. The software then stacks these, effectively giving us an image with a higher signal to noise ratio. We then use post processing techniques to enhance this image still further, maximising contrast, sharpening etc. Now, if you're an obsessive imager like me (or others on this forum - you know who you are!) then you're always keen to find ways to push the boundaries of your image to get the best quality you can. In addition to the techniques I have stated above there is one more, and that is to use a full aperture external ERF on the objective of your scope.

What is the reason for doing this and why does it work? Well, all that energy we talked about before passing through the objective whilst also 'heating' the silver filter on the quark (or the UV/IR cut, or the 35nm Baader) also heats the column of air in the telescope tube that it passes through, and this heated air causes turbulence in the tube and this turbulence manifests itself as worse seeing. To stop this the simplest way is to keep all of this 'energy' out of the telescope tube and use an external ERF. A night time astronomy analogy is that people let their scopes to cool down to an ambient temperature as this offers the best views.

Now like mentioned before, for an exacting imager the goal is to get the best final image possible, constructed from sub frames with only the best ones used from a few milli seconds worth of time. Visual is a totally different thing. Daytime seeing means that other from a few fleeting seconds at a time you are rarely getting the true resolution your scope is capable of, more so the larger the scope. What you are getting is the benefit of a larger image scale - it can sustain a higher magnification compared to a smaller scope. Now, if you live somewhere in the world where you have near perfect daytime seeing conditions (think where the worlds solar observatories are located) then visually here a full aperture ERF would likely benefit you, however, if like me and you have jet stream seeing visually it will not. However, like have alluded if you are an obsessive demanding imager and you are looking for those 50 perfect sub frames in an imaging run then a full aperture ERF will help you achieve this. If you are an occasional imager who is priamrily visual then the 35nm Ha will more than suffice. But, for perfection there is a cost, a large Baader ERF is optically flat, and while it might just look like a 'coated piece of glass' there has gone a lot of work to get it to work the way it is designed.

So to answer your questions, I reckon the blurb above covers 1 & 2. Regards question 3, the 152mm f5.9 may be a bit brighter than the 80mm f6, but it is the focal ratio here that is key to brightness, not aperture, and as they are the same focal ratio then brightness should be the same. What will be different is image scale and resolution - you will see a smaller part of the sun at a larger image scale and finer resolution with the 152 compared to the 80. Yes, you could increase the magnification with the 80mm to get the same image scale as the 152 but the image will be much dimmer and softer.

Regards question 4, Daystar have their own UV/IR cut filter (although I also thing they recommend the IDAS brand too?) which they recommend for scopes up to a certain size, then above a certain size they recommend an external ERF. I guess they want people to buy their own filters rather than other brands if they can help it.

Hope this helps.

Mark

[DJD]

Excellent reply Mark.
Best wishes,
David

[FRAZ]

Thanks for original question and spectacular response

Fraz.

[grimble_cornet]

The other problem with a full aperture ERF for a 152/900 scope is actually getting hold of one
Mine has been on order and paid for since April with two revised dates so far for 'expected delivery' from Baader - currently 'early August' but I'll believe it when I have it in my hand

[Ewan]

Sorry to hear that Mike, I think if I go full ERF I would like not to wait that long.

Mark, I will go with a full ERF as I really do want the best from my gear but most of all its the safest option all round.

Thanks for an in depth answer which was a very good explanation.

Ewan

[Derek Klepp]

Great topic.Mike Valery is well versed on ERFs or larger aperture.

[Sunwatcher]

Stepping out of the shadows for a moment to compliment this thread! There is a light at the end of the my tunnel (telescope!) - my observatory will be functional again in a couple months after more than a years hiatus. Sure would also like to see such an in-depth thread covering the latest method of remotely operating a telescope. Thank You

[Astrograph]

There are some caveats here.

When you put something in front of the main objective you potentially affect its optical performance. Wavy old Astrosolar film does very little because its so thin. A lump of glass like an ERF has to be very flat. Visually a normal front absorption type filter has a surface accuracy of 1/4 wave. Using one compared to an internal ERF like a UV/IR cut or Ha filter is visibly worse even though the internal filter is also 1/4. This is because the image is affected less the closer it is to the focal plane. The Baader D-ERF is 1/10 and visibly as good as an internal filter.

Ultimately its about money.

[fjabet]

@Mark : the 12mm front filter in the Quark is the BF with the bandpass, not an ERF

[Ewan]

There are some caveats here.

When you put something in front of the main objective you potentially affect its optical performance. Wavy old Astrosolar film does very little because its so thin. A lump of glass like an ERF has to be very flat. Visually a normal front absorption type filter has a surface accuracy of 1/4 wave. Using one compared to an internal ERF like a UV/IR cut or Ha filter is visibly worse even though the internal filter is also 1/4. This is because the image is affected less the closer it is to the focal plane. The Baader D-ERF is 1/10 and visibly as good as an internal filter.

Ultimately its about money.

'The Baader D-ERF is 1/10 and visibly as good as an internal filter.' ok and photographically ?

So are you saying Rupert that an internal Ha 35nm filter will still give very good images & not allow any damage to either the front objective in a 152mm Achro, the Quark or my eyesight ?

[Astrograph]

Hi Ewan

ERF's of any type are not actually there to protect your eyes, they are there to protect the filter. An Ha filter is only going to pass the Ha band. Basically red light. Ironically, etalons don't like heat and all types of filter do a better job when they have less light to filter. This is why double stacking works.

The light that does the damage is UV because its high energy. A focused beam of that from a telescope will damage the etalon. A blocking filter is a basically another type of ERF. It also only lets Ha light pass but its not as narrowband.

So you eyes are not under any threat. They are only going to be exposed to a very narrow band of Ha light which is so faint it does not appear that bright so no worries about that either.

Your lens is only subjected to unfocused light. This has no warmth. You feel the warmth of the sun on your skin because your skin is sensitive to IR light. A lens does not care. Leave it looking at the sun all day. It will not get warm.

Inside the tube the light is converging. It is still cold. Only if you interrupt the concentrated beam will you experience the energy in it. The beam itself will not radiate heat.

All ERF's have some limitations. A front mounted ERF must be optically very good or it will reduce performance. An internal ERF is being subjected to a converging beam therefore it does not perform as well on its edge. This is because filters work best with light which is perpendicular to their surface. There is always a compromise somewhere. In reality this means it probably leaks a bit more light than its 35nm wide spec suggests, but we don't really care because it does block all the high energy light. The important thing is that the system is safe.

[Ewan]

Hi Ewan

ERF's of any type are not actually there to protect your eyes, they are there to protect the filter. An Ha filter is only going to pass the Ha band. Basically red light. Ironically, etalons don't like heat and all types of filter do a better job when they have less light to filter. This is why double stacking works.

The light that does the damage is UV because its high energy. A focused beam of that from a telescope will damage the etalon. A blocking filter is a basically another type of ERF. It also only lets Ha light pass but its not as narrowband.

So you eyes are not under any threat. They are only going to be exposed to a very narrow band of Ha light which is so faint it does not appear that bright so no worries about that either.

Your lens is only subjected to unfocused light. This has no warmth. You feel the warmth of the sun on your skin because your skin is sensitive to IR light. A lens does not care. Leave it looking at the sun all day. It will not get warm.

Inside the tube the light is converging. It is still cold. Only if you interrupt the concentrated beam will you experience the energy in it. The beam itself will not radiate heat.

All ERF's have some limitations. A front mounted ERF must be optically very good or it will reduce performance. An internal ERF is being subjected to a converging beam therefore it does not perform as well on its edge. This is because filters work best with light which is perpendicular to their surface. There is always a compromise somewhere. In reality this means it probably leaks a bit more light than its 35nm wide spec suggests, but we don't really care because it does block all the high energy light. The important thing is that the system is safe.

I understood a lot more after Marks reply earlier Rupert.

I just have to find out now how much the full ERF will be & where to get one from or look at a slightly stopped down version.
Ideally I would like one that just slips over the dewshield & tightens with the 3 thumb screws but am I right in thinking that part is called the cell & it comes seperate as I have seen some small print saying 'cell not supplied' ?

[Astrograph]

PM sent about this

[GUSdlp]

Hi Ewan

ERF's of any type are not actually there to protect your eyes, they are there to protect the filter. An Ha filter is only going to pass the Ha band. Basically red light. Ironically, etalons don't like heat and all types of filter do a better job when they have less light to filter. This is why double stacking works.

The light that does the damage is UV because its high energy. A focused beam of that from a telescope will damage the etalon. A blocking filter is a basically another type of ERF. It also only lets Ha light pass but its not as narrowband.

So you eyes are not under any threat. They are only going to be exposed to a very narrow band of Ha light which is so faint it does not appear that bright so no worries about that either.

Your lens is only subjected to unfocused light. This has no warmth. You feel the warmth of the sun on your skin because your skin is sensitive to IR light. A lens does not care. Leave it looking at the sun all day. It will not get warm.

Inside the tube the light is converging. It is still cold. Only if you interrupt the concentrated beam will you experience the energy in it. The beam itself will not radiate heat.

All ERF's have some limitations. A front mounted ERF must be optically very good or it will reduce performance. An internal ERF is being subjected to a converging beam therefore it does not perform as well on its edge. This is because filters work best with light which is perpendicular to their surface. There is always a compromise somewhere. In reality this means it probably leaks a bit more light than its 35nm wide spec suggests, but we don't really care because it does block all the high energy light. The important thing is that the system is safe.

Hi Astrograph,

A couple of questions, I hope they're not too many!

You say erfs are meant to protect etalons so I'm confused, why in images of pst parts the etalon comes before the uv filter and the blocking filter and is therefore directly exposed to the beam? Also, in a double stack assembly, what is being double stacked, the etalon or the erfs? (I take it it's the erfs)

Which front mounted ERF do you recommend and why, daystar or baader d-erf?

You mentioned in another post that the baader h-alpha 35nm ccd filter is a better option than the baader uv-ir cut because it's cooler, so, expense aside, would the Baader Narrowband Enforced 3.5nm H-Alpha Filter w/ LPFC ($537 on optcorp.com 2") or the baader h-alpha 7nm narrowband be even better options or is this a wrong assumption?

thanks

[Astrograph]

Hi

Sorry for the delay in replying.

An ERF is there to protect the etalon but whether that needs doing is also effected by the size of the objective in the main scope.

The blocking filter quoted in solar systems is basically an ERF that only allows the target wavelength to pass. These are usually dielectric mirror types so reflect the UV and IR wavelengths. Although narrow by filter standards they still have a relatively wide bandpass at 10A. These reject enough of the high energy light to allow the etalon filter to reject the rest without any issues.

As an example if you look at a Daystar filter you will see the front of it is like a mirror. That is the initial blocking part. If using a Quark on something like a 60mm scope, this filter on its own is enough so no additional ERF filter is needed. With apertures of 80mm+ though a pre filter is needed.

A PST will have something before the etalon but as it only has a 40mm aperture it can be fairly basic as it is not doing much.

As far as H-Alpha vs UV/IR cuts go. The Baader UV/IR cut does not seem to be that good. We used them initially but while experimenting with filters behind them discovered it was letting something pass it shouldn't. Basically we had a polariser behind one to see the effect it would make on contrast. It appeared to do nothing but when checked the polariser has been scorched and was scrap! That was on a 100mm scope. After that we used Daystars own and also Astronomiks. These also seem to feel cooler if you put you hand in the beam behind the filter. Unlike the Baader they also have a window at the calcium wavelengths.

The 35nm H-Alpha filter seems a very good ERF. On a 6" scope the beam is cool. I understand the dielectric coating is the same as used on the D-ERF but I have not been able to confirm that. Using a 3.5nm Ha filter adds only cost and no benefit because the block and etalon that follow will take out any residual high energy wavelengths.

The internal filter should not be thought of as a complete replacement for a traditional front ERF. It does the job and is much, much cheaper but has practical limits on the size of the objective (I think 6" is the limit). While visually to me it appears better than a basic front mounted glass ERF it is not as good as a 1/10 wave D-ERF or one of Val's. This is because the front ERF is encountering light perpendicular to the filter where the internal filter encounters a converging beam with light at an incident angle. There is therefore some aberration introduced. Its not enough to stop my enjoyment but if your seeking perfection, there is no short cut to the cost of a front mounted filter.

[Valery]

To be as brief as possible:

1. If one does looking for the best possible performance then he need a good full size
external ERF filter.

It is very difficult to imagine that one bough a Quark, larger scope - 152mm, larger mount and agree for compromises!

2. I got several (6 or 7) full size ERF filters for 6" refractors. They are 1/16 - 1/20 wave front filters 13mm thickness.

They are immediately available at $1300 + shipping.

To see how such filters works one can search here for my images taken with 150mm refractor. All will be clear. Somewhen last year I published comparison pictures taken with Quark through full size ERF and with sub aperture 35mm Baader. Full size ERF wins clearly resolution and clarity wise.


Valery.

[GuillermoBarrancos]

Isn't it clearly specified in the specs of the Quark, that you can only use an internal IR/UV rejection filter up to an aperture of 120mm, but anything larger will require a front mounted ERF?

I have 120mm APO and use a 2 inch Baader UV/IR rejection filter in front of my diagonal and on warmer days I clearly noticed that quite some heat (thermal) still builds up around the front lens of my scope, making me consider saving up for a front mounted ERF in future to reduce thermal stress, to get better imaging result.

I would not dare using a 152mm aperture without front mounted ERF (especially not visually), but that's just me.

I guess when purely imaging, you might get away with it when using very short sessions, but I still wouldn't risk such a scope on it. Even a 152mm achromat isn't exactly pocket change.

[Astrograph]

Your front objective is perfectly safe. All light passing through it is unfocused.

Daystars recommendations are conservative and as I stated, I do not completely trust the Baader UV/IR. I would not use a UV/IR over 100mm personally but a 35nm Ha is fine up to 6". Used them for hours like that and with ND filters behind them.

Using an internal filter like a UV/IR or Ha filter means the high energy light is being reflected back out of the tube. As it was converging on the way to the filter it continues to converge on the way out. It is therefore recommended to place the filter where it will encounter the widest beam and also consider where it is likely to converge to focus. The last thing you want is it focusing inside your lens cell!

If you place the filter in the focuser this normally best. If, using a Quark, your scope focuses with just a diagonal then obviously put the filter there. If you need an extension in front of the diagonal, put it on that.

A front ERF will avoid any doubt, BUT the basic glass ones sold are only normally 1/4 wave and they definitely have an adverse effect on optical quality. A D-ERF or one of Val's is more expensive but ultimately much better.

[GuillermoBarrancos]

I have mine screwed in front of the diagonal, so it's in the focuser tube. Though, I still notice with longer sessions that there is quite some thermal heat still building up around the front lens. It's not like it's boiling, so I am not afraid for my scope, but the thermal heat does start to affect image quality and usually an indication that I just have to stop and say it's enough for the day.