Baader ERF alternatives

[JohnM]

It's been a while since there have been any discussions about proper ERFs here.
I got away from the stage 2 mod ideas for quite some time, mostly because of the cost.
I was looking at gear accumulated and have everything needed now for my 6" f8 achro but for the prohibitively expensive D-ERF.
Have there been any recent developments for other alternatives?

[swisswalter]

Hi John

not to my knowledge if you want to make Ha recording

[robert]

Putting the ERF inside the scope allows you to use a smaller diameter which is cheaper.
Robert

[MjrTom]

I use one of these for my 100mm PST mod and they work great.

http://www.ebay.co.uk/itm/221261345569? ... 1423.l2649

The person which sells these hangs out on this forum from time to time.

Mark

[JohnM]

I know placing a smaller ERF inside the OTA is done. I'll have to think about it. Heat build up is a worry.
As long as I had everything dismantled I went back together with my old 60mm Denk/Spectrum objective.
For some reason mine has never been able to reach a good focus, so I guess I'll go back to my 40mm.
With that I can get consistent good results.

[MjrTom]

Thanks! I couldn't remember his name when I posted earlier.
I am more than happy with the quality of the ERF.
A bit cheaper than the equivalent Baader one.

Mark

[JohnM]

I can go with a 90mm Baader at somewhat below stratospheric price.
I need to do some scale drawings and figure out where it would need to be placed in the tube.

[Merlin66]

John,
keep the ERF as close to the objective as possible. Remember that 99% of the incoming energy will be reflect back towards the objective (ignoring the minimal tilt you need to prevent ghosting) and come to a focus.
There's no evidence of significant heat build up with sub diameter ERF's.

[Pablito]

Can you please tell me what is reasonable minimum size of sub ERF? I mean, what should be subERF/objective ratio to have it still safe for internal etalon.

Lunt in LS100THa and LS152Tha puts quite small ERFs. Do they use something else than Baader style ERF?

Thanks in advance for that info

[Merlin66]

Pablito,
I usually recommend that the sub ERF be >50% the clear aperture and placed as close behind the rear of the objective as possible. The reflected energy (lots) doesn't want to end up focused on the objective!
This prevents any unwanted heat build up.
Can't comment on the Lunt arrangement.....

[Cschur]

IS there any reason you just cant use Baader ND3.8 solar film? it produces a very bright image, with equal cuts across the band for IR/UV. ???

[Merlin66]

Chris,
I haven't tried it...
but I would think having a ND3.8 filtration on the Ha wavelength would not allow enough light for good results.
Normally the ERF blocks everything but provides 90-95% transmission at Ha.

[JohnM]

I tried a Baader 3.8 film once to see what the effect was.
Blocked everything including Ha.

[MapleRidge]

Hi everyone...

Here is a little thought that popped into my mind one day, and this is sort of the topic to toss it out for criticism.

Could you not make an ERF out of something like the Baader photographic film on the front of the scope and then place a photographic deep sky Ha filter ahead of the etalon assembly and have it work? Is the photographic film either too poor an optical substrate or blocks too much light? The Ha filter would cut the bandpass down, but avoid the need for a large, expensive ERF.

Yes, no, maybe...just thinking

Brian

[Merlin66]

Brian,
I think John has already answered this one.
The attenuation of the solar film is just too great....

[MapleRidge]

Hi Ken...

Ok, whack me in the head...I missed the second line.

Brian

[peter drew]

I've built several PST mods from 80mm-220mm aperture, some with full aperture ERF's and some with subdiameter internal ERF's. I found no visual difference and no evidence of heat build up with the internal ERF versions. Using a small internal ERF gives the opportunity to use a high quality unit at minimum cost. A 6" 8 would be working at 5" F10 without further optical modification, a 75mm Baader D-ERF would be adequate in this instance.

[PlaneSkies]

"I found no visual difference and no evidence of heat build up with the internal ERF versions."

Can I ask whether the case for imaging as well please?
I am current trying to decide between a full aperture ERF and sub-diameter internal ERF for a PST Stage 1 mod (thanks to Ken's great instructions :thanx: )

Sorry my first post here is a question - but I'm keener than ever to get started on the modification after seeing all the amazing images with the PST modification!

Cheers

Steve

[peter drew]

Steve. I'm not an imager so I can;t comment on this with any authority, the visual images though are as good as the visual images seen through Ha telescopes used by others for their fine processed results. The Baader D-ERF's do seem to reflect most of the heat rather than passing it further down the tube. Those using Herschell wedges manage good images with nothing between the objective and the wedge.

[marktownley]

IS there any reason you just cant use Baader ND3.8 solar film? it produces a very bright image, with equal cuts across the band for IR/UV. ???

Plumb in the maths here. If we are imaging without the ND3.8 to give us the perfect histogram - say for arguments sake this gave us a 1/10000 second exposure, the ND filters work logarithmically, so, in the above example if we put in the ND3.8 the resultant exposure to give us the same histogram would be somewhere between a 1/10th and a 1 second exposure. This is why it doesn't work.

[swisswalter]

"I found no visual difference and no evidence of heat build up with the internal ERF versions."

Can I ask whether the case for imaging as well please?
I am current trying to decide between a full aperture ERF and sub-diameter internal ERF for a PST Stage 1 mod (thanks to Ken's great instructions :thanx: )

Sorry my first post here is a question - but I'm keener than ever to get started on the modification after seeing all the amazing images with the PST modification!

Cheers

Steve

Hi Steve

welcome to that wonderful site. Have sun fun and mod