H'alpha - double pass module

sullij1 · Post by **sullij1** » Thu Mar 29, 2012 3:59 pm

Your offer is generous; however, I am inclined to make no commitment to buy at this time as I am not convinced that this is the least expensive route. Please see Neo's post # 4138 in this thread.

I think I may pursue a route similar to this one. Link to thread:

http://solarchat.natca.net/index.php/en ... -equipment

Catalin Fus · Post by **Catalin Fus** » Thu Mar 29, 2012 5:22 pm

There is no problem!

When I'll see that the project can be completed, I guess I can ask for a final answer from each person that showed interest.

Cata

colinsk · Post by **colinsk** » Thu Mar 29, 2012 10:19 pm

"IMHO, as I mentioned before, the filters should be in a parallel collimated beam and a re-imaging lens would be required...... "

Ken - The double pass only has one filter, or are you calling the KG3 a filter that s/b in the collimated beam?

Vanter, Adding a 1/4 wave plate before the mirror and changing the beam splitter to a polarizing cube will give you much less problems with back reflections. The splitter would need to be AR coated. Then you take the other half of the light and put it in a CaK filter and EP...

Catalin Fus · Post by **Catalin Fus** » Sat Mar 31, 2012 12:44 pm

Colin, you just put my brain to work!! Good idea!!
Has any of you built such a CaK scope based on available filters like Omega one?

Cata

sullij1 · Post by **sullij1** » Sun Apr 08, 2012 7:58 am

Just to keep things going and for my own edification; I am modifying an old 90mm Objective, 1000mm/F11 scope to use the Omega filters. I will incorporate Groskie, Smie and Lugli ideas into the design. More like Luglis. I am still debating on the front ERF. I was going to use Neo’s ERF idea and am waiting on the filters to arrive. Been thinking, (Dangerous) I have some Surplus Shed yellow 5mm thick filters. Could one use window Solar film to back the yellow filters to create an ERF? Have read that the coatings are not really necessary and have recently been being applied (Have read Thousand Oaks system was not a coated filter http://www.thousandoaksoptical.com/halpha.html). Still want to incorporate some form of reflective. I am supplying a link to the films to show their properties. So my questions are:

Will this work?

If maybe so, what film would work? a lighter film with a higher light trough put and low IR through put or low light throughput and low IR throughput?
Link:
http://www.gilafilms.com/pdf/Residentia ... mSpecs.pdf

colinsk · Post by **colinsk** » Mon Apr 09, 2012 2:38 am

Anytime you block all incoming light you start to worry about the H-alpha signal getting too dim.

sullij1 · Post by **sullij1** » Mon Apr 09, 2012 3:47 pm

Thanks Colin. I have several cheap UV filters that can be covered in various films that I may experiment with. I will have the KG3 other filters in place by that time. So things will be acceptably safe.

Catalin Fus · Post by **Catalin Fus** » Fri Apr 13, 2012 5:19 am

Hi,

I have been silent in the past time (too much work and a trip for Easter back home in Romania...trip which I'm still into) but I have some answers regarding the ERFs.
As we speak, the only issue is the UV-IR coating to be applied after polish. The options are :

1 - coater in Poland - maximum raw diameter remains ~ 155 - 160mm, price wise, slim chances to stay in the target with less than 250E/ piece (still waiting for final offer but preliminary discussion's conclusion is that price/piece is higher than expected)
2 - coater in Romania - maximum raw diameter is 143mm, price wise, surely in target, less than 250E/piece.

Those of you who are really interested, please give me a feedback regarding the above options..

Thanks,
Cata

Post by **Merlin66** » Sat Apr 14, 2012 8:55 am

Cata,
IMHO to be realistic... the thought of a 160mm ERF is exciting, but.....
The number of observers who would have the funds and equipment to use it would only be <10% of the already small number of Ha observers...
The 143mm would allow the PST mod to a 150/1200 scope and operate at 120mm clear aperture...that I think, is more in line with the majority of users.
What do the others think??

Catalin Fus · Post by **Catalin Fus** » Sat May 19, 2012 10:02 am

Now, to get back to the topic as the ERF issue has been resolved I will like to ask if anybody around here has the ebay 0.15nm filters at home and is willing to share a photo of the filters and of the transmission graph received with them. The highest the resolution, especially for the graph, the better.

After reading some internet infos and talking to an optician friend, it seems that if a pair of such filters is used in double pass configuration, the equivalent will be like 4 filters stack -> 0.5Angstrom, which sounds good.

My friend also has finished the design for a telecentric lens that can be used with 650mm focal length refractors. If anybody is interested in such a telecentric, please write me as I have already ordered the lenses and maybe more can be done if that's the case.

Also, I was thinking of a system with internal tilt filter, just as proposed by Bob Yoesle here, with some SM-xx etalon put into a parallel light but with everything kept smaller and using an SM40 with telecentric lens.
I'll update today/tomorrow here this topic with the design.

Regards,
Cata

colinsk · Post by **colinsk** » Sun May 20, 2012 12:38 am

Cata,

Can you publish the design? I am more interested in a f/30 telecentric for a 127mm objective of any f number. I am still reverse engineering telecentric designs because I can not afford modern optic software so I have to do all of the aberration calculations by hand.

Catalin Fus · Post by **Catalin Fus** » Sun May 20, 2012 6:55 am

Colin, here you are.

I hope everything is understood! The FL of the last lens is 80mm and I guess it can be replaced with something to suit the needs.

Material for all is BK7.

If you need something special, please let me know your telescope and I'll gladly ask the optician if he can make a design.

Regards
Cata

Attached files Telecentric-80mmbf.pdf (31.5 KB)

p_zetner · Post by **p_zetner** » Sun May 20, 2012 6:07 pm

Hi Cata.

I've just started playing with the Omega Optical CaK pair. The attached photo was taken with a Stellarvue SV80S apo (overkill!) 80mm f6 refractor. I used a Chroma 380nmCWL 40nm bandpass (32mm dia) as an "ERF" ($106.00). Additional optics used: 2 Edmund Optics achromats - one neg 150mmfl as collimator and one pos 160mmfl as refocuser separated by 57mm to give telecentric operation for my system. Omega filter pair mounted behind the Edmund optics pair. The overall effective focal length was 512mm. Imaging with DMK camera (1/3in sensor).

Contrast on the image is not too great - possibly the Chroma filter or lack of tracking. The image is from a single video frame, no flat field, no tracking and exposure time was 1/3333 sec. per frame. I also realize now that telecentric collimator / refocuser is not what I should use for a fast system! Also, I was hoping for a full disk view and am configuring a new system for this based on Stellarvue 60mm finder scope.

Cheers.
Peter.

PS. As ERF, I also use a 4" x 4" Schneider UV-IR "True Cut" filter in front of the objective!

p_zetner · Post by **p_zetner** » Sun May 20, 2012 6:34 pm

Hi Colin.

The design of a telecentric lens is not that complicated. The attached notes (Gene Baraff) give the idea. I think Bob Yoesle referred to this document before but the link he gave is dead. The working equation is 15(b). For lenses L2 and L3 I use Edmund Optics or Surplus Shed achromats.

Cheers.
Peter.

Attached files DesignTelecentricLenswithEtalon.pdf (323.9 KB)

Catalin Fus · Post by **Catalin Fus** » Sun May 20, 2012 6:43 pm

Thanks Peter for your reply!! That picture is not looking bad...

Do you have CaK filter 'set' in the stack? Which one are those?
May I ask, the filters are behind the telecentric lens assembly or between the lenses of it?

As far as I know, the Cak doesn't need to operate in parallel light, as the H-alpha needs...but I'm no expert in this.

Regards,
Cata

p_zetner · Post by **p_zetner** » Sun May 20, 2012 6:54 pm

Hi Cata.

It's the "thick (blocking) and thin (clean up)" pair I purchased somet time ago. I've attached the transmission curves. The filter pair was placed behind the telecentric lens. I realize now, though, that the f6 (approx) system is too fast for this to be effective. Telecentricity reduces the beam angle to zero but a significant cone angle remains because of low f#. For short objective focal length (480mm in my case), a collimator of relatively large (negative) focal length (-150mm in my case) keeps the beam angle already small (small angular magnification of the half degree solar disk). In the next (fast) system, I'll place the filter pair between the lenses. I'll keep the telecentric spacing to improve focal error tolerance. I want to keep a relatively short system focal length to get whole disk images on my DMK camera.

Cheers.
Peter.

Attached files CurveCaKPair1.pdf (155.1 KB)

p_zetner · Post by **p_zetner** » Sun May 20, 2012 6:58 pm

Hi again Cata!

Forgot to mention - you are probably correct, the CaK line is significantly broader than the H alpha line so the filter tuning and collimation through the filters is not as critical. I went "whole hog" because I will also use the omega H alpha pair.

Regards.
Peter.

Catalin Fus · Post by **Catalin Fus** » Sun May 20, 2012 7:00 pm

I've ordered myself one filter from Omega, but I'm still waiting for it. Do you have a link to yours? (if they are from ebay)

Regards
Cata

p_zetner · Post by **p_zetner** » Sun May 20, 2012 7:10 pm

Hi Cata.

I did purchase this pair on eBay but it was last year. I just checked on eBay and the pair doesn't seem to be available at present - just a single filter of 4nm bandwidth. You could email Bob Johnson and ask if he plans to offer the pair.

On a side note, the 4nm bandpass of the eBay filter is better than the 8nm bandwidth of the Baader CaK filter - should give more contrast. It has all the required out-of-band blocking and may be good enough to give very good views! Personally, I'd like to tune my pair to about 0.5nm or so, if possible.

Peter.

Catalin Fus · Post by **Catalin Fus** » Sun May 20, 2012 7:17 pm

Peter,

I bought one that should be better than the one on ebay. I paid some dollars more for that, to be narrower than the 4nm one.
It is a DF10 => should have 2nm bandpass.

I'll wait for it and when it comes, I'll check the graph and see what I can do :-)

Regards
Cata

p_zetner · Post by **p_zetner** » Sun May 20, 2012 7:29 pm

Sounds good, Cata!

Who makes / sells the DF-10 ?

Cheers.
Peter.

Catalin Fus · Post by **Catalin Fus** » Sun May 20, 2012 7:33 pm

Omega Optical but is not listed on ebay.

Ask Bob directly and he might give you some more info :cheer: . I know just that from DF15 (3nm) which is the one currently on the website, he offered me for some dollars more, the option of DF10 (2nm) and I accepted it.

Cata

Catalin Fus · Post by **Catalin Fus** » Sun May 20, 2012 9:12 pm

Peter,

I think I have confused something. The standard ebay CaK filter has 15nm HBW and the improved has 10nm. Not 4nm and 2nm.

My mistake! I didn't bought the expensive one, with 4nm bandpass.

Cata

p_zetner · Post by **p_zetner** » Sun May 20, 2012 11:56 pm

Hi Cata.

The eBay listing that I'm looking at (Item# 150811106688) says "Optical Filter Astronomy 394NB4 CaK filter 1.25 Eyepiece". Bob's usual filter designation shows the centre wavelength 1st (394nm), the "NB" stands for "narrow band" and, I believe, the "4" stands for the filter bandpass in nm? This seems to be the usual notation. He doesn't specify further in the description and I can't read the bandpass easily from the graphs since they are logarithmic. Did he tell you that this filter has 15nm bandpass?

Peter.

Catalin Fus · Post by **Catalin Fus** » Mon May 21, 2012 5:56 am

Peter,

the standard one is 34$ if I'm not mistaken. That one is 394BF15 -> 15nm.
http://tinyurl.com/bl7mb59

About this one I was talking. Sorry again for the confusion. This one can be upgraded to 10nm.

Regards
Catalin

p_zetner · Post by **p_zetner** » Mon May 21, 2012 4:19 pm

Hi Cata.

I think you have to be careful about this one and ensure that it is "image quality". An image quality filter should give lambda/4 (per inch at 632nm) transmitted wavefront distortion or better.

Peter.

Catalin Fus · Post by **Catalin Fus** » Mon May 21, 2012 5:36 pm

Peter,

I have bought the other two units like HeII and TiO with normal specs and they are working great.

I'll try the CaK received today...probably in the next days, when it will be clear again...and I'll gladly publish the results :-)!!

Regards
Cata

H'alpha - double pass module

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