Make a native F30 telescope better for Solar

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AndiesHandyHandies
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Make a native F30 telescope better for Solar

Post by AndiesHandyHandies » Sat Apr 21, 2018 7:05 am

Hi

This is an Omega Bobs filter project. We know they are not brilliant but they are cheap and its a project.
We had to send the first set of filters back because of excessive reflections from the combined ERF and H-Alpha.
The replacement seperate 40nm ERF and h-alpha pairs are free of the problem. Needs a KG3 for long IR blocking. And a red filter for off-band leakage.
The ERF works as well as a Baader 35nm filter. Supplied scan shows better transmission.

1. Collimator

We have tried a Dave Grosky type collimator with lenses to hand and its better than F30.
I noticed that the PST Etalon is within the native light cone, and seems to use a diverging lens.
So not a long lashup hanging out the back of telescope. Which means offset sweet spot.
How do I calculate the lens needed to produce a 20-22mm output? And where would I get one. I assume its fairly long focus which may be an issue?
And converge it again.
Its a long time since I did physical optics. I should find the book.

Note.
We are still in development so a few overlapping ghosts to deal with, but its OK on prominences. Its hard at the moment to assess disk performance.

Andrew

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Re: Make a native F30 telescope better for Solar

Post by marktownley » Tue Apr 24, 2018 5:28 am

Hi Andrew, Need the optical parameters of the scope before any working out can be done - objective diameter, focal ratio.
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Re: Make a native F30 telescope better for Solar

Post by AndiesHandyHandies » Tue Apr 24, 2018 6:38 am

Hi Mark,

Its a doublet refractor stopped down to 77mm and a resulting focal ratio of F30.

It has a large rear focussing tube.

We have about 20mm apertures for the ERF blocker and 2 H-alphas. As you will know.

Thank you. Andrew.

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Re: Make a native F30 telescope better for Solar

Post by AndiesHandyHandies » Wed May 02, 2018 10:32 pm

Hi

The focal length is 2250mm. 6" F15.

Stopped down for F30.

So what diameter and negative focal length lens would we need for 20mm filters?

The spacings for the filter module to get reflections out of the path are 35nm pre-filter - 100mm - First H-alpha - 120mm - second H-alpha.

Andrew.

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Re: Make a native F30 telescope better for Solar

Post by marktownley » Wed May 02, 2018 10:54 pm

For a collimated beam you are looking at a 25mm diameter negative lens with a focal length of -600mm. This will need to sit 600mm inside focus. Refocusing lens to balance aberations etc needs to be a positive lens 25mm diameter 600mm focal length. Engineering wise this is going to be interesting! That 40mm ERF needs to sit half way down the tube ideally.

Mark
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Re: Make a native F30 telescope better for Solar

Post by AndiesHandyHandies » Thu May 03, 2018 8:26 am

Hi Mark,

Thanks for the information.

I cannot re-call mentioning a 40mm diameter ERF for my mates option?
" That 40mm ERF needs to sit half way down the tube ideally."
Ah, the seperate Bobs ERF blocker to protect the filters is 40nm bandwidth and 20mm in diameter, we have new set(s) with the blocker removed from one H-alpha as we had excessive reflections.

He is using a stop on the 6" objective and trying various UV-IR or coloured filter options there.

At the moment he has a positive collimator hanging off the back of the focuser at right angles with balance weight!

I assume the lenses you mention are easy to get, is there a preferred UK source.

Cheers. Andrew.

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Re: Make a native F30 telescope better for Solar

Post by marktownley » Thu May 03, 2018 11:20 am

Misread sorry - read 40mm erf not 40nm erf.

I've just had a quick look on thorlabs and edmunds optics and couldn't see negative focal length lenses in the spec you need. It's far from a conventional or ideal mod you are trying to build here.
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Re: Make a native F30 telescope better for Solar

Post by AndiesHandyHandies » Thu May 03, 2018 3:37 pm

Hi

My mate is having seeing issues at the full focal length.

He has been using a focal reducing lens before the positive collimator.

He has tried various configurations with filters inside the focuser at native F30 and the current one outside with reducer.

From his experiments a lens up the focuser give him vignetting issues with the filters.

I think he will stick with the current set up.

He has found using the collimator to be best.

Thanks for your advice on all fronts.

Andrew.

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Re: Make a native F30 telescope better for Solar

Post by christian viladrich » Thu May 03, 2018 6:35 pm

Hi,
I suspect there might be a misundertanding here.

There are two ways to mount an etalon on an instrument :
- telecentric beam : with F/D > 30 for mica-spaced etalon, and F/D> 45 for air-spaced etalon.
- collimated beam : F/D can be anything

Beware, the PST is an airspaced etalon associated with two lenses. The front lens is intended to make a collimated beam for F/10 setups. The second lens is used to refocus the beam.

So, if you want to use your PST etalon in a telecentric beam, you have to remove the two lenses attached to it, and use a telecentric setup giving F/40 or higher.

The easiest way would be to use the PST in a collimated beam. This is the "PST Mod" where the two associated lens of the PST are kept, and you use an F/10 refractor.

More on this here :
http://astrosurf.com/viladrich/astro/in ... lar/FP.htm

Hope it helps.
Christian Viladrich
Co-author of "Astronomie Planétaire"
http://www.astroplanetes.com/
Co-author of "Astronomie Solaire"
http://www.astronomiesolaire.com/

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Re: Make a native F30 telescope better for Solar

Post by AndiesHandyHandies » Thu May 03, 2018 8:36 pm

Hi Christian,

I have two etalon options to play with,

The PST
and
Omega Bobs two 25mm 1.5nm H-alphas which are dielectric and a 25mm 40nm blocker he provides to protect them.

I will take Marks advice and try my PST on the back of the Vixen VMC110L as is. I have a Lunt 110mm DERF so I was intrigued to see if a compact PST mod could work having found the scope looking round for a 100mm or 110mm F10 scope. There may need to be some mod to attain the PST position in respect to focus. Moving the mirror/secondary or a weak negative lens. I understand as a MC there will be a softer image than a refractor. My mate said he knew I would buy the scope abyway. A poor mans Questar.

I also have a Meade 127mm F7.5 Triplet, visually identical results to my mates 6" F15 Coude(60 year old mirrors). Intend to stop down to F15 and try a ES 2x 'TC' with the Omega Bob filters, to compare with my mates 6" F15 stopped down to F30 and using a positive lens collimator with a second set of Omega Bobs filters I bought for my mate, in exchange for some engineering help on mine. Using a collimator at F30 works better than just at F30. Mark says that though they are inteference filters index 2 he found they worked better in a collimated beam. As they are 'New old Filters', cosmetic seconds from production runs so variations across the surface. Due to a return I may have been lucky to get good production filters so they may be better in TC. If that does not work can try a collimation set up.

I will try the Omega filters as a prominence scope with the blocker and one H-alpha and use the other H-alpha as a continum filter on the PST.

Something will work.

My mate e-mailed mean image from solar chat today he photshopped to show his visual result today and he thinks its OK.

After a few weeks research I know more about solar scopes than I did to start with!

Andrew.

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Re: Make a native F30 telescope better for Solar

Post by marktownley » Thu May 03, 2018 9:30 pm

Hi Andrew,

You will get by far the best results using your 100mm DERF with the 127mm Meade and a PST etalon. This setup will be far superior than any of the others you have suggested.

Mark
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