Solar mod ERF suggestions

[Moonshane]

Personally I'd use a good birch ply. It' more stable than most timber and machines well. Thst said I would not make it to too tight a fit in case it absorbs a little moisture and swells slightly. You could use a  single bolt to hold it in place which would not be too detrimental to the value of the ota 

[spaceboy]

I had thought about it's nature to swell due to going in and out the flat with different temperatures and air moisture but would this still happen with kiln dried timber?

[Moonshane]

Doesn't matter how dry it Is today, it can still soak up moisture tomorrow. Seal it well and this is reduced but yes basically. What about delrin ?

[Uplooker]

Hi Spaceboy,

I would not get too caught up with having the largest ERF possible. I internally mounted a Baader 75mm ERF in a Thorlabs 75mm lens holder, I think the same one you linked to. Just make sure it is the metric one.

Here is some info I posted on SolarChat regarding my Mod.

As other have said, the ERF will not get hot as long as it is not at the focal point. I calculated the diameter of the light cone so that I would not cause any vignetting where n I internally mounted the ERF

https://solarchatforum.com/viewtopic.php?f=9&t=5852&p=59616#p59616

Good luck

[Moonshane]

Nothing worse than big netting when doing solar ?

[Moonshane]

Can't see the pics in the linked thread though?

[Uplooker]

4 minutes ago, Moonshane said:

Nothing worse than big netting when doing solar ?

Blooming autocorrect, that will teach me for replying on my phone.

Ah, maybe the photos are not visible as it is an old thread and solar chat have gone through a few server changes.

I will see if I can dig out the original images

[Moonshane]

Lol

Would be good to see ?

[spaceboy]

On ‎12‎/‎11‎/‎2017 at 15:05, Moonshane said:

Doesn't matter how dry it Is today, it can still soak up moisture tomorrow. Seal it well and this is reduced but yes basically. What about delrin ?

I'd already looked in to Delrin as it would be easier to work with, have some shock absorption, be lighter and already be black but oddly the cost is more than that of aluminium.

[spaceboy]

On ‎12‎/‎11‎/‎2017 at 15:15, Uplooker said:

Hi Spaceboy,

I would not get too caught up with having the largest ERF possible.

As other have said, the ERF will not get hot as long as it is not at the focal point. I calculated the diameter of the light cone so that I would not cause any vignetting where n I internally mounted the ERF

Good luck

It was not so much having the largest one that was causing me the concern but having one that gets the energy back out of the OTA the quickest. I do get rather involved in solar observing and when I had the https://www.firstlightoptics.com/telegizmos-telescope-covers/telegizmos-solar-observing-hood.html an Hr can pass by in a blink of an eye. All that UV energy has to go somewhere and I'd rather it back out of the scope than building up inside it. I honestly cannot see there being enough room for the 110mm. I drew out a couple circles to give an idea of how a 110mm erf would sit inside the tight 113mm OTA and there really isn't as much wiggle room as you might think. The 90mm is now my only option but comfortably with in budget and while it leaves me with faffing trying to find a cell I'd rather go with this than the 75mm which may end up 65-68mm when in a cell. With the OTA being shortened to achieve focus my guess is a 68mm filter will end up far closer to the etalon than the objective.

[Uplooker]

Your right that the ERF will be further down the tube, from the objective. I think mine was just over half way.

Your right again, the clear aperture of a 75mm filter will be reduced, but not significantly, maybe down to 70mm.

As long as the ERF is not near the focus point it will be fine and not get hot.

[Peter Drew]

I usually remove the eyepiece end baffle, the forward ones are ok left "as is" because they get proportionally larger as the tube is shortened.  

[spaceboy]

I will have to remove the one focuser end when shortening the OTA anyway as I'm led to believe the etalon protrudes some way on lunt's, but my estimates are that the 90mm ~ 85mm ERF should sit nicely behind the middle baffle. Although the middle baffle isn't quite middle of the ota I don't think ??

[Moonshane]

I put mine 250mm down from the objective which seemed rightvper a light cone I drew to scale. Be interested if anyone disagrees. It' a 90mm Derf with open aperture of 85mm like yours in a 100mm f10  Tal.

[Peter Drew]

Here's an aluminium 90mm ERF holder I made earlier, in fact I made another today out of 3/4" MDF.