PST mod - donor scope advice

[Merlin66]

Remember you also have the tried and proven Lyra / TS 102mm fll OTA......

[allcart]

The Lyra's have gone up in price now as the suppliers were apparently putting their prices up. Its still an excellent scope but almost twice the price of the Tal.

With respect to the Tal 100rs, I chopped about 100mm off my tube, and I retained the original R&P focuser. I did find that the drawtube is long enough that it can be extended again and still used as a normal scope. It would be advisable to measure the length of your drawtube to make sure it is actually longer than the amount you may chop from the tube.

[allcart]

Forgive me for jumping in on this one.

Allen (Allcart) mentioned the Tal 100rs has the optimum focal ratio for a PST mod. Does this mean that the tube does not have to be cut so the scope can be used for normal stargazing if required?

Cheers

Cliff

The optimal focal ratio is F/10. This has been investigated and confirmed by modding veterans like Merlin and Peter Drew. The scope tube still generally has to be shortened, but it is possible that you can simply remove the pst assembly and the ERF, stick on a diagonal and ep and use the scope normally. This works for my Tal but I don't know if it works for every scope.

Other mods have been tried and tested with very good results using other focal ratios. I am but a newcomer to the world of modding , so I can only relate my experience to the Tal so far, but more mods are on the cards.

[Stu]

I think I'm going to try to get an adaptor made up so I can avoid cutting the tube. The Vixen has a 1.25" diagonal but given that you don't actually use the focuser it seems expensive to but a new 2" one.

Not quite sure how you use the original 5mm blocking filter Peter, I thought you had to buy a larger one built into a diagonal for stage 2? Can you explain it for me? By pm if necessary to avoid hijacking the thread!

Many thanks

Stu

[allcart]

Your Vixen diagonal plays no part in a stage 1 mod as it is not used. I suppose the deciding factor in keeping the Vixen focuser or not, is whether it has a 2" drawtube. If you haven't got Merlins PST modding pdf, then ask him for it. There is tons of info in it that explains most everything. Also, Kev (photosbykev) did an excellent blog article about his Lyra mod that is worth a read. http://www.photosbyk...-modifications/

Or you could lose yourself for several weeks reading about modding at MarkT's blog. http://brierleyhillsolar.blogspot.co.uk/

.

There are ways to use the 5mm blocking filter from the pst in a 1.25 diagonal for the stage 2 mod, but I haven't gone that route so I'll let others explain the procedure.

[Merlin66]

Guys,

the BF5 from the PST is "optimised" for a 400mm focal length, BUT it has successfully been used as an "interim" solution in Stage 2 mods.

When you unscrew the eyepiece/ BF5 section from the "black box" you'll find that some of the Celestron 1.25" diagonals have exactly the same thread on their eyepiece section (!!?) the PST BF can then be fitted and used until you sell more kids into slavery to pay for a Coronado/ or Lunt BF10/ B1200 etc.

[Stu]

Thanks, yes, I understand that the standard diagonal isn't used in the mods, I was referring to the blocking filter which I believe is needed for a stage 2. These seem quite pricey so if it is possible to use the 5mm (from the PST?) then it would help.

http://handsonoptics.com/index.php?main_page=product_info&cPath=1_72&products_id=1991

I've got Ken's document which is great, plus have read Kev's info and plenty of other stuff. Getting my head around it slowly.

Another question, if using an internal 75mm D-ERF, can you use the same formula used to show the size of the light cone in Ken's document to calculate the optimum position?

ie diameter = x/10 + f/100

Where x = distance from the focal point and f = focal length

Hope that makes sense.

Stu

[Merlin66]

Stu,

If you're using a different focal ratio then:

diameter = x/focal ratio + f/100

IMHO the sub-diameter ERF MUST be greater than 1/2 the aperture and positioned as close to the rear of the objective as possible.

[michael.h.f.wilkinson]

I am also going for the 75mm D-ERF in my 80mm scope mod.

[Stu]

Thanks Ken. I'm assuming you lose some aperture on the D-ERF by having to hold it securely? If making, or getting your own holder made, what is a safe amount to hold it by? For instance, a 3mm lip all around makes your 75mm D-ERF into a 69mm one.....

[JamesF]

Thanks Ken. I'm assuming you lose some aperture on the D-ERF by having to hold it securely? If making, or getting your own holder made, what is a safe amount to hold it by? For instance, a 3mm lip all around makes your 75mm D-ERF into a 69mm one.....

I had assumed that wouldn't be an issue if it were fitted sufficiently far back in the tube that the 69mm (or whatever) was larger than the light cone. I think it was Ken who suggested that it could be as far back as 50% of the focal length if necessary?

James

[Stu]

I think that so long as the light cone is smaller than the D-ERF then all should be well, but you need to consider the tube heat currents and also the total energy going through it. Positioning the D-ERF so far back that the energy is all concentrated in a small area doesn't sound wise.

My question was more around how much to overlap when holding the D-ERF as this obviously affects the available aperture.

Stu

[Peter Drew]

I use a 1mm lip on my 75mm holders which reduces the ERF aperture to 73mm, more than adequate for mods up to 125mm aperture. I use a 110mm internal ERF in my 150mm F10 mod.

[Merlin66]

Caution:

The ERF acts as a highly refective mirror...if it is mounted close to 50% the focal length it will reflect a focused beam back onto the objective!

[JamesF]

Caution:

The ERF acts as a highly refective mirror...if it is mounted close to 50% the focal length it will reflect a focused beam back onto the objective!

I can see why that probably wouldn't be a good plan

Is there any sort of "ideal" placement for the ERF? For example, as far forward as it will go whilst allowing a few millimetres clearance around the light cone?

James

[Merlin66]

Basically, yes.

As long as it doesn't vignette or act as an aperture stop.

[Sqyff]

Hmm, now you've all got me thinking again.

I am picking up an Evostar 90 tomorrow and was going the full aperture D-ERF route but now I'm wondering if it would be better to go for the 75mm filter now (£100 or so saved).

Would it be a good idea to mount an internal filter just before or after a baffle? I'm thinking that if the baffle hole is smaller than the filter then you would be well inside the light cone for an internal mod.

[JamesF]

It had occurred to me that mounting the ERF at a baffle would give a nice controlled way of adding a small tilt to the ERF to prevent image ghosting.

James

[Sqyff]

I was just looking at the Thorlabs filter holder and noticed that it states that the max optical thickness is 7.6mm.

I take it that the 75mm Baader D-ERF is still made to a 5.6mm specification?

[Merlin66]

The Baader spec sheet actually now says 5.5mm......

[Peter Drew]

James. If you look back to the picture of the 75mm D-ERF holder that I made for Keitatrochdale's Tal conversion, a Thorlabs filter holder can be mounted in the same way. A suitable slot is cut into the Tal OTA and the filter unit complete is inserted like a CD, This can then be easily removed for either white light solar work or normal nightime astro observations. The outer tube attachment strip and a similar one as a slot closure when the ERF is removed were both made from the tube shortening offcut.

[lukebl]

Well, the first part of my mod has arrived.

This lovely TAL arrived from FLO within 24 hours of ordering.

....but how can I possibly put a hacksaw to such a beautiful piece of engineering? Especially as I'm a bit cack-handed at DIY.

[JamesF]

That's partly why I've been waiting to see if one might come up second hand. It hurts to put a saw through something like that when it's brand new

James

[trev27]

Scary isn't it!!!! 'tis well worth it in the end though! I wouldn't be surprised if the Tal is more solid than the Evostar, given their reputation, but cutting the skywatcher was quite easy to do by wrapping a bit of masking tape around the tube, marking the cut point and then clamping the two tube rings to the point with only enough space for a hacksaw to get through. Didn't take long at all and the rings kept the blade on target.

I know what you mean though - that Tal is really nice!

[Stu]

That's why I'm hoping to be able to come up with an adaptor to replace the focuser which avoids the need to cut the tube. Not sure it's possible but will certainly explore this before cutting!

Good luck, looks a lovely scope

Stu