Skywatcher Evostar 90/PST stage 1 mod

[Sqyff]

Thinking about the stage 1 mod for my PST and was wondering if this scope would fit the purpose optically.

I want to keep the cost down and FLO are doing this scope on an EQ3-2 for £235.

With a 90mm Baader D-ERF at about £318 this would keep the total cost to under £600.

Also I'm no engineer and so would like to keep the chopping around of the tube to a minimum as I can't even cut bread straight.

Your thoughts would be most welcome.

Cliff

[Merlin66]

Cliff,

The f10 ratio would be good...

A 90mm ERF will end up close to 80mm clear after fitting to a cell...

The PST etalon must be positioned 200mm inside prime focus. I don't know if there's enough space without removing the focuser and fitting an "end block"....Plan B would involve cutting back the OTA.

(EDIT) I see the EVO90 has a 1.25" focuser...most available PST nosepieces are for 2"...

Options: As per #1, fit a simple end block with a central 2" hole

#2 Cut the OTA and fit a 2" focuser....

[Sqyff]

Thanks Merlin66.

I would most probably go for the 2" focuser as this would give me some adjustment.

What sort of improvement in viewing would this mod provide over a standard PST?

What I'm trying to work out is 'bangs for bucks'.

Would this be a good price to mod the PST in this way or would a larger scope be better? Of course more aperture is always good but in my case this would necessitate also purchasing a new (and more expensive) mount to use the scope plus the increased cost on the donor OTA and D-ERF. Basically at which point would you say the two lines of cost and performance cross?

Cheers

Cliff

[Stu]

As a hopefully related question to this thread....

.... Would you lose anything by using a 1.25" focuser assuming you could find or make an adaptor for it? Would it vignette the image at all?

Thanks

[Peter Drew]

In addition to the already good advice, if you would be prepared to fit an ERF internally then the 75mm Baader D-ERF would save you about £100. As far as performance goes, I have a 90mm F10 mod and the results are far superior to the standard PST and a 60mm Coronado.

[Sqyff]

Thanks Peter.

I think this 90mm is going to be my final format (until I get aperture and stage 2 mod fever) .

The internal 75mm D-ERF sound a good plan but as my DIY skills would not only endanger me but anyone within a 5 mile radius I think I'll probably stick to an externally mounted D-ERF. That way I can at least know if there is a light leakage problem and also for ease of tilt adjustment.

I think Mr. FLO and Mr. Teleskop-express are going to be getting an order from me very soon.

Thanks to everyone.

[Sqyff]

Just been checking the Skywatcher website to see if I could glean the prime focus of this scope from any of their online information.

http://www.skywatcher.com/swtinc/product.php?id=35&class1=1&class2=103

It states that the tube length is 86.5 cm (865mm) and the focal length is 900mm. Does this mean that I would have to cut back the tube by 165mm or am I just not getting the concept of this part?

Also, if this is correct would getting a 2" low profile focuser (and cutting back the tube so as to take the extra distance in to account) help with adjusting the etalon into the right position?

[Merlin66]

Re the 1.25" PST nosepiece....this is workable...consider a 40mm long nosepiece with an internal diameter of 29mm; in a f10 beam the clear un-vignetted circle at the etalon would be 25mm - slightly larger than the 20mm etalon size.

The tube length quoted probably includes the focuser...yes, your arithmetic is sound. Do the final measurements using the sun's focused image on a card.

You can add a 2" focuser, but please remember when the etalon is positioned at -200mm IT WILL NEVER MOVE AGAIN! - a bit of a waste of a 2" focuser. If and when you go for a Stage 2 mod you'll need to add a focuser between the etalon and the CCD/ eyepiece.

[Sqyff]

Thanks Merlin. Looks like Mr, Evo's going to be for the chop when I get him.

I think I'll try and get a 'plug' made up with a 2" hole and some sort of screw to hold the PST adapter.

I wonder how much an aluminium plug is going to cost me to get made up.

[allcart]

I have recently chopped my Tal 100RS for my stage 1 mod. I kept the focuser on the tube. Merlins method of calculating the -200mm is more accurate than my method, but working on the priciple that the Tal is 1000mm FL, I just used basic maths and a tape measure.

I removed the diagonal and extended the drawtube by 10mm. I then put a tab of masking tape on the end of the metal tape measure and lowered it down the inside of the tube until it sat on the objective. The tab of masking tape is to ensure that I don't scratch the objective. I measured from the objective to the end of the drawtube and used this measurement to work out how much to chop off the tube so that the end of the drawtube is 800mm from the objective.

Assuming my first measurement to the end of the drawtube was 900mm, that told me that I needed to remove 100mm to place the end of the drawtube at 800mm. The purpose of extending the drawtube by 10mm was to give me some latitude for fine tuning the etalon position.

First light this weekend was fantastic. I am now moving straight on to stage 2 and have also picked up a celestron omni 120mm XLT for my next solar mod.

I should have said earlier, that the reason I kept the focuser on the tube is that I can remove the pst and ERF, extend the focuser by 100mm again and use it for white light with a Hershel wedge, or even Astro use. One scope, 3 formats with minimal changes between them.

[Sqyff]

Hi Allen

That's what I was wondering when I mentioned the focuser.

I would like to be able to use the scope for white light viewing as well but need to work out how this would be possible if I cut back by 165mm.

Gotta get my brain around this.

[allcart]

Looking at the tube and FL measurements it seem that they are only allowing 35mm for the lightpath through the diagonal and EP. Seems a bit tight to me. I would measure the tube in the way I described, or more accurately as Merlin66 described. Once you have accertained the proper measurements you can work out how to proceed.

I'm sure you can retain any focusser on the tube if you want to, but to make use of it afterwards for WL it will need to have a drawtube longer than the amount you are going to cut off the tube, so that you can extend it again after the pst is removed.

In your case, you would remove the aluminium plug and re-attach the focusser, or replace the 1.25 focusser with a 2" and then do the measurements before cutting. With my Tal, the drawtube length is more than enough to compensate for the 100mm of tube I chopped off.

[Merlin66]

For the TS102 I machined up an adaptor which allowed the PST etalon to be fitted at the -200mm without the focuser..This adaptor can quickly be removed and replaced with the original focuser for the Baader Herschel wedge.

[allcart]

Nicely done Ken. Its times like this that I wish I had access to a machine shop.

[Sqyff]

Thanks guys. I guess I'm a bit nervous about screwing this up with the financial outlay and all.

I'm especially nervous about dismantling the PST as I have only had it since October, but aperture fever is a sly and stealthy creature that jumps you when you least expect it so I am going ahead with the project.

Cheers

Cliff