Focusing DMK41 on PST

[keithatrochdale]

I just opened up my PST black box. The prism was a bit tilted, presumably from my earlier efforts at moving focus more than it would do. Have twisted that back a bit, although it wants to spring back to the tilted position.

I think you find that is quite normal.

Could it be to reduce some of the ghost images?

[Photosbykev]

The top of the prism should be parallel, in both axes, with the underside of the top of the black box

[King]

The only other thing I can mention is that certain barlows don't seem to let you achieve focus. For instance I can use a x2 Cemax or TeleVue 2.5 just fine but it won't work with a TeleVue x3.

[Peter Drew]

If you have to resort to a Barlow, you don't have to use the whole thing. Unscrew the Lens from the body and fit just this to the nosepiece of the DMK. This should give you enough focus range without increasing the image size by much.

[Peter Drew]

If you have to resort to a Barlow, you don't have to use the whole thing. Unscrew the Lens from the body and fit just this to the nosepiece of the DMK. This should give you enough focus range without increasing the image size by much.

[thenews24]

While we're talking about the pst, although not related to dmk...was wondering if anybody has used a Daystar erf, instead of the Baader D-erf on their donor ota?

Is it any good? Sounds like most people say the Baader d-erf takes the cake....but being in the U.S. myslef and somewhat imaptient, I could get my heads on a daystar a lot quicker then waiting for the baader to arrive. Is it worth the wait? Is the Baader that much better then the Daystar? Plus for me, the Daystar would be cheaper, i don't know.

[Merlin66]

The Daystar ERF (both the old style and the new) have been used successfully in PST mods.

The Baader has the added advantage of extra UV-IR protection and a narrower bandwidth.

[GlassWalker]

Success! The barlow module did its job and I got focus, although now the sun is a bit too big to fit the sensor in one go. I just managed to grab a couple of videos between the clouds and will have a play with those later. First impressions are it is giving a lot more detail of the prominences than a DSLR ever did, and that's without stacking.

Would it be too twisted to put a reducer after the barlow to try and get the image size down again...?

[Astroblagger]

I have difficulty focusing also, will a 1.5 barlow reduce the image size abit?

[Merlin66]

Using a barlow will increase the plate scale ie larger image at the sensor....

You can't use a reducer ( well certainly not easily!!!) to "correct" back from a barlow - they work by reducing the backfocus! You'll end up where you started.

[Black Knight]

Success! The barlow module did its job and I got focus, although now the sun is a bit too big to fit the sensor in one go. I just managed to grab a couple of videos between the clouds and will have a play with those later. First impressions are it is giving a lot more detail of the prominences than a DSLR ever did, and that's without stacking.

Would it be too twisted to put a reducer after the barlow to try and get the image size down again...?

Glad it's working for you now. Did you use the entire barlow, or just the lens to achieve focus?

[GlassWalker]

As I wrote yesterday I extracted the optical assembly, which happened to have eyepiece thread on one side. I flipped the optics around so they're still facing the right direction when screwed onto the camera. Previous attempts with the whole barlow were unsuccessful as I think it was also too far out.

[Black Knight]

Ahh, OK. Sorry I missed that

[GlassWalker]

Time to revisit this... yesterday I had a good run with the barlow to extend the focus, but I still really want to get the sun in one go rather than mosaic it.

Preparing for possibly getting a saw out to chop a couple mm off the eyepiece holder, I unscrewed it, then also the lower section. If I understand correctly, the upper section has the blocking filter which looks a dim red looking through it. The lower section is an energy rejection filter which passes a bright red image on axis, going orange off axis. This made me think, if I move or use an alternate ERF I can ditch the lower section of the eyepiece holder assembly and gain a load of focus range. Since it looks like the existing ERF isn't going to be easy to extract, I'm thinking more about using a substitute.

What is the desired characteristic of the ERF? For example, looking at a Baader D-ERF as a general reference, it has a broad passband around 600-700nm. Is there any reason I couldn't use an Astronimik 12nm H-alpha filter instead? The Astronomik curve doesn't give as wide a response chart as the Baader D-ERF, and indicates some UV might get through, but I can further stack a generic UV filter there.

So how does the above thinking sound, or should I go back to plan A and just saw off some of the eyepiece holder? I would further add, if I went ahead with the filter mod I wouldn't use this visually. I'm only interested in imaging.

[Merlin66]

If you are only imaging...then you can dump the bottom section and the mini-ERF....

For the amount of exposure you'll give the BF during your imaging sessions there will be no appreciable problem. You will obviously get a brighter image, which also helps, giving shorter exposures.

[GlassWalker]

I take it the BF could degrade with cumulative increaded energy applied to it? Removing the ERF would be a quick fix solution assuming the BF degradation would be far enough off. I'm still thinking I could use another filter of some kind elsewhere in the chain to help reduce the out of band energy before it gets there.

As for increased brightness, would that be significantly so? I would have thought the passband of the ERF is the significant part there, maybe introducing only a little attenuation. The BF would still take out the unwanted wavelengths that a missing ERF would pass?

[Merlin66]

The mini-ERF seems to have about a 80% transmission, so you "gain" about 20% more light in and through the final BF.

If you replace the mini-ERF is replaced with say a DSO Ha/ UV-IR combo....where do you put them? How do you tilt them to reduce the internal reflections???

You'll end up back where you are....

[GlassWalker]

There's a load of space between the etalon and prism... I did line up my EOS-clip H-alpha filter to see if it would fit in there, but it's just a fraction too big to fit. I could even use a front filter although having said that the nearest I have that's big enough is a red lighting gel which would pass a lot of IR anyway, so probably not enough benefit to be bothered with.

Right now I think I'm about 50/50 if I should cut the eyepiece holder back or run minus ERF... will probably give the latter a try before I start cutting anything!

[Merlin66]

Try without the mini-ERF first......

[GlassWalker]

I did get a try imaging without the ERF section today. Didn't go well. The DMK output went white vertically - is this blooming? Focus, etalon, moving the camera in/out and capture settings didn't help at all. Just overloaded I guess.

Think I'll go back to thinking about shaving the eyepiece holder.