Pentax 125SDP vs Taks & others for AP?

[AnakChan]

Hi all,

Tomorrow, I'm going to take a look at a Pentax 125SDP that's currently held reserve for me. I've done my googling and know the specs of this model, and also tried to reach out to the few owners who actually had one but so far haven't had a chance to get a response from any of them.
 
My intention is to have a wide(ish)-field scope for AP, and my camera is the CDS Astro A7s (12MP Full Frame - therefore, approx ~8.4microns) so I've narrowed down my preferences to scope specs that have a large image circle with all the usual whole shebang such as flat field, CA correction, etc.
 
I'm particularly curious on how a 20 yr old 125SDP refractor would compare against the newer FSQ-106ED's and even the FSQ-130ED's (I'm aware  there are also very very few FSQ-130EDs out there).
 
Does anyone have any thoughts or guidance they're able to provide?

Cheers,
Sean
P.S. The 125SDP is in excellent condition going for approx £3230. I assume that it is a reasonable price?

[rl]

FWIW, I've just bought a s/h Pentax 75 SDHF for astrophography. It's a smaller version of what you're going for, and, like yourself, I did the google spadework first. Most people put it almost in the Tak category with the qualification that the Chromatic correction was not quite as good, but the flatness of field (over a 6cm circle if that's relevant nowadays) a bit better.

I was out looking at the moon Sunday night, in particular at Gassendi and the Mare Humorum area, using a 4.7mm Ethos at just over 100 times mag. I was very struck by the contrast and lack of CA on the terminator in the middle of the FOV. The detail visible in Gassendi was remarkable, with the 2 central peaks very clear and the circular rings on the crater floor very apparent. The thought was going through my head at the time,"How can a Tak be any better than this?" but you'd have to ask someone who has used then side by side. It was pretty much like using a reflector without the diffraction spikes. The only colour visible with the image in focus was a ring of fire from the periphery of the FOV which is a known ethos trait. Out of focus I could get a pale green edge to the limb towards the edge of the field.

Flatness of field; pick your widest good eyepiece. Focus on any one star in the field and all the others are in focus straight away. You wouldn't notice it necessarily at first glance but it's true when you check like no other scope I have owned. Your eye simply does not have to accommodate the difference. If that's not the case then your eyepiece has a curved plane....I bought this scope in desperation after having owned an ES triplet + field flattener and having got very poor results for AP. This scope takes very nice pictures indeed using an APSC sensor 24mm* 16mm.

These scopes are intended first and foremost as astrographs and, on the 75mm, many eyepieces won't reach focus because there is not enough in-travel. Use of a 1.25" diagonal helps a lot. .

The focuser is an old-fashioned rack and pinion that has no 10;1 reduction but it works extremely well and will hold a heavy camera in the vertical.

It avoids another pet hate of mine; retractable dew caps that surrepticiously slide down over the night. This one stays properly in place.

It looks really tiny alongside a lot of other 3" scopes. No surplus fat, it does exactly what it was designed to do very well.

Downside; the eyepiece fittings are eccentric sizes. Expect to be seriously nice to your friends who can use a lathe. By modern standards it's a bit slow photographically. To speed it up you need a reducer that doesn't flatten . ..such things do exist but are not cheap. I take the point of view that you can always add hours to a good image that's a bit faint, but you can't really correct a fast camera with terrible edge distortion.

In conclusion, it is well-made with no modern frills and none the worse for it. The unsung hero in Japan who designed it 20+ years ago really loved his job. I paid £550 for mine and wondered if I was paying over the odds (it's got a few cosmetic dents where the previous owner overtightened some clamps) but I can't see myself selling it in the near future..the results I'm getting are rather good. on the basis of what this scope can do I've just got my camera modded (this week) so we will see if the CA gets any worse with the improved red sensitivity.

The step up to the 125 is well out of my league both for cost and portability and the mounting. Good luck1

Regards, RL

[NickK]

I have a Pentax 105 SDP that many have seen at SGL

The 105 gives obscenely good imaging and visual - giving 3D views of planets with televue and Naglar optics. I just use Hyperon and a range of Televue 2x & 5x with ATIK cameras on the back. Optically it can quite easily (visually) separate the trapezium. Field is flat as a pancake for everything I've done. I use a borg helical focuser.

I would say that you'd need to look long and hard for the differences to Taks - the main aspect that Olly has pointed out is a small amount of colour focal differences but that can also be down to the effect of cooling - and yes the scope is affected by cooling unlike the little fluoride 85s. It is likely that you will want to put a robe-focuser on it and this is where the "odd" shape of the scope has problems.

The other aspect of the scope is that it is designed for AP.. hence it's back focus is good but I can't get an elbow and EP on the scope with the borg focuser on. Hence my garden pier is higher up..

I've white light imaged the sun at 6500mm and dawes limit does blur a little but it's remarkable considering the scope aperture.

Woo... if I wasn't married and had a job at this time.. I'd be collecting that 125 I got mine for 1800 IIRC which was a steal.

Edit: addition - you will not be able to use a white light style wedge on the scope due to the back lens set.

More info.. I've had a large number of cameras on the back of it .. including the ATIK 11000. The 383L has no problem with light coverage, the 11000 needs 2" filters. However I didn't have major issues at the time with light coverage of the sensor (or at least any that couldn't be undone with flats).

[AnakChan]

Hi all. Thank you for your response. I appreciate it. The point of the finicky adapters is good to know. About the the image circle and light falloff, yes it does seem the Pentax does fill it nicely. Thank you for the confirmation.

Just recently (after I started this thread), someone did tip me about the a strong red shift during AP. I've read about the similar issues to the 100SDUF II and I'm wondering if that these scopes are just so old that they're designed for film AP and less suitable for digital AP? This would naturally pose to be a problem if I intend to use this scope for AP for about 90% of the time.

[AnakChan]

Hi all. Thank you for your response. I appreciate it. The point of the finicky adapters is good to know. About the the image circle and light falloff, yes it does seem the Pentax does fill it nicely. Thank you for the confirmation.

Just recently (after I started this thread), someone did tip me about the a strong red shift during AP. I've read about the similar issues to the 100SDUF II and I'm wondering if that these scopes are just so old that they're designed for film AP and less suitable for digital AP? This would naturally pose to be a problem if I intend to use this scope for AP for about 90% of the time.

[sorry no edit rights to my own post yet] On the other hand, someone who owned both the 125SDP and FSQ106ED feels the 125SDP produces higher detail than the Tak and didn't notice any aberration. So, getting rather polarising opinions. Maybe one of them had a dud?

[NickK]

Old school film has nothing todo with focus positions for different wave lengths as the film media doesn't have different layers for different wavelengths. In terms of the colour - I've not noted any issue. The only issue I have noted is that, when taking LRGBA, the cooling atmosphere here in the UK causes a shift in focus over time.. and if no refocusing is done then you naturally get a loss of focus but that is purely down to the temp change not a colour problem.

I tend to use an IR/UV cut filter on the filter wheel nose piece so if there's any difficulty in bringing those wave lengths to focus then that may be why I never see it if it ever exists at all.

The 125 may product better detail but that's down to the 125mm objective rather than the 106mm objective. Dawes law dictates that. Either way the 105 is impressive in this aspect and I'd expect the 125 to be even more so. The speed of the imaging is the achilles heel with the 105 being ~f/6.2 so that's not too bad but not instant compared to the f/2 or f/4s around.

The main issue with the 105 is the 1:1 speed focuser - hence the addition of the borg. If I was robo focusing I would be tempted to have a spaced plate system made up so that the 1:1 can be left un used and the borg removed (it works but can be difficult to turn with a heavy camera on the back). The plate would then clamp through to the mount plate rather than hang off the scope. The 125 has a large helical and I have seen a robo focuser attached to one that works well.

3K is a large amount and certainly worth looking through and possibly testing with a day of imaging using your cameras. Take the images home, have a look and then decide.

PS.. there's a spooky side when attending a star party.. you feel watched - don't worry that's all the tak owners

[AnakChan]

NickK, cheers for encouragement. Good to know about no coloration of your 105SDP. About temp causing focus shifts, seems that's quite normal and is even more pronounced with the fast scopes like the FSQ106 & VSD100 (and therefore presumably the 100SDUF too).

Anyhow, I walked in to the shop feeling 50/50 about the 125SDP. The unit looked really beautiful. The shop keeper mentioned about the proprietary components and all I could think of was @rl's comments of how the (ring/adapter) components had eccentric sizes. Together with someone else's report of the red shift, as beautiful as the product looked, I was hesitant.

So I walked out of the shop empty handed, sat in my car and googled up more pix of the 125SDP and came across a post which showed a Baader adapter allowing greater possibilities. I walked back into the shop, chatted to the shop keeper a little more and this time, walked out with the 125DSP in my hand . Unfortunately I cannot "try before I buy" nor is there any shop warranty for a 2nd hand product so it'll just have to be a risk I'll have to take. Fingers crossed it performs well. I'll post up pix of the 125SDP later. (Aside from guide scopes) My 1st refractor in 38 years.

[ollypenrice]

Nick, I think the issue of colour correction for film does still apply. Of course, film has only one workable plane in the same way that an OSC does, but the point is that the correction of any scope is imperfect and the designers have to make a decision as to where in the spectrum the lens will be optimized. (For example, if it had been designed for imaging my TEC140 would be optimized a little closer to blue. As it is, it's optimized for visual. The difference is slight but some people do pooh-pooh the TEC as a CCD lens.  I don't, I think it's fine, but some blue stars do sometimes 'grow' a bit...)

A lens optimized for modern CCD has to be very well corrected into, and just beyond, the visible blue. I would expect this to be the Achilles' heel of the Pentax. I don't think that, in the days of film, optical designers had to worry about 'blue bloat' as they do nowadays. It will doubtless be a good scope but my worry would be blue. I haven't seen one of these scopes and don't recall any images from them either so this is conjecture.

(A century ago it was common for well heeled gentleman astronomers and universities to have two parallel achromats, one optimized for visual and one for film. UCLAN have one such dual instrument today.)

Olly

[NickK]

Olly's experience greatly passes mine

My Pentax 105 made it's way down to you one winter! However considering my AP isn't great (I no longer bother with flats etc) and focus more on mono because of time constraints, I remember you made a comment previously about this but refocusing between LRGBA tends to reduce it enough for me that I don't notice when I actually do LRGBA

Edit: Sean - just to give you a background, Olly has considerable professional AP experience with lots of experience with Taks and other scopes. Possibly one of the more capable persons to give a good response

Even still - I'd still be extremely tempted by the 125 or if I won the lottery a 150

[rl]

Plenty of images taken on the 75mm version by Nik Szymanek (hope I spelled it correctly), have a look at his website.

Of course, you are never quite certain what been massaged out I the processing; I wish my attempts looked like that!

Like Nickk, I use a Baader uv/ir filter. So far I have not seen any evidence of blue bloat on the 75mm (owned it 6 weeks), but I don't know how much that would transfer to the 125.

Modded camera arrived today; I will put something up in the near future if the sky obliges.

RL

[ollypenrice]

Olly's experience greatly passes mine

My Pentax 105 made it's way down to you one winter! However considering my AP isn't great (I no longer bother with flats etc) and focus more on mono because of time constraints, I remember you made a comment previously about this but refocusing between LRGBA tends to reduce it enough for me that I don't notice when I actually do LRGBA

Edit: Sean - just to give you a background, Olly has considerable professional AP experience with lots of experience with Taks and other scopes. Possibly one of the more capable persons to give a good response

Even still - I'd still be extremely tempted by the 125 or if I won the lottery a 150

I remember it, Nick, and another guest also has one from his days as a film astrophotographer. I'm not knocking the scope for an instant, just expressing where my concerns would lie. The comparison with the smaller one may not tell us too much because the colour correction of refractors becomes increasingly difficult with increasing aperture. That's why, as they get bigger, comparable refractors also tend to become slower - or much, much more expensive - or both! 

Olly

[davew]

As far as I remember, and the last time I lusted over a Pentax scope was 15 years ago, the 75 and the 125 were different scopes. The 75 had three element in three groups and the 125 had four elements in four groups. 

A minor point but may be worth mentioning  for future posts.

Well done on the purchase. I envy you

Dave.

[NickK]

As far as I remember, and the last time I lusted over a Pentax scope was 15 years ago, the 75 and the 125 were different scopes. The 75 had three element in three groups and the 125 had four elements in four groups. 

The 105 has a doublet at the front, and a doublet at the rear (from memory). There was a link with the lens sets: http://www.baader-planetarium.de/pentax/pentax.htm

In fact this PDF: http://www.baader-planetarium.de/pentax/pdf/pentax_sdp_31012006_baader.pdf

It shows the 105 and 125 the same with an SD and ED in the rear. Plus it has the screen shot results of the optical tests done for both the 105 and then later the 125 for your enjoyment Sean

As Olly has said - nothing like getting a eye in... so have a look

[NickK]

So I walked out of the shop empty handed, sat in my car and googled up more pix of the 125SDP and came across a post which showed a Baader adapter allowing greater possibilities. I walked back into the shop, chatted to the shop keeper a little more and this time, walked out with the 125DSP in my hand . Unfortunately I cannot "try before I buy" nor is there any shop warranty for a 2nd hand product so it'll just have to be a risk I'll have to take. Fingers crossed it performs well. I'll post up pix of the 125SDP later. (Aside from guide scopes) My 1st refractor in 38 years.

Hehe excellent

The Baader 4-in-1 device I have - that works really well and replaces the back pentax russian doll cone:

You may need a back space to get focus like I have here but this should help you understand the 4-in-1.

I got the borg adaptor made, if needed I can pass on details of that to you, should you need an adaptor made.

Either way - I look forward to your pics and I hope that the scope fulfils your expectations

[AnakChan]

I have to say I'm still a little nervous as there's very little hands on experience on this OTA and of the few that do I have a split 50/50 opinion about it. Thank you very much for the pics of the compatible adapters! That is going to be extremely useful for me this weekend as I go hunting!

Olly, don't know if this will help but here's what I found so far about the 125SDP, note that this is from Pentax themselves so possibly a little bit of marketing involved. I'm not going to pretend I understand everything of the graphs below and a lot of them are comparisons against the other Pentax designs. Not certain how these would compare to say the likes of the Tak FSQ's.

[source: http://pentaxplus.jp/archives/tech/tels_sdp/index.html]

[AnakChan]

Nick, in your 2nd photo, does the adapter introduces any vignetting? I'm wondering if the adapters taper off too quickly that it blocks the peripheral of the light cone? I guess I should ask what size sensor you're using to to give  the question a little more relevance.

[NickK]

Nick, in your 2nd photo, does the adapter introduces any vignetting? I'm wondering if the adapters taper off too quickly that it blocks the peripheral of the light cone? I guess I should ask what size sensor you're using to to give  the question a little more relevance.

This was during some testing with the drivers with the 11000M - note that this was focusing on a target at the end of the garden hence the extension at 2x650mm.

I've always used a 2" filter and back setup for the 383L with minimal vignetting and standard flats nail it. On the large 11000 flats worked fine although the 2" would be better with a larger diameter through to the filters, the main thing is the weight of the camera at this size vs the borg that can cause tilt (the borg is really a medium weight system). The 11000 has a large 54mm thread rather than a t-thread.

[NickK]

Spot diagrams (all likely to be tak sourced):

FSQ 85: http://www.takahashi-europe.com/en/FSQ-85ED.optics.spots.htm

FSQ 106: http://www.takahashi-europe.com/en/FSQ-106ED.optics.spots.htm

FSQ 130: http://telescopes.net/store/takahashi-fsq-130ed-astrograph.html

This may be interesting: http://forum.astronomie.de/phpapps/ubbthreads/ubbthreads.php?ubb=showflat&Number=255953 ( http://homepage3.nifty.com/cz_telesco/refracter_test.htm)

[NickK]

Perhaps I should explain the 4-in-1:

On that picture:

left - the last ring of the focuser that connects and allows manual rotation of the entire camera assembly without unscrewing anything.

middle top - the adaptor ring I had made for the borg, it's a screw fit into the back of the scope and then the borg screw into that.

right - that's the complete 4-in-1 which adapts a few scope types to a 2" push fit with a single grip and brass collar inside. It works, it's solid but with heavy weight and like any push fit, it can leave the camera a little off centre. The borg is better because the push fit I use on that is closer to the camera (the borg acts to replace the 50mm 2" extension I needed with the 4-in-1). The borg (at the top off the screen you can see it's actually got a larger than 2" on one side and then actually only drops to the 2" push fit on the other at the last moment so it's actually better than the 4-in-1 for light cone avoidance too - the borg is nicely flocked inside too).

[AnakChan]

Nick, cheers for the explanation. Think I'll need to unscrew & measure the various Pentax rings (as yet, everything is still in the box unpacked - been a little too busy to take to take it out). I think I have an idea where the Baader PX ring fits though.

[NickK]

Nick, cheers for the explanation. Think I'll need to unscrew & measure the various Pentax rings (as yet, everything is still in the box unpacked - been a little too busy to take to take it out). I think I have an idea where the Baader PX ring fits though.

Be careful because there's M68 P1.0 and P.075 where the pitch changes means you need to know the size and the thread pitch.

[AnakChan]

Be careful because there's M68 P1.0 and P.075 where the pitch changes means you need to know the size and the thread pitch.

Cheers! That's one thing I didn't think about. That's a very good tip! What I may do is to take out the whole cone and to the shop to see what will fit. I've got to get not only the right sizes but the right length too so that I'm within focus range.

Finally I did unpack the scope after supper only to find out that I can't mount it on my Vixen AXD GEM. So I'll need to do more than just eyepiece/diagonal and camera adapter shopping but for baseplate shopping too. Here's a pic of the 125SDP with my µ-250CRS.

[AnakChan]

Hi @NickK,

I was wondering if the Baader 4-in-1 M84 to 2" clicklock would be of any use for a diagonal like the Takahashi 2"? The reason being is that it seems that it doesn't allow inward-travel of the focuser according to the posts in the thread below.

http://stargazerslounge.com/topic/187899-pentax-scope-is-coming-back/?p=1961867

Cheers,

Sean

[AnakChan]

Scored a 2nd hand RC0.77x67P on auctions that cost me bomb:-

[NickK]

On 18 December 2015 at 03:31, AnakChan said:

Scored a 2nd hand RC0.77x67P on auctions that cost me bomb:-

 

Very nice.. just be concerned about the rear elements for the perzval design. Sorry I've been absent - been rather busy!

Next for me is to work out a robot focuser..