TheSkySearchers

Thanks to generosity of Mark (John Donne) in memory of late Henry (Hankmeister3) I am a proud owner of iOptron 150mm Maksutov-Cassegrain!
Was thinking of trying out a large Mak for a while, but never managed until now. Thank You so much Mark and Henry!

As you may know most of mass produced Maks on the market are of the Gregory design, where aluminized spot on the inner face of the corrector plate serves as the secondary mirror. 150mm iOptron is of the Rumak design, where a separate secondary mirror is mounted on the inner surface of the corrector plate. This provides extra degree of freedom in configuring Mak optics. You can see the secondary glued on inside, but interestingly the corrector also has aluminized spot. Could be that they are utilizing Gregory Mak corrector blanks?
Mak-Casses in general are praised for small central obstructions. For example, 150mm Orion/Synta Gregory Mak has central obstruction of ~10% by area. iOptron 150mM Rumak is a different beast with central obstruction of ~14%, same as in 150mm/6” SCT. 6” SCT baffle opening is 27mm. I could not find any info on the baffle opening for the 150mm Synta Mak, but I would assume it to be like 6” SCT. This means that both scopes were designed for use with 1.25” EPs only. I have measured iOptron 150mm Rumak baffle opening to be at least 35mm. This is close to 8” SCT baffle tube opening.

My only explanation is that iOptron 150mm Rumak was designed to be 2” EP-friendly, compromising somewhat light gathering power and contrast. It will be fun to try 2" EPs in it, so I decided to go with Altair Astro 2” prism for the first light.
Interestingly, most of iOptron Rumak owners report sharp and high contrast views, which makes me think that central obstruction size is just one of many optical system parameters determining contrast, and scope performance can't be solely judged just by the central obstruction size.

iOptron Rumak comes across as a well-built instrument. It is on the heavy side at ~14lb compared to ~12lb 150mm Synta Mak and 8.4lb 6” SCT, which says quality to me. While it is comfortable to handle, I feel that extra pounds place it beyond your typical grab and go mount like ES twilight I or SW AZGti, but it should ride well on SW Skytee 2 and iOptron AZ Mount Pro I have.

iOptron Rumak comes with a couple of nice features including two speed focuser and metal dew shield. I would certainly put the metal dew shield to a good use, however over the years I have mastered making my own custom dew shields from craft foam sheets and prefer them over metal ones.

I saw a few discussions on forums about use of insulation to overcome slow acclimation of large Maks and SCTs. That sounds reasonable and I even purchased some of the reflective foam insulation but never got around to try on my SCTs. This seems to be as good occasion as any. I wanted to make the insulation easily removable because I saw reports of moisture getting under and causing some issues. I came out with two parts: wrap-around with holes for the finder shoe and mounting bar secured in place with Velcro strips, and dew shield covering the front portion of the OTA. The dew shield has matte black flocking inside.
Here is the Mak sitting alongside SV102ED on iOptron AZMP, waiting for the first light.

Congratulations on your new acquisition. Your post raises some interesting points which I'll address. I'm the very happy owner of an Intes MK66 Rumak Maksutov.

Bigzmey wrote: ↑Wed Nov 10, 2021 2:11 am
....

As you may know most of mass produced Maks on the market are of the Gregory design, where aluminized spot on the inner face of the corrector plate serves as the secondary mirror. 150mm iOptron is of the Rumak design, where a separate secondary mirror is mounted on the inner surface of the corrector plate. This provides extra degree of freedom in configuring Mak optics. You can see the secondary glued on inside, but interestingly the corrector also has aluminized spot. Could be that they are utilizing Gregory Mak corrector blanks?

That is doubtful, the corrector blank is unsiivered. When producing a Gregory Mak corrector the blank is ground first and the silvering is done after completion. The radii of curvature of the two sides of the corrector are slightly different and optimized for the f ratio of the design. Rumaks are faster than Gregorys at f12 or even faster. Gregorys need f13 or f15 or even longer.

The silvering is odd.... For a long time I was persuaded that the iOptron was not a Rumak but a Gregory from the marketing photos. My MK66 has screws for collimation on the secondary support.

Mak-Casses in general are praised for small central obstructions. For example, 150mm Orion/Synta Gregory Mak has central obstruction of ~10% by area. iOptron 150mM Rumak is a different beast with central obstruction of ~14%, same as in 150mm/6” SCT. 6” SCT baffle opening is 27mm. I could not find any info on the baffle opening for the 150mm Synta Mak, but I would assume it to be like 6” SCT. This means that both scopes were designed for use with 1.25” EPs only. I have measured iOptron 150mm Rumak baffle opening to be at least 35mm. This is close to 8” SCT baffle tube opening.

My only explanation is that iOptron 150mm Rumak was designed to be 2” EP-friendly, compromising somewhat light gathering power and contrast. It will be fun to try 2" EPs in it, so I decided to go with Altair Astro 2” prism for the first light.

The central obstruction is better expressed in linear % rather than area % terms. The use of area is perhaps a marketing ploy to generate a smaller number. The linear central obstruction seems to be a better metric for quick assessment of contrast, etc. The MK66 has a 33% linear obstruction and so handles 2" eyepieces well.

Interestingly, most of iOptron Rumak owners report sharp and high contrast views, which makes me think that central obstruction size is just one of many optical system parameters determining contrast, and scope performance can't be solely judged just by the central obstruction size.

This is true. If you look at the MTF or point spread function it behaves interestingly for obstructed optics. Low spatial frequency contrast is depressed but high frequency contrast is enhanced compared to an unobstructed scope. See figure 104 at https://www.telescope-optics.net/obstruction.htm. It shows that obstructed scopes are better than unobstructed at very high spatial frequencies..... Better than perfect even. That comes at the loss at the low frequency end. Fracs and Newts for fuzzies, Maks and aplanatic SCTs for planets and detecting faint galaxies.

iOptron Rumak comes across as a well-built instrument. It is on the heavy side at ~14lb compared to ~12lb 150mm Synta Mak and 8.4lb 6” SCT, which says quality to me. While it is comfortable to handle, I feel that extra pounds place it beyond your typical grab and go mount like ES twilight I or SW AZGti, but it should ride well on SW Skytee 2 and iOptron AZ Mount Pro I have.

It is then comparable in mass to my MK66. It rarely sits on my Vixen Porta II for terrestrial use but needs the deft touch of a neurosurgeon or safecracker to balance. It's more usually on my CG5.

iOptron Rumak comes with a couple of nice features including two speed focuser and metal dew shield. I would certainly put the metal dew shield to a good use, however over the years I have mastered making my own custom dew shields from craft foam sheets and prefer them over metal ones.

The MK66 has a single speed focuser and a dew shield.

I saw a few discussions on forums about use of insulation to overcome slow acclimation of large Maks and SCTs. That sounds reasonable and I even purchased some of the reflective foam insulation but never got around to try on my SCTs. This seems to be as good occasion as any. I wanted to make the insulation easily removable because I saw reports of moisture getting under and causing some issues. I came out with two parts: wrap-around with holes for the finder shoe and mounting bar secured in place with Velcro strips, and dew shield covering the front portion of the OTA. The dew shield has matte black flocking inside.

Interestingthat you and John Barrs go this route. I've never insulated my Mak. I prefer to cold soak in advance of an observing session.

Great report Andrey.
I like the insulation and dew shield.
I look forward to your first light report.
Clear skies soon !

Thanks Mark and nFA!

I have figured out the mystery of reflective spot. If you look close it is set back relative to the corrector. It is indeed a separate secondary cemented to the corrector. They left the back of the secondary untreated so you see the reflective layer through the secondary glass and the corrector glass. Since the secondary is fixed the scope is collimated at the main mirror.

Here is a fun pair of scopes: 6" F12 Rumak and 6" F5 achro. About same size and weight, compliment each other nicely as far as strengths and weaknesses go.

Bigzmey wrote: ↑Wed Nov 10, 2021 8:56 pm Thanks Mark and nFA!

I have figured out the mystery of reflective spot. If you look close it is set back relative to the corrector. It is indeed a separate secondary cemented to the corrector. They left the back of the secondary untreated so you see the reflective layer through the secondary glass and the corrector glass. Since the secondary is fixed the scope is collimated at the main mirror.

Here is a fun pair of scopes: 6" F12 Rumak and 6" F5 achro. About same size and weight, compliment each other nicely as far as strengths and weaknesses go.

Interesting. The MK66 is (rarely, 2-3 times in 20 years) collimated at the secondary.

Bigzmey wrote: ↑Wed Nov 10, 2021 8:56 pm Thanks Mark and nFA!

I have figured out the mystery of reflective spot. If you look close it is set back relative to the corrector. It is indeed a separate secondary cemented to the corrector. They left the back of the secondary untreated so you see the reflective layer through the secondary glass and the corrector glass. Since the secondary is fixed the scope is collimated at the main mirror.

Here is a fun pair of scopes: 6" F12 Rumak and 6" F5 achro. About same size and weight, compliment each other nicely as far as strengths and weaknesses go.

Discovery is a lot of fun Andrey.

If it ever needs collimating maybe it will be easier to do from the rear of the telescope.
As was mentioned it probably won't need collimating very often.

I am looking forward to the compare of these telescopes.

John Donne wrote: ↑Wed Nov 10, 2021 9:50 pm

Bigzmey wrote: ↑Wed Nov 10, 2021 8:56 pm Thanks Mark and nFA!

I have figured out the mystery of reflective spot. If you look close it is set back relative to the corrector. It is indeed a separate secondary cemented to the corrector. They left the back of the secondary untreated so you see the reflective layer through the secondary glass and the corrector glass. Since the secondary is fixed the scope is collimated at the main mirror.

Here is a fun pair of scopes: 6" F12 Rumak and 6" F5 achro. About same size and weight, compliment each other nicely as far as strengths and weaknesses go.

Discovery is a lot of fun Andrey.

If it ever needs colonating maybe it will be easier to do from the rear of the telescope.
As was mentioned it probably won't need colummating very often.

I did a brief visual check and the rumak appears collimated. Star test will show for sure. As nFA stated Maks and SCTs hold collimation quite well. Even if it goes off typically just a slight adjustment is required.

Hi Andrey. Congratulations on receiving the iOptron 150mm Rumak telescope from Mark which was part of Henry's legacy. And you and nFA did a nice tutorial on the differences between the Rutten and Gregory Maksutov optical design telescopes. You should really enjoy using this telescope for many years and as I have mentioned on other threads, it is a great companion to the short focal length wide field 6" f/5 refractor on observing sessions. Thanks for your equipment report Andrey and we hope to see a first light report from you soon.